Our ProMaster DIY Camper Van Conversion — Electrical and Solar

Our aim for the camper van electrical system is to be able to do without electrical hookups indefinitely, and to be able to be away from any power source for a day or two without much sun and without having to run the engine.

To achieve this we have tried to keep electrical loads down by choosing efficient gadgets, avoided some high power consumption electrical devices, included some extra battery capacity, and a large solar panel to charge the battery when sun is available.


This section goes over the design of the electrical system for the camper conversion, the selection of components, and the installation.

Back to the ProMaster camper van conversion main page…

Safety Warning and Disclaimer

There are serious safety issues involved with wiring your own system.  The voltages  are high, and potentially lethal. Doing the system incorrectly can lead to serious consequences down the road.

PV systems have the added hazard that even when the grid power is turned off, the system can be “live” and present a serious shock hazard.

If you don’t feel like you want to put in the time to learn how to do this correctly, then find an electrician that you can partner with to do this part.

I want to make it very clear that I am not an electrician, and I take no responsibility whatever for the correctness of the wiring hints below — you need to do your own homework!



We do not want to be tied to electrical hookups at RV parks, so we incorporated a good sized house battery and tried to minimize our electrical loads. Most of our loads are DC and run directly off the house battery with only a couple of modest AC loads that can be powered either by our onboard inverter or shore power.

The house battery can be charged from the van alternator, or from a large solar panel on the roof, or from shore power if available. The battery can supply a couple of days (or more) with typical loads, and on a sunny day, the solar panel is sized to fully recharge a depleted house battery.

At this point (Jan 2017) we have used our van for about 2 years and the electrical system has performed well without any significant problems.

wiring diagram rv conversion


At the heart of the camper electrical system is the house battery. It can receive charge from three separate sources: 1) the van alternator when the engine is running, 2) the roof mounted solar panel, and 3) the inverter/charger when plugged in to shore power.

The house battery powers all of the DC loads (like lights, fridge, furnace fan, water pump) via the DC distribution panel. It also powers an inverter to provide 120 Volt AC house power to our small number of AC loads. Most of the electrical devices on the van run directly from the 12 volt house battery.

The Battery Isolator prevents the RV loads from running down the van starting battery, the PV panel charges the house battery via the Charge Controller, the DC and AC distribution panel safely distributes power to the RV electrical loads, the Inverter/Charger supplies 120 AC power from the house battery and also charges the house battery when hooked up to shore power.

All of this explained in much more detail below.



These are our choices for the main components.

Item Description Cost
House Battery 2 @ 6 volt, 200 AH golf cart batteries (Costco) $180
Inverter/Charger Triplite APS 1250 Inverter/Charger $406
Battery Isolator PAC-200 Battery Isolator 200 amp $46
MPPT Charge Controller Midnite Solar KID Charge Controller $280
AC/DC Distribution Panel PD5000 AC/DC Power Control Panel $58
PV Panel SolarWorld SW315 PV panel $368
Total (not including fuses, wire, …) $1338


We used two 220 amp-hr, 6 volt golf cart batteries from Costco connected in series for 12 volts. This gives us some reserve for cloudy days..

We thought about using an AGM battery, the advantages of the AGM battery is that it does not need water to be added (low maintenance), it vents less hydrogen during charging than a flooded battery, and there is no chance of spillage.. On the down side, they are quite a bit more expensive for the same capacity, and appear (from the Trojan Battery data below) to have a shorter life.

The conventional flooded lead-acid golf cart batteries we used are less expensive and very readily available, but will require checking the water levels from time to time, and the battery compartment will have to be vented to the outside to prevent hydrogen buildup. But, flooded batteries vent very little hydrogen when charged at the correct rate.

Just as an approximate comparison — some data on two of Trojan Batteries offerings of flooded and AGM batteries.

Type Model Volts amp-hr
Len Wid Ht weight Cycles
(80% discharge)
Flooded T-105 6 225 10.3 7.11 11.07 62 750 $110
AGM 6V-AGM 6 200 10.3 7.08 10.74 65 500 $295

Trojan recommends a charge rate of 10 to 13% of the 20 hr amp-hr rating, so about 22 to 30 amps for our golf cart batteries — lower charge rates are OK, but higher charge rates will result in more gassing and shorter life. They also recommend 3 stage chargers. Charging voltages for flooded and AGM batteries are different, so the charger should be settable to the type of battery you have.

Both types will have a longer life if they are not discharged as deeply, but, it seems like for a camper van that might only be used 30(?) times a year that discharging to 80% should still give a very long battery life. Designing for 80% discharge allows the use of a smaller, lighter, more compact, and cheaper battery compared to (say) designing to 50% discharge.

The batteries live inside the van in a case that is vented to the outside.

Another option that is just becoming practical is to use Lithium batteries (as used in electric cars). They would reduce weight and size by quite a bit over the lead acid batteries, but are still expensive, and would likely take some careful homework to get right.

AC and DC Distribution and Fuse Panel

After quite a bit of looking around, I found this nice and not very expensive distribution panel for RV’s that handles both the AC and DC distribution in one fairly compact package.

It provides for up to 12 DC circuits and up to 4 AC circuits.

rv conversion DC distribution panel

Battery Isolator

This gadget allows the van alternator to be used to charge the house battery, while preventing the van starting battery from being drained by powering loads in the RV. It isolates the van starting battery from the house battery when the ignition switch is off so that RV loads only discharge the house battery. The idea is that even if you discharge the house battery overnight, the van starting battery will still be fully charged.


This relay based unit appeals to me because: 1) it does not have the voltage drop that the diode based isolators do, 2) it does not get wired between the alternator and the starting battery as the diode ones do (so less interference with the van wiring), and 3) It is easy to hook up because it just needs a wire from the starting battery and an 12V source that goes on with the ignition switch.

Not sure if I’m reading this right, but it seems like when you start the van that this unit puts the starter battery in parallel with the house battery. I guess this could be good if your starting battery is low, but maybe not so good if your house battery is low?

In hindsight, a unit with a lower amperage rating would have been fine. The unit is inline with a 50 amp circuit breaker, so anything over 50 amps would probably be fine. The maximum recommended charging current for the golf cart batteries is about 30 amps.

Another option would be to use a Voltage Sensing Relay. This is a relay similar to the one above, but the relay is activated just monitoring the van battery voltage to determine if the engine is running or not. It requires less wiring.


The inverter/charger performs two jobs. When you are not hooked up to shore power and are being powered by the house battery, it provides limited 120 volt AC power for the van from the house battery. When you are hooked up to shore power, it turns off the inverter function and passes the shore power through to the AC loads that are connected to the inverter, AND it charges the house battery using a full 3 stage charger.

We selected the Tripp Lite APS1250, which provides 1250 watts of AC from the battery, and when in charger mode, provides up to 30 amps of battery charging using a 3 stage charger.


These inverter chargers are made by several companies. I chose the Tripp Lite mostly because I have a larger one I bought several years ago and it has held up well.

I like this kind of inverter/charger because: 1) it eliminates the need for a separate battery charger to charge from shore power, 2) it has 3 stage charging, so its easy on the battery, 3) it automatically switches from the inverter powering AC loads to the shore power when you plug into shore power, so you can hook your AC loads up to the inverter and they will be powered whether you are on shore power or on house battery power.

Like most of these inverters, it draws some power from the battery whenever the inverter is turned on even if nothing is plugged into the inverter, but it can easily be turned off manually when no AC power is being used, so this is a nuisance, but not a serious one.

Update 1/1/2016: One thing I’ve noticed is that pure sine wave inverter/chargers have dropped in price some, so if you plan to run AC loads that might need the pure sine wave output, you might want to upgrade to a pure sine wave inverter — one example is the Xantrex Freedom HFS 2000 Inveter Charger.

Solar Charge Controller:

The charge controller goes between the PV panel and the house battery. It transforms the PV panel output voltage down to a voltage that is suitable for charging the battery, and it prevents the solar panel from overcharging and damaging the battery.

I am using the MinNite Solar KID charge controller. This is a relatively new design that has quite a bit of flexibility for small solar systems. It includes MPPT (Maximum Power Point Tracking) which is more efficient than the PWM models.

The unit is very nicely made in the US. The manual is written in an informative and down to earth style that is refreshing compared to the awful manuals that come with so many products. I’m told the phone tech support is very good.

