Sizing the Electrical Components for Your Camper Van

In order to build your electrical system, you will need to know things like: what size battery do I need? What size alternator? How large should the wire be? What size fuses should I use?  etc.   This page provides some simple help on sizing the electrical system components.

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 material below — you need to do your own homework!

Estimating Energy Use and Battery Size

To estimate how large a capacity your house battery should have, you need to take into account: 1) what your electrical loads are, 2) how long you want to be able to go without recharging the batteries, 3) what limit you want to set on how deeply you discharge the batteries.

Estimating your electrical loads: Work out what electrical devices you want to run in the van and how much time you think you will use them each day.  You will also need to know roughly how much power each device uses.  The spreadsheet below has data for the common loads.

How long do you want to go without recharging: If you camp out in the “boondocks” for several days at a time and you don’t want to be running the van a lot to charge the batteries, then you will want a larger set of batteries that can last for several days.  Solar charging can be help in this regard as the PV panels will charge the batteries.

How deeply will you allow the batteries to be discharged?  Lead acid batteries last longer if they are not routinely deeply discharged.  A battery discharged only down to 20% might last 3000 cycles, while if discharged down to 80%, it will only last about 750 cycles.  50% maximum depth of discharge is a level that is recommended quite a bit for long battery life.  These numbers will vary some with battery type and brand, but you get the general idea.

On our conversion, I used 80% depth of discharge to size our batteries.  The logic for this is that the cycle life at 80% depth of discharge is still 750 cycles, and with an RV (maybe) only seeing 30 or so cycles a year, the battery