The unit includes basic monitoring of the system, and they sell an add on unit for more extensive monitoring if desired.

Midnite Solar has a nice online tool that lets you see if the PV panel(s) you are planning to use are compatible with the KID.

Price is about $285.

Update: March 4, 2015: They have just raised the price up to $440. I think that this is kind of steep, and I’d probably look for a better option.


PV Panel:

I decided to use just one large solar panel. This seems easier than mounting and wiring together several panels.

The panel I selected the SW315 from Solar World. Its a 315 watt, 72 cell panel. The 72 cell design provides enough voltage (36 volts) to work with the KID charge controller as a single panel.

I ordered the panel from the local Platt Electric — their price was competitive and if you pick it up at Platt there is no shipping charge. With a little discount they gave, it came to $368 — about $1.17 per peak watt.


I have not mounted the panel yet. Thinking about mounting it as low as possible and aft of the Maxx Fan on the centerline of the roof.

Still thinking about the best way to bring the two wires from the PV panel into the van. I’m wondering if I can bring them through the backup camera housing?

Update August 2015: Solar panel mounted …

Update: Full details on the solar panel mounting and wiring here…

Update: March 2017 – new product to look at

Ran across this house battery charger.  It will charge from the van alternator or solar (or both at same time).  Its a full three stage charger that will charge the house battery optimally regardless of the alternator output voltage or solar panel voltage.  It has charge profiles for all the common lead-acid batteries.  The solar charger accepts only 12 volt PV panels, but it is an MPPT charger.  It appears to be a new product, but definitely worth a look.



For most of the DC and AC circuits, I used regular 14 gage Romex house wire. I used nearly all of one 250 coil. There are a couple runs that use 12 gage Romex due to higher demand and/or to reduce voltage drop. I’m aware that some sources recommend using stranded wire for greater vibration resistance, but the Romex is widely used in RV’s and does not appear to cause an problems. The outer plastic protective sheathing and its wide availability at low cost are pluses.

There were a few places where larger gage stranded THHN wire was used (e.g. between inverter and battery and between van and house battery). I got this all at Lowes where they will cut pieces to the length you want.

For the most part, I used this ampacity table to determine the maximum capacity of a given wire gage, and this voltage drop table (the 12 volt section) to determine the minimum wire gage for less than 2% voltage drop.




While the overall wiring diagram for the whole system gets a bit busy (and more than I am up to drawing), it really consists of a few simple functions that are largely independent of each other. This section goes through the details of each of the functions, which are:

  1. House battery charging from the van alternator
  2. House battery charging from the solar panel
  3. Satisfying 12 volt DC loads (DC distribution panel)
  4. Satisfying 120 volt AC loads and Shore Power


House Battery Charging from Van Alternator

This is the electrical subsystem responsible for charging the house battery from the van alternator when the van is running.


When the van engine is running, the relay closes allowing the van battery/alternator to charge the house battery.

The battery isolator relay connects the van battery to the house battery only when the van ignition switch is on. This prevents house loads from discharging the the van battery and resulting in a vehicle that has a flat battery in the morning and won’t start.

The barratry isolator relay is turned on by connecting it to a 12 volt power source on the van that is only on when the ignition switch is on. I used power from the 12VDC outlet near the driver side back door as it is only powered when the ignition is on. I ran a 14 gage wire from the 12 VDC power socket to the battery isolator. The other relay terminal on the battery isolator is hooked to any handy ground. The isolator I used is a PAC-200 (see above) — the 200 amp rating is quite a bit more than is required.

The 50 amp DC circuit breaker protects the wire between the van and house batteries. The circuit breaker can also be manually switched off, so it acts as a disconnect when you want to be sure there is no connection between the van and house batteries.  The breaker is sized to be a bit above the maximum recommended charging current for our batteries so that if the breaker trips we will know the battery is being charged at too high a rate.  The maximum recommended charging rate flooded lead acid batteries is in the range of C/10 to C/5 (where C is the capacity of the battery in amp-hrs), so about 33 amps for our batteries.

The van alternator is the source of power for both charging the van battery and the house battery.

The wires between the house and van battery are 8 gage — this provides the 50 ampacity with less than 2% voltage drop for the about 10 ft run.

Battery Charging from Solar Panel

This is the part of the electrical system that charges the house battery using a roof mounted PV panel.

solar charging of rv house battery

Charging House Battery from Solar Panel


The solar panel is a large one from Solar World that provides 315 watts under full sun. It is a 72 cell PV panel so, it has a higher output voltage (36 volts) that is compatible with the input voltage range of the Midnight Solar charge controller with only the one panel hooked up.

The charge controller is from Midnight Solar — its is described under the Components section. Basically, it transforms the DC voltage put out by the solar panel to the voltage needed to charge the house battery (so the PV panel might be putting out 36 volts and the charge controller transforms this to 13 to 14ish volts needed to charge the battery. The charge controller can be set for charging either AGM or flooded batteries. Its a 3 stage battery charger. The charge controller also prevents the PV panel from overcharging the house battery, which would damage it.

The 30 amp DC breaker between the charge controller and the house battery is what Midnight Solar recommends as this is the maximum charge current that the charge controller can supply. It also protects the #10 wire. Right now this breaker is a model that does not have a manual shut off switch built into it, and I plan to replace it with one that does when I find one.

The PV panel provides standard MC4 PV panel connections (a positive and negative). To connect this to the charge controller, I bought a 50 ft long MC4 extension cable and cut it in half. I used one half of it to connect to the plus MC4 terminal on the PV panel and the other half to connect to the negative. The extension cord is #10 wire, and will probably end up being about 15 ft long. The maximum power point current for the PV panel is 8.7 amps. The voltage drop for 15 ft of #10 wire at 8 amps is about 2%, or about 0.7 volts under full sun conditions — this seems OK to me.

The wires connecting the charge controller and the house battery are #10, which has the required 30 amp ampacity. The wire run is only a couple feet long, so the voltage drop is low.


Satisfying 12 Volt DC Loads

This is the part of the electrical system responsible to safely supplying power to the campers 12 volt load:

DC distribution panel for RV conversion

DC Distribution Panel

The DC distribution panel takes 12 volt power from the house battery and distributes it to the various 12 volt loads in the camper. It provides individual fuse protection for each 12 volt circuit.

I am currently using 11 of the 12 available fuse positions.

The 60 amp breaker in the line from the house battery to the DC panel is somewhat more than I think our total DC usage will ever be, and is well within the 100 amp maximum rating of the DC panel.

Note that for our system, the DC and AC distribution panels share the same housing, but are separated by an internal partition.

Satisfying 120 Volt AC Loads and Shore Power Battery Charging

This part of the electrical system is responsible for distributing power to the 120 VAC loads in the camper, and also manages whether the source of the AC power is shore power or the camper inverter. It also covers charging the house battery from shore power.

AC distribution panel for RV

AC Power Distribution


The AC panel distributes power to the 120 volt AC loads in the camper in the same way as the DC panel (above) distributes power to the DC loads. We have planned the camper to minimize AC loads and only have a couple of AC outlets and a circuit to power a small microwave (we ended up not getting a microwave).

The inverter/charger plays the key role in how the AC power is supplied. If the camper is not hooked up to a 120 VAC shore power connection, then the inverter/charger goes into inverter mode, and takes power from the house battery and transforms it to 120 VAC. It will supply up to 1250 watts in this mode. If the camper is hooked up to shore power, then the inverter/charger shuts down its inverter, and just passes the shore power through itself and to the AC distribution panel. It also starts up its battery charger and if the house battery needs charging, it charges it from the shore power — it supplies up to 30 amps of charge current.

For the wires from the inverter/charger to the house battery Tripp Lite recommends #2 AWG for this model as long as the length is 16 ft or less (ours is only about 3 ft). It says to use a fuse in this line that is located close to the battery and rated at least to the maximum amperage listed on the inverter, which is 127 amps. I am using a 150 amp breaker to protect the #2 wire.

Since our AC loads are low, the shore power receptacle is just a 20 amp regular 120VAC plug. I think this will be fine and it allows us to just use a regular 12 gauge extension cord to hook up to the shore power tower. To hook up the shore power receptacle, I just cut the plug off the end of the inverter/charger power cord and wired it to the shore power receptacle.


This section provides some pictures showing how I installed the components in our van.

The batteries plus all the main components take up an about 2 by 2 ft space under the bed on the drivers side.

electrical center for RV conversion


The breakers can also be used as switches to disconnect each function.

The distribution panel on the right serves as both the DC and AC distribution panel — AC on forward end and DC on aft end.


The battery compartment is vented to the outside to prevent any buildup of Hydrogen in the compartment.

The vent is 3/4 inch PVC  that runs from the top of the battery compartment in a short run to the side of the van.  A hold is drilled in the side of the van and a louvered aluminum cover keeps the water out.

An about 7/8 inch hole was drilled through the front face of the battery compartment near the top (left picture).  This was connected to the hole and vent cover in the van sidewall via a short 3/4 inch PVC pipe.  The right picture shows the PVC vent pipe.  Since the vent cover was larger than the 3/4 inch vent pipe, a PVC adapter fitting was used between the two.

An inlet vent is also required, and this was placed in the inboard wall of the battery compartment about half way down.  It is just a 3/4 hold drilled through the battery compartment wall into which a short piece of 1/2 inch PVC pipe is sealed in place.

Inlet vent.

Inlet vent.

Another view of finished vent.

Another view of finished battery vent along with furnace vent and the shore power hooked.

I have to admit to a little personal skepticism about the necessity of the vent system — it seems to only come into play when the charger malfunctions and severely over charges the battery producing Hydrogen — but, better to error on the safe side.

The battery compartment lid looks a bit like a shoe box in which the bottom of the box forms the lid of the battery compartment, and the top of the box is the forward end of the drivers side bed platform.

The battery box lid.

The battery box lid.

The battery box lid sides down into place between the inboard and outboard vertical walls of the bed platform and is held in place with two wing bolts (for easy access to the batteries. There is probably a better way to do this, but this does work OK.

I still plan to install an EPDM rubber liner that will go most of the way up the sides of the battery compartment to contain any spills and form a corrosion barrier between the already quite corrosion resistant MDO compartment walls and the battery.

Shore Power Cord

Its handy to have the things like the cord to hook up to shore power, the hose to fill with fresh water, and the hose to drain the grey water where they are easily accessible from the outside (where you use them).  An outside compartment (say under the van) would be ideal for this, but I could not find a good spot.  I decided to stow them on the back face of the bed platforms where they can be easily accessed from the outside via the back doors.


The shore power cord (50 ft extension cord) and the fresh water fill hose are hung on the back of the passenger side bunk, and the grey water drain hose hangs on the back of the driver side bunk.  This has worked out nicely.

The hangers are homemade out of short sections for 3 inch PVC pipe and screwed to the bed platform back wall.

Note Comments section at the end of this page — Tyler shows a way to put the shore power receptacle in the ProMaster bumper — so, no hole in the side of the van.  He also shows a very simple way to do a single AC circuit for the van with one very simple and cheap fuse and switch arrangement that you can buy at Home Depot.

Alternate bumper location for shore power socket

Alternate bumper location for shore power socket

Here is another method of doing a “stealth” shore power cord…

Distribution Panel

This pictures shows the AC and DC distribution panel (top) and the solar charge controller (below). They are mounted on driver side bed enclosure facing the aisle between the two beds.

AC and DC Distribution panel RV conversion

The combined DC and AC distribution panel mounted above the solar charge controller.


This shows the AC/DC distribution panel with the cover off. The DC distribution panel is to the right (with the car type DC fuses), and the AC distribution panel to the left with the AC circuit breakers.

The DC panel provides for up to 12 DC branch circuits, of which I am using 11 (some of the fuses have not been installed yet).

The main breaker is the top one (it shuts off all AC). Up to 4 branch circuit breakers can be installed below the main breaker (I’m only using 2). Note that the 30 amp main breaker is going to be replace with a 20 amp later.



Switch Panel

I just recently added a small panel that adds on/off switches to the furnace and to the pump, and provides some extras for future use. It also provides 12 volt DC and USB charging outlets as well as a small volt meter to provide for easily monitoring the house battery voltage.



The switch panel mounts next to the fuse panel on the vertical face of the driver side bed platform.

The two switches on the left are pump and furnace, and the remaining switches are spares for future use. We have always found it to be a good idea to turn the pump off when not in the RV, as any kind of leak or faucet left on will cause the pump to run and drain the tank and them damage the pump. The switch on the furnace allows you to completely turn the furnace off and still have other DC gadgets powered up.

The top row has a 12VDC outlet, then volt meter, then a dual USB outlet.

The blocks of wood on the top and bottom are to keep things from hitting the switches and accidentally tripping them.

I have not updated the wiring diagram to show this panel, but the switches are in the plus lead between the fuse and the the furnace or pump, and the switches should be rated to at least the same current as the fuses.


Solar Charge Controller

The charge controller goes between the PV panel and the house battery and regulates and optimizes the charging of the battery. It also prevents the PV panel from overcharging the battery.

The MidNite Solar KID charge controller is mounted in the bed enclosure just below the distribution panel. This means you have to get down on hands and knees to read the status, but it does keep the wire runs short and the electrical bay compact. If we decide to do upper cabinets (above the windows) in the van, I may move the charge controller to one of the upper cabinets for easier reading — I’d convert the hole for the current position into an air vent to ventilate the electronics area.

There are several status screens that display various info about the solar charging — e.g. PV panel voltage, battery voltage, PV panel watts, …



Solar Panel Install

Full details on the solar panel mounting and wiring here…

See the full description of solar panel mounting and wiring at link just above.

See the full description of solar panel mounting and wiring at link just above.

Shore Power

Since our AC loads for the camper are small, I decided to just use a 15/20 amp shore power connection — this is just a regular 12 gage extension cord that plugs into an ordinary 120 VAC outlet. Most RV’s now use 30 amp or 50 amp shore power connections, but with our small AC loads we just don’t need that.

This is the through wall connection we used:



A bargain by RV part price standards at only $15.

These pictures below show installing the shore power receptacle.


Connection to Van Battery

The connections to the van battery are shown below. I just used some unused connection points on the ProMaster battery terminal lug.

Connection to positive terminal of van battery

The connection with the Mega Fuse is the one that goes to the house battery via the battery isolator.

The ProMaster battery terminals have metal plates attached to them that serve as an attachment point for several wires and fuses.  I used an unused position on the plates to attache the wire that goes to the house battery.  This wire is fused with a Mega Fuse that attaches directly to the plate.


Adding A Battery Monitoring System

A Battery Monitoring System basically tells you with good accuracy how much juice is left in your battery at any time.  It works by first sensing when the battery is at full charge, and then monitoring all the current going into and out of your battery and summing this up to let you know how much capacity is left.

This is not a must item at all, but it does provide some useful information

Victron Battery Monitor

We recently added a Victron Battery Monitoring system to our conversion — all the details here…

If you think you might want to add a monitoring system at some point in the future, then I would reserve a space for the shunt — see the link above for details.

Securing Heavy Components

The batteries weigh about 60 lbs each and the Inverter/Charger is also heavy. I was concerned about these breaking away in a crash and coming forward to injure passenger or driver. So, this end of the bed box is bolted down through the floor with five 3/8 inch steel bolts. The other end of this bed houses the propane bottle and is bolted through the floor with several more bolts. The bed boxes are made out of a premium quality MDO plywood with all joints glued and screwed.


Bolts that secure battery compartment through the van floor.

The picture shows how the through floor bolts are anchored under the floor — the steel plate provides tear out resistance. The brown paint is Rust Oleum rust primer.


Electrical Loads Estimate

I did a rough estimate of the power use while in camping mode, and estimated the battery size and PV panel size needed to support this load for a couple days.

I have assumed an 80% battery discharge level in these estimates. I know that batteries will last longer if only discharged to 50%, or even less, but if you look at the Trojan estimate for number of charge/discharge cycles for an 80% discharge, its about 750 cycles. For an RV that only gets used maybe 30 days a year, 750 cycles is about 25 years, and I’m sure the battery will die of something else before 25 years. Maybe I’m missing something?

The spreadsheet also includes a rough heat loss calculation to get the furnace size and the power drain for the furnace fan and electronics. This came out to about 3000 BTU/hr with 32F outside and 68F inside the van — this is for an insulated small van.



If you have something that will view Excel Spreadsheets, download Excel Spreadsheet here…

Otherwise, table below is copied out of the spreadsheet, but its hard to read because of the formatting.

Van Conversion — Electrical loads Estimate
These are loads for one 24 hr period.
Available Power From Battery
Battery size (amp-hr at 12V) discharge depth voltage Energy avail (wh)
100 0.8 12 960 Trojan SCS150 RV/Marine battery: 100 ah, 11.3 L X 6.7 W X 9.8 D
200 0.8
1920 This could be two of the Trojan 6V golf cart bats (or two Costco golf cart bats) in series
Might want to just leave space provisions for the 2nd battery.
Assumed below that things that are on all the time (CO detector, fridge) will run off the battery for 16 hours — that engine running or solar will take care of the rest)
Solar Panel:
Based on the stuff below, you would want the solar panels to put in about 500 wh over the day.
Electrical Loads:
Item Usage Time (min) Wattage Energy (wh)
Lights three LED at 3.5 watts each? 180 10 30.0
water pump 6 40 4.0
TV 60 20 20.0
Hair Dryer 4 500 33.3
Furnace fan 210 22 77.0 This is based on 14 hours per day when heating is needed at 25% load factor (see below) with 12K furnace, or 210 min
computer 90 40 60.0
Fantastic fan 30 21 10.5 low 1.3 amp, hi 1.75 amp (note that this would be much higher for summer, but furnace would be lower)
CO detector 960 1 16.0 the 1 watt is just a guess — should find out
LP gas detector 960 1 16.0 the 1 watt is just a guess — should find out
Fridge THIS ONE NEEDS REFINEMENT 960 12 192.0 This is roughly based on Jeff Yago’s article times about 3X — still seems kind of low.
Total 458.8 watt-hrs per day
Optional added loads
microwave minimal use if not on shore power 0.12 700 84.0
* with this setup, and 80% discharge, the 12 V – 100 AH would be good for about 2 days, and the  2 6V 225 AH would be good for about 4 days (both probably less in the real world)
*  JY says that good fridges use 2 to 3 ah/day per cf — conservatively use 4 ah/day-cf.
A 1.7 cf fridge then uses (4ah/day-cf)(1.7 cf) = 6.8 ah/day, or (6.8 ah/day)(12 volts) = 82 wh/day, with an average draw of 3.5 watts (seems low)
Dometic says 40 watts, but this would be 365 KWH per year, which seems high for a tiny RV fridge.
Need a better source for power used by the Danfoss compressor fridges.
* At 0 F outside temperature, the furnace would be on twice as much and furnace watt hours would be 150 ish.
* In summer, more fan operation might add as much as 200 or so wh, but furnace would drop to 0 (-62 wh)
* adding a microwave even if operated on battery would not be too awful as long as time of use is limited.
Van heat loss :
Item area R value Tavg outside Tin Heat loss (BTU/hr)
walls 210 6 32 68 1260
Floor 78 9 32 68 312
ceiling 72 6 32 68 432
windows single 30 1.5 32 68 720
windows double 18 3 32 68 216
infiltration 468 32 68 82 at 3.0 ach
Total Heat Loss 3022 BTU/hr
So, a 12K BTU/hr furnace would be running (3022/12000= 25% of the time)
Propane burn per day would be (3000 BTU/hr)(24 hrs)/(92000 BTU/gal) (0.8 efic) = 0.97 gal/day

Correction: The power drain for the 700 watt (nominal) microwave should be more like 1000 watts.

Update June 20, 2015: Did a test on our Norcold 3 cf fridge with danfoss compressor…  The power usage turned out to be about 570 watt-hrs per day –more than twice what is in the table above.   And, this is a relatively efficient and small fridge.


Solar Panel Charging:

Using PVWatts as a rough way to estimate output…

For a 315 watt panel mounted horizontally in Billings MT, the average output per day is:

1575 watt-hrs per day in July

440 watt-hrs per day in October

300 watt-hrs per day in December

With tilt equal to latitude (46 deg), the July number goes to 1450 wh/day and the Dec output goes to 800 wh/day.

This is with average weather for Billings — you can, of course, have full overcast days with very little production.

Our estimated use per day is about 400 watt-hrs, so for most of the year, on average, a 315 watt panel would easily meet our daily needs even with the horizontal tilt. For the deep winter, the panel only meets about 75% of our daily usage, but if it could be tilted, it would meet our daily needs with a bit of margin.

The above are with a PVWatts service factor of 0.77 — the real world one may be a bit less than this with the battery charge/discharge inefficiency included.

Note that PVWatts has a bug that results in inaccurate output with arrays smaller than 1 KWH, so you have to put in a larger array, and then just scale the output numbers down.

How Has It All Worked Out?

After three trips in the camper van, I’d say the electrical system is doing fine.  It runs all of our loads fine, including the two main loads: the fridge (about 42 amp-hrs per day) and the furnace (about 20 amp-hrs per day).  There have been no malfunctions, and the only maintenance was to add water to the battery once.

The solar panel has only been in place for the last trip, and its performance has been fine.  While I have not carefully analyzed this, I think that for Spring, Summer, and Fall trips the solar panel will be able to provide all of the juice needed on sunny or part sunny days.  Dead of winter trips are harder because the sun is lower and there are more cloudy days.  I’ll try to log some actual data on how the solar charger performs with no charging from the van alternator on the next trip.

At some point, I may add a TriMetric  system to keep better track of the battery pack state of charge, but this is probably more out of satisfying curiosity than actual need.

Weights, Costs, Labor


Item Description Weight (lb) Cost ($)
House Battery 2 @ 6 volt, 200 AH golf cart batteries (Costco) 120 $180
Inverter/Charger Triplite APS 1250 Inverter/Charger 23.2 $406
Battery Isolator PAC-200 Battery Isolator 200 amp 2? $46
MPPT Charge Controller Midnite Solar KID Charge Controller 6 $280
AC/DC Distribution Panel PD5000 AC/DC Power Control Panel 2? $58
PV Panel SolarWorld SW315 PV panel 49.6 $368
Circuit breakers 2? 64
Wire #14 and #12 Romex 220 ft #14 Romex, 25 ft #12 Romex 14.6 $40
Other wire #2, #8, #6 short lengths 15? $30?
Terminals 1? $20?
Shore Power receptacle 1 $15
Inside lights, … TBD TBD
Total 236 $1507


I did not keep track of the time spent installing the electrical system as it was spread out over a month. It was probably roughly the equivalent of 3 full days.


September 18, 2014

Questions?  Comments?

I’d be happy to hear any ideas, suggestions, corrections, or questions.


  1. Hi there, what part did you use on the exterior of your van so that you could plug in your solar panels to an exterior plug of some sort that is weather proof to get your solar juice into your van? Like a marine or rv solar flow through maybe Lol?


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  3. Welcome to the Promaster family. I too basically cloned Gary’s plan and used a Trip-lite inverter/charger. I got to say that it is not that powerful, it won’t run a coffee maker or small microwave. As far as the charging of house batteries it was a huge pain plugging it in all the time. Since I added 2X100 watt solar panels
    I haven’t used the charger at all.

    • Hi Yenne,
      Its not a large inverter, but we have run a small 700 watt microwave on it OK. I guess the coffee pot would depend on the wattage. Its been an ideal size for us, but we don’t use our 120VAC powered stuff much.

      Not sure what you mean by plugging in the charger — it should start charging the battery automatically as soon as you hook up to shore power. When you hook up to shore power it should automatically start charging the battery from shore power and hook up your 120VAC outlets in the van to shore power and stop running the inverter. That is, it has an automatic transfer switch built into it.


  4. Hi Gary,

    Great job with this guide — it has been immensely helpful as I work on my own Promaster conversion.

    I’m planning to use the same TrippLite inverter/charger that you recommended, and was hoping you could elaborate on how you handled the unit’s “main ground lug”. Did you connect this to your chassis ground?

    Also I was hoping you could say more about how you did the chassis ground. Did you drill through the floor, run an 8 gauge wire through, and then attach it somewhere on the frame underneath the van? Or does the Promaster by any chance have a good factory grounding point somewhere handy?



    • Hi Steve,
      I went out to the RV to remind myself what I did, and I’m really embarrassed to say that I had not hooked up the ground at all — oops!

      I have some #8 wire on hand and will run this over to the chassis ground point today.

      You can see the simple chassis ground point I used for other grounds on this picture: http://www.buildagreenrv.com/wp-content/uploads/2015/06/COOCELElecBayWithLabels.jpg
      Its visible on the extreme left — the green ground wire just goes to a large sheet metal screw through the nearby van vertical rib. For the inverter ground, I’ll press a round terminal onto the end of the #8 wire and put this under the same sheet metal screw. I think this fine, and probably better than trying to ground to the van frame where it would be more subject to corrosion.
      Self tapping sheet metal screws are good for grounding in that you are guaranteed good contact between the screw and the van metal because it cuts its own threads (unlike a bolt through a drilled hole). I also scrape off a little paint to bare metal where the wire will touch it.

      This is a safety ground and is not required for the unit to operate normally, but comes into play if there is some kind of internal short or fault that could result in something like the case going hot. So, its important to hook it up.

      Thanks for asking the question and catching my goof!


  5. Why not just connect your alternator/starter battery (with an isolator) directly to your DC fuse distribution panel?

    Why run that current through the aux battery, instead of connecting both batteries separately and individually to the DC fuse bus?

    • Hi Brian,
      The wire from the starter battery and the wire to the DC distribution panel meet at the plus terminal of the house battery, so they are all hooked together.

      I guess the logic is that most of the time its the house battery that powers the DC panel and the inverter because the starter battery is not connected to the house battery at all due to the isolator, so both the DC panel and the inverter are hooked to it.

      Does that make sense?


  6. Hi Gary,

    I concur with your response on keeping things simple, especially on smaller capacity systems. However, I had a bad experience on my last van. The house battery did not get properly charged. This resulted in me having to buy a new house battery yearly.

    If you are going to go without a charging unit, I would recommend the following. Use wiring at least one size heavier cables than your existing battery cables. Also use a cable for your ground, rather than depending on a vehicle ground. Locate your house battery as close as possible to the front of the van. The point here is to eliminate any added voltage drop that will prevent the house battery from getting fully charged.

    Another consideration is to ensure that your house battery charging profile is the same as your vehicle battery. This is usually different with different types of batteries. A properly setup charging unit will automatically ensure the right charging profile. This is especially important if your house battery(s) has a larger capacity than your vehicle battery.

    For northern use this becomes more critical, because the solar system charging is far less reliable, especially in the off season.

    One added note. If your van is not being used being regularly, be sure to use a proper battery tender. Trickle chargers just don’t cut it on modern vans that have electonics that continuiously drain the battery.

    • Hi Bob,
      I would 2nd the notion that you want to have a way to keep the battery fully charged when its sitting in the driveway between trips.

      I was relying on my solar charging system to do this last year, and it works find as long as the PV panel is not covered with snow. I let this get away from me last winter by not keeping the PV panel clear of snow. The house battery gradually lost charge and was at the point of starting to freeze before I caught it. Now, I keep the PV panel clear of snow, and if I can’t, I plug in the shore power connection to the house so that the Inverter/Charger keeps the house battery fully charged.

      Since RVs typically spend 90+% of the time in the driveway, its probably more important how you treat the batteries between trips than on trips 🙂


  7. Hi Greg,
    Regarding your concern of the charging profile, the following links have some added info:
    Alternator charge enhancer
    Alternator charging is done with a charge enhancer, the Pro Alt C Alternator to Battery Charger 12V 130A by Sterling:

    This video will explain it best:

    More info on my electrical system here:

  8. I have learned a ton reading your posts. We are preparing to convert our Ford Transit van and the design of the electrical system has started. Our systems are similar but looking to use a Blue Sea (7620) 12v DC 500A automatic Charging relay, bit more expensive but allows to share the starting and auxiliary battery in the event of a low starting battery, since the stereo uses the starting battery. The other difference is going with a AIMS Power Pure Sine Inverter Charger and the installation of it shows using a shunt between the auxiliary battery and the AIMS unit. There is a remote panel to shut the unit on and off and it will also read the battery status, power use, etc but it connects via a cat 5 and I do not understand why there is a shunt involved in the installation. You do not discuss the use of shunts and wonder why it would be used in this installation? We would also like you opinion on the automatic charging relay please..

    Great idea on the shore power connection and standard extension cord!

    Thank you,
    Greg and Paige

    • After further investigation and questions in other places we are going to wire the Inverter/Charger off the vehicle batteries to charge the batteries and not directly charge the house battery via a circuit off the vehicle battery and alternator. It was brought to our attention that the charging profile from the alternator may shorten the lifespan of the batteries. Your opinion?

      • Hi Greg,
        Congratulations on the van conversion!

        There has been a lot of discussion on the issue of charging the house batteries from the vehicle alternator on the ProMaster, Transit, and Sprinter forums and other places. Some rather complex and expensive schemes have been proposed and used to allow the house batteries to be charged using a more correct charging profile. Personally I think this is overkill and adds a lot of complexity, less reliability, and cost for not a lot of gain. I can’t really prove this, and I suppose that the battery life may be shortened a bit, but there are literally millions of RVs out there that use direct charging from the alternator — I’ve owned 4 of them myself that use the direct charge method. I just don’t see any dramatic down side to the direct charge method.

        Are you planning to have solar charging? Some people find that solar takes care of nearly all of the house battery charging and they use the alternator charging only on a few occasions where the solar is not providing enough charge. The solar charge controllers typically use three stage charging profiles. At other times, they just shut off the alternator charging with a switch. I’m planning to add the manual switch on my setup.

        If you use something like the Blue Sea unit (which seems fine) between your vehicle and house battery, I would also find out what the maximum charging rate your battery manufacturer recommends, and put a breaker in this line that is somewhat above the maximum — this way you will know if the batteries are being charged at too high a rate. On my two golf cart batteries, the maximum charge rate is 38 amps — this is a long way from the 500 amps the Blue Sea unit is rated for. You don’t want to charge your house batteries at too high a rate as this will effect their life.

        Shunts are normally used as a way to measure current flow in order to track the actual number of amp-hrs into and out of a battery so that its State Of Charge can be estimated and reported — that is, its part of the battery monitoring system. This is a nice thing to have — I’m planning to add a battery monitoring system to my setup one of these days.


    • Hi Bob — Thanks,

      25 watts would amount to amount to almost 50 amp-hrs a day out of a 12 volt house battery. Seems like enough that it is worth just turning the inverter off when not in use.


  9. Hi Gary,

    First off, thanks so much for everything on this site. It’s been making my conversion process a thousand times easier. I was just wondering where you ran the wires from the Van battery back to the house batteries. Did you go through the firewall or under the vehicle? And what gauge wire did you end up using for that connection?

    • Hi Brian,
      The ProMaster has the battery under the drivers side floor just in front of the drivers seat, so basically its in the van passenger space and there is no need to go through the firewall. I ran the wire under the seat over to the sidewall and then through the wall cavity space back to the house battery — about a 8 ft run. I did run the wires inside conduit in the seat area.

      I used 8 gage wire.

      Trojan recommends that their flooded lead acid golf cart batteries be charged at no more than 13% of their 20 hour discharge amp-hr rating. For the 220 amp-hr batteries I got, this would be about 29 amps. I was not sure how much charging current the PM alternator would provide, so I put a 50 amp breaker in this line so that I would know if the charge current got much over the 30 amp max recommendation. The #8 wire is the smallest gage that will do 50 amps, so that’s where the #8 wire comes from. The run on the PM is pretty short, so there was no need to go to a bigger wire to reduce voltage drop.
      These two links give max amperage and voltage drop vs wire gage:


      If you are using AGM batteries, or even have a larger flooded lead acid house battery than mine, then you might have to go to a higher capacity wire. The AGM’s have somewhat lower internal resistance, and may end up drawing more current from the alternator.

      Long, confusing answer. I’d start with what your battery manufacturer recommends for max charge rate for your house battery. You don’t want to routinely go above that. Knowing the max current the line should be carrying to not damage the batteries, pick a breaker to go with that (maybe 20% more than the max recommended charge rate?). Then pick a wire gage from the ampacity table link above that will carry the breaker current rating (or more). Then check the wire for voltage drop using the 2nd link, and go to a bigger wire if you need to to get the voltage drop down to 2% or less.

      A tricky thing about this wire from the house battery to the van battery is that it has to have a breaker or fuse at both ends — one near the van battery, and the 2nd near the house battery. That is, there are large current sources on both ends of this wire, so a short in the middle needs breakers or fuses on each end to protect for overcurrent.

      I would recommend running a full size wire connecting your van negative terminal to your house battery negative terminal — that is, do not rely on a chassis connection to carry the return current.


      • Thanks! Went with 8 gauge as well. So far so good!

        Also, I’m curious if you leave your inverter on all the tIme? Does it put a drain the battery even when no ac is being drawn? Looking into getting a remote for the inverter to turn it off when not in use.

        • Hi Brian,
          I leave the inverter/charger in the “charge only” position almost all the time. In this position the inverter part does not run at all, and if I want to hook up an AC load, I have to switch the inverter/charger to “auto”.

          The inverter does use some power when in standby waiting for an AC load to be applied, which is why I go with the “charge only” mode. I’ve been meaning to measure how much, but have not gotten around to it yet.


  10. Hey Gary

    I’m just wondering two things about your battery setup. Do you have your main negative terminal on your battery series wired to the van chassis?

    I noticed there is a green ground wire running from your distribution panel to a screw in the chassis. Is this from the AC side of the panel or the DC side?

    and lastly, do all of your negative wires from your DC loads run to the ground bus on the distribution panel or do some bypass the bus and go right to the vehicle chassis?

    Thanks for your great wiki on the van conversion. It has been a huge help!

    • HI Travis,
      I have a gathering point for the ground wires just below the distribution panel — its a cylindrical thing that you can run many wires into and then tighten down a bolt on the whole bunch of wires. Most of the grounds including from the distribution panel and the battery go to this connector, and a single wire goes from this connector to chassis ground.

      None of these ground wires carry any current under normal circumstances — that is, I run two full size wires to each load so the chassis is never required to carry any of the normal circuit (or battery) load. The grounds are there for safety only — that is, if some sort of short from a hot wire to (say) a metal case should happen, the case is grounded and the fuse on that hot wire will blow and remove the shock hazard.
      I don’t like to use the chassis to act as the return path for circuits because 1) its hard and time consuming to do good chassis grounds, and 2) the ground connections tend to deteriorate over time and cause problems that are hard to track down — this comes from working on old cars 🙂
      I used Romex wire for all the AC and DC circuits, so it no extra work to string the full size return (plus ground) wire for each circuit.


      • Oh okay, so you have one of the wires running from the negative battery terminal into your ground wire gathering point, and by having the ground bus on the panel wired to the same point you are completing the circuit.

        Also I was wondering how using the Romex with DC works…how do you include the bare copper ground wire in the circuit? On the panel there is only a positive bus and a negative bus

        • Hi,
          Yes — that’s right on the grounds.

          For DC circuits: The Romex does indeed have a black (which I use for +12V), a white (which I use for the negative return), and a full size ground wire. Seems like I terminated the white and ground wires on the same negative bus on distribution panel. Then a wire connects the negative bus to the common ground connection point I described before.

          I like the Romex because it makes running all the wires for a circuit easy, keeps them together, and the sheathing provides some extra protection. In a small number of places I enclosed the Romex in plastic conduit — places where it looked like some added protection might be needed.

          Some people feel that its important to run stranded wire in RVs due to the vibration causing problems with the connections. I looked around on this subject and could not find any actual cases where the solid wire caused a real problem and that RV manufacturers (at least some of them) use Romex — this includes past RVs we have owned. So, Romex is very common in the RV world, but in the marine world most people use stranded.

          Again, I’m not an expert on any of this stuff. Doing a little googling on RV wiring will connect you with many “experts” 🙂


  11. Hi Gary,

    I wanted to update you with a cool tweak to your battery isolator method. I used your method and bought the 80 amp Stinger relay, wired “true ignition” from the rear DC receptacle, through a switch, to the relay. I just added a 5A time delay relay between true ignition and the switch, it works excellent. I purchased the gadget linked below and programmed it to 30 second on delay. Now I don’t have to worry about my starter and house batteries being in parallel when I start the van!



    • Regarding this time delay relay:
      Is the 5A rating on the time delay relay of sufficient capacity for the current going through that path? Wouldn’t that path be seeing full amps from the alternator to the house battery?
      Also, what is “true ignition” and how do you find it? What is the purpose of the switch that you wired the relay to?

      • The time delay relay is in the path of the control signal for the relay, which is low current.

        “True ignition” is a term automotive parts like to use that means an electrical source that is only ON when the ignition of the vehicle is ON.

        So if you simply wired a true ignition source to the relay control input, the relay would close and connect the circuit immediately when you turned your key. If you add a simple switch between true ignition and the relay control input, you can choose to disable the relay so it does not close when you turn your key. Adding the time delay relay on the switch path, means that if I turn my key AND the switch is on, the relay will close 30 seconds later.

  12. Gary,

    In regards to your AC-DC fuse panel, did you add a ground bus bar to the back panel? I purchased to same distribution panel. On the inside there is the neutral and ground bus bar and I noticed you have which appears to be a ground bus bar on the back. Please elaborate….Thanks!

    • Hi Lance,
      The panel came with the ground bus on the back for the DC circuits — I used it just the way it came.


  13. Gary,

    Where did you purchase the copper wire connectors that are in your, “New connection,” pictures on the the pos and neg terminals? Thanks

  14. Hi Gary,
    I have fully read our disclaimer and understand you are not liable in any way for any advice you giver me and it is completely up to me to thoroughly research my van’s electrical system. I would appreciate your review of my plan since we have the same van and tripplite unit. Can I send it to you?

  15. Hi Gary,

    Thanks so much for the write up, it is the best electrical write up I have come across! Did you breaker the shore power input? If so, what was your solution? I am trying to tackle this issue now.. trying to figure out how to do an inline 20A AC breaker without adding an additional breaker box of some sort.

    Thanks again.

    • Hi Tyler,
      So, I used the APS 1250 Tripplite charger/inverter.
      The shore power hooks to the APS via the line cord on the APS — I just cut off the 120 VAC plug on the APS line cord and wired it to the shore power connector on the van wall. There is no breaker in this line, and to my mind its just like plugging the APS into a wall outlet (or the shore power outlet).

      The 120 VAC output from the APS goes to a combination AC/DC distribution panel. The AC part of the distribution panel has a main breaker that basically protects the panel and the wire from APS to the panel. It also has two breakers for AC branch circuits in the RV. This is the panel I used: http://www.bestconverter.com/PD5000-30-Amp-ACDC-Power-Control-Panel_p_27.html#.V4O2ibgrLb0

      The APS unit senses when shore power is connected and uses the shore power to charge the house battery and connects the shore power to the AC panel to supply the AC plugs in the RV. When there is no shore power, the APS supplies power to the AC panel using its inverter (as long as its set to that mode).
      One thing I was a bit concerned about is that the APS inverter might send power to the shore power connector and make it a shock hazard, but it does not.

      In hindsight, we use the 120 VAC in the RV very little, and a single circuit would have been fine without the AC distribution panel. But, would have had to figure out some kind of inline breaker for that single circuit. For the amount we actually use the AC in the RV, just plugging an extension cord into the APS 120 VAC output would probably have done the job.

      Bear in mind that I don’t claim any expertise in this whole electrical area, so do your homework 🙂


      • I am using a Magnum inverter that requires a breaker between the shore power receptacle and AC input on the inverter. I have the same distribution panel you used, it was a good find! I am thinking I can just route the shore power input through the control panel on a dedicated breaker (not tied into the main AC breaker), then to the inverter AC input.

        I am going to go ahead and wire up the shore power receptacle, although like you, I’m not sure we will use it much. If you put the receptacle in the rear bumper, there is an existing wire path into the cargo area that is there for the tow package add on, so no holes!

        Thanks for the reply and again for the great write up,

  16. Hi Gary, I really liked this thread. Question about the AC distribution, where did you connected the ground (green cable) for your AC panel side? And, where did you buy the DC panel board in the Distribution Panel? Thanks, Ben

    • Hi Ben,
      I bought the combination AC and DC distribution center here: http://www.bestconverter.com/PD5000-30-Amp-ACDC-Power-Control-Panel_p_27.html#.V4E_5rgrI-U
      Seems like this is one of the better deals around, and it worked out well and saves a bit of space as well since you don’t need separate DC and AC distribution panels.

      The AC side of the distribution center has a ground bus and a neutral bus. I connected the ground and neutral wires coming back from each AC circuit to these buses.
      I noticed in looking at it to answer your question that I did not connect the ground bus to the chassis ground, which I will do next time I’m doing some wiring — seems like it should be connected to chassis ground in the same way you would connect the ground/neutral buss in a house panel to the earth ground rod.

      We have found that we basically don’t use the 120VAC much at all. Pretty much everything we have runs off the 12 VDC — this is nice in that I usually can just turn the inverter function on the Triplite off, so there is no “idle” current draw from the inverter.


      • Great! Thanks for the link. Planning to have my Chevrolet Express Van converted by September this year, and I can tell you, your conversion has helped me with a lot of ideas. I will use the converted Van for my mountain bike trips, I hate driving back tired after a full day biking. Now, I will be able to stay in my van, rest, and return the next day. The best thing is that
        I will save in hotels… lol
        Take care, Gary.

  17. Another question: you note that “The battery isolator relay is turned on by connecting it to a 12 volt power source on the van that is only on when the ignition switch is on. I used power from the 12VDC outlet near the driver side back door as it is only powered when the ignition is on. I ran a 14 gage wire from the 12 VDC power socket to the battery isolator.”
    Question: did you install this outlet independent from the rest of the system? If so, direct off the van battery?

  18. This is an amazing and thorough explanation! Much appreciated. I’m leaning towards the lead acid batteries but am worried about the venting letting in moisture and letting out heat in the winter. How did you vent yours?

    • Hi,
      The vent on the battery compartment has a 3/4 inch PVC pipe that goes from the top of the battery compartment to vent out through the side of the van. There is a louvered cover on the sidewall vent on the outside to shed rain.
      There is a small inlet vent near the bottom of the battery compartment that opens to the inside of the van.

      These are pretty small vents and I’ve not noticed any cool air current coming from the inside vent.

      We actually used the van for several trips before adding these vents. If the batteries are charged at the correct rates, there is very little H2 generated. Its over charging or charging at too high a rate that makes for more H2.


    • This was a 12 volt outlet that came with the ProMaster — not one that I added.
      The isolator relay does not take much current to activate it so I thought it would be OK to power it from the wire going to the 12 Volt outlet.

      There is another type of isolator called the VSR type that does not need any extra connection for the relay. This kind of relay senses the van battery voltage and assumes that when the voltage rises above a certiain level that the engine is on and and the voltage is high because the alternator is charging the van battery and its safe to connect the van and house batteries with no danger of draining the van battery with house loads.
      As far as I know they work OK.


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  20. Gary – great great write up!!
    Before I clone your system (no solar for now) I’d like to ask if you would do anything different?
    How did the Trimetric combo Inverter/charger work for you?
    I see some reviews that mention a problem with the unit issuing a smell.
    Also – will this system work with two 6v AGM deep cycle batteries?

    • Hi Laird,
      I plan to add a Trimetric one of these days. Not sure its really necessary to a good electrical system, but I would just like to see a good accurate state of charge at a glance. I had not heard of the smell problem, but something to look into.

      The basic system will work with AGM batteries. I would up the capacity of the wires from van battery to house battery as AGMs will likely charge at higher currents under some circumstances. The battery isolator and the fuses or breakers in this line should also be good for somewhat higher current (although the isolator I show should be good for just about anything). I think it would be worth a call to Trojan Battery tech support to see if you can get any specific advice on maximum charging currents for — just look up their equivalent AGM battery so you have a specific battery to talk about.

      About the only other change I’d make on the conversion would be to include more noise treatment, or at least to look into the subject in more detail to try to determine if you really get a worthwhile drop in moise level.



  21. Gary..

    I’ve really enjoyed your write ups and they have given me a lot of ideas as I design my Van conversion. The one hang up in my mind for the electrical system is properly sizing the cable and determining the Amperage for the cables from the van battery to the house battery. I see you have assumed 50 amps which makes for a managable cable size but in many places I see 100 amps which makes a huge cable. Is there a good way to estimate this current?

    • Hi Brian,
      The logic for my cable is that the maker of the golf cart batteries similar to mine (Trojan) said that the batteries could be damaged by currents higher than about 40 amps.
      I decided to assume that it was unlikely that the car charger would generate charging currents large enough to damage the battery, and I put in the wires sized for about 50 amps and a breaker to protect the wire at 50 amps. I felt that the breaker tripping would tell me if the charging currents were higher than 50 amps and therefor would be likely to damage the battery — at that point I’d have to figure out another solution (maybe add some resistance?).
      But, the breaker has never tripped and when I measure the charging currents with a clamp on DC current meter and while I don’t remember the exact numbers, they were low enough that I’m not concerned about damaging the batteries.

      The story might be a bit different if you have AGM house batteries as they can take somewhat higher charging currents.

      Not sure if this would be worthwhile or not, but I guess you could test your actual batteries with your van charging system by draining the batteries to (say) 50% charged, and then hooking them up to the charging system with short large gage cables — even a heavy set of jumper cables might work. Then measure the charging current with a meter. This would give you a good charging current value to use for your particular batteries and your van’s charging system. You would also want to check with the battery maker to make sure you don’t exceed their maximum charging rate recommendation.

      I have seen discussions on some of the van forums where people have gone to a system in which they run a 120VAC battery charger (a good 3 stage one) off an inverter that is connected to the battery system. The idea is that this provides carefully regulated charging of the house battery. To me, this kind of system seems rather complex, inefficient, and expensive and its doubtful whether its necessary to go to these lengths.

      I’d like to hear what you end up doing.

      • Gary..

        Been I while since I contacted you regarding how to control voltage to the house batteries. Finishing up my conversion of a Ford Transit so thought I would finally give you an update on my electrical decisions.

        Initially I had thought of doing a simple battery separator (Voltage Sensing Relay). The Ford Transit has conversion power points on the battery to connect to for this sort of thing. With my single factory battery set up this is fused at 60 amps. This would be plenty to recharge the house batteries on fairly short drives, however, if the fuse blows it seems to be a bit of a pain to replace. Thought about “protecting” the fuse with a 50 amp breaker (belt and suspenders protection for the van battery) but in a large overcurrent situation I wasn’t sure there was any way to know that the 50 amp would necessarily blow first.

        In the end I went with a Marinco DC to DC charger. Serves two purposes, isolates the battery and limits the current. It also provides the correct voltage profiles for the AGM battery bank. Which may extend the life. It only charges at 40 amps max., however, with a battery bank of 220 amp hours that I’d like to keep above 50% discharge (lower in rare circumstances), that should still fully charge our house batteries in a few hour drive. For our uses that should be fine, even if we can’t plug in when we arrive to our next destination.

        Heading out for our first adventure this Wednesday. Due to conflict between where we need to park to plug in and garbage pick up we will run down the battery overnight keeping the fridge cold and stocked before we leave. Should be somewhere in a 10% to 15% draw down. We’ll see how it recharges on our first short (2 hour) drive to our initial destination.

        Maybe more information than you needed but it’s an option for you and other converters to think of.

        Your site has been a huge help in my conversion. Wish I could link you to my website for pictures and further explanation but spent my time working on the van instead of doing computer stuff. Thanks for all the great work on your site.

        • Sorry, mispoke, it is not a Marinco charger. Got mixed up with other components. Unfortunately, for the life of me I can’t remember the brand off hand right now and I’m away from the van. I’ll try to get the correct info posted as soon as I can.

        • Hi Brian,
          It does seem like the DC to DC charger is a cleaner way to charge the house battery and solves the issue of the house battery not getting the proper 3 stage charge that it gets from a good charger. And, as you say, keeps the charge rate from getting excessive and potentially damaging the house batteries.

          Its got all those advantages and I can certainly see going that way. For me personally its a bit to complicated and expensive, but that’s just me. It may be that having had 3 RVs before the ProMaster conversion that all used some form of simple VSR or isolater relay that I’ve become somewhat comfortable with that approach.

          I’m kind of a solar nut, and I did put some effort into trying to get the solar charging up to where it would do almost all of the charging job, and this has proven to be the case, and the solar charger does treat the house battery nicely with a 3 stage charge.
          What I probably should do is put a manual cutoff switch in the line from the van battery to the house battery that I could keep switched off almost all the time and just use the solar.

          Please let us know how the DC to DC approach works — maybe a picture or three?

          Thanks — Gary

          • Gary..

            Figured out the charger I installed, it is a Promariner DMC 12V to 12V, more info here: http://www.promariner.com/en/05504

            I’d say I wasn’t uncomfortable with the relay set up, just worried about blowing the fuse on the battery. The charger was an added expense but pretty simple to install.

            We’re likely going to add solar in the future, don’t know if our mainly northern tier travels will allow us to go solely solar. We’re going to use the van for awhile and see how we can integrate it in, to extend “off the grid” non-running time when we get an idea of real world electrical use.

            Let me know where I can send pictures and I’ll get a few over to you.

          • Hi Brian,
            You can send stuff to me at buildagreenrv@gmail.com

            I don’t think you will have any trouble with solar up north in the summer as we do fine in MT. If you do a winter camping, that’s likely to be a different story as the days are short and weather is more cloudy. PVWatts is helpful for working out how much juice you will average in summer and winter for a given size rig.

            One thing that did surprise on the solar is how much a bit of snow on the panels cuts down the solar charging. With my 315 watt panel, something like an inch of snow won’t maintain the batteries just sitting in the driveway between trips with no real load. I thought a fair bit of light would get through the snow, but not so.


  22. I have maybe a dumb question, but where do you ground the house batteries? Do you just run back to the chassis battery so that they share the common ground of the chassis itself? If not, can you ground your house batteries directly to the chassis in line with the chassis battery? I’ve been reading up a lot on 12vdc to help with my school bus conversion and this is one thing that has eluded me so far. I hope I’m not over thinking it!

    • Hi Zack,
      I ran a wire from the negative terminal of the house battery up to the negative terminal on the van battery using the same gage wire as goes from positive terminal of house bat to positive terminal of van battery. So, basically the chassis ground is via the van battery.
      I do have a 12 gage wire to a chassis ground screw back near the house battery to ground something that was supposed to have a chassis ground (maybe the DC panel?), but the main ground path is back through the van battery.

      All of the circuits from the DC panel out to lights etc have a full sized negative and positive wire — I did not use the chassis to carry the negative side of the circuit for anything.

      Some people do use the chassis to carry the current for the negative side of the circuit. I don’t think this is a good idea in that the connections to the chassis on both ends of the circuit have to be good and stay good (not corrode over time).


      • Hi Gary,
        You recall what gauge wire you used for the ground run back to the van negative terminal? Im pricing cables and the diff between 4 and 2 AWG is huge. Yours looks significantly smaller and I would think a ground wire shouldn’t need to be a standard battery size. I also noticed that the negative terminal on my van battery is not 5/16ths, like the positive one – maybe 1/4″?


        • Hi Pat,
          I decided to put a 50 amp breaker in the positive line from the van battery to the house battery to insure that the house battery would not receive too much charging current, which could damage it.

          The wire gage is picked for the 50 amp maximum current that it will need to handle as the breaker will pop after this. I can’t remember exactly what gage I used, and I’m away right now and can’t look at it. From this ampacity table, it looks like #8 would be OK as long as the wire is not to long.

          I’m sure I used the same gage for the negative side wire, as there is no need for it to be larger than the positive side wire.

          Remember that you do want to have a breaker or fuse on the positive wire near the van battery terninal and a 2nd one near the house battery positive terminal.


  23. Hi Gary…what would be the best way to locate this 12v signal at the back door area.
    thanks….Gerry in Michigan 248-882-4981

    I used power from the 12VDC outlet near the driver side back door as it is only powered when the ignition is on. I ran a 14 gage wire from the 12 VDC power socket to the battery isolator.

    • Hi Gerrry,
      I remember that I pulled off the black plastic louvered panel, and that this exposes a few wires. I can’t remember how I identified the 12 volt line that turns off with the ignition — maybe tracing where it goes to, or maybe poking through the insulation with the probe of my meter.
      Once I found the right 12 volt wire, I striped the insulation off of it for a half inch, wrapped the new wire around the stripped half inch of the old wire, soldered it, and then taped it carefully with electrical tape.

      If I can find a picture, I’ll add it.


  24. Hi Gary, yennes here from the Promaster forum with a question about the performance of your electrical system. I have basically cloned your system and was wondering what your full charge voltage is. I ask because the inverter will not run my coffee maker unless the van motor is running even though the inverter is rated at 1250 watts and the coffe maker is rated at 1160 watts.
    I have thought about a battery monitor but they are kind of expensive.
    Thanks in advance for your response.

    • Hi Yennes,
      I look at the battery voltage gauge fairly often, but my leaky memory is not coming up with a number for fully charged — I think that 12.8 might be pretty close. This is after a charging period, but with no charging current coming in when you look at the meter.

      I’d guess that when the coffee maker is on, it pulls the battery voltage down some. Maybe its close enough to the inverter capacity that the drop in battery voltage is enough to have the inverter trip off?
      It seems like the fact that you can run it with the engine on means that it is the drop in battery voltage and the alternator adds enough juice to keep the battery voltage up to where the inverter can handle the large load? (maybe 🙂

      Would like to hear what you conclude is going on and how you fix it.

      I think I might get myself a Trimetric for Christmas — probably don’t really need it, but it would be nice to know more about what’s going on.

      Best — Gary

  25. I notice your diagram does not show a transfer switch. Is that because this function is included in the combined inverter/charger unit? If that is the case, does the presence of shore power take precedence over the solar in charging the batteries? BTW, I am not referring to the transfer switch that is used when you have both shore and generator power available. I’d appreciate help in understanding this. Thks, Bob

    • Hi Bob — Right, the inverter/charger automatically transfers over to shore power when you plug into shore power.
      This has worked fine in our experience.

      You can also turn off the inverter function with a switch that is included on the unit, and we usually do this unless we specifically want to run a 120 VAC appliance because the inverter at idle uses some battery power and we like to avoid this. We don’t have much in the RV that needs 120 VAC so the inverter can be off most of the time.

      Sorry about the slow reply — I missed your comment when it came in.


  26. Pingback: Solar Accessories - Ford Transit USA Forum

  27. Hi Jean-Paul,
    The way I have the top of the battery box closed off is not very clear in the writeup — I’ll try to put some more pictures in.

    Basically, the “shoe box” fits over the batteries with the bottom forming the lid of the bat box, and the top of the shoe box forms the bed surface.
    The shoe box is attached to the bed platform sides with two bolts (one on each side) that are wing nutted for quick access.

    I may be wrong about this, but it seems like high vertical (upward) g forces are not nearly as likely in typical crashes, and I put less effort into designing for them than for the forward g’s. But, its certainly possible for the van to experience some significant vertical g’s in a roll over kind of situation.

    I just added the venting to the battery compartment and will add some pictures of that as well.

    The no propane system sounds interesting — maybe you could take some pictures as you go and we could put them up on the site.

    Your English is just fine — a whole lot better than my French 🙂


  28. Hi Gary,

    I’ve read all your site about your conversion and was very impressed by the quality of your presentation and of the solutions you used. It’s very inspiring for me which begin my second conversion on a new gas PM (the first one on a Econoline is presented on my web site).

    In that new one, I dont want use propane, so heating and hot water will be on the Prestone circuit, using a Webasto (DualTopC) ou Espar (Hydronic) water heater, stove on alcohol (Dometic Origo A200), all the others equipment on electricity. So I’am installing 2 big solar panels (Canadian Solar, 255 W each) and 3 batteries for a total of 345 A. I’m putting the 3 parallel batteries (Kirkland Deep Cycle 115 A Group 27 DC) in a ventilated and galvanized steelbox embedded and fixed under the floor, just in front of the sliding door and behind the driver cabin, between the cross-pieces. A trap in the floor with an hermetic seal gives access to batteries from the inside of the PM. Many advantages (lower gravity centre, no gasses inside, frees space inside…) but I am concerned by security issues (see my discussion with Taylor on PM Forum).
    I have read your discussion with ikruper about the security risks in case of crash and I think I’am OK for side and forward g forces, but I’am not satisfied of my protection for «vertical g loads». In your discussion I have difficulty to figure the description of « bed platform with a sort of shoe box » you propose. Do you have recent pictures, as this system don’t seem to be installed on these pictures showing you battery box in your PM.


    Ps. sorry for my poor english, I’am nearly just french speaking here in Montréal… and I was born in Normandie !

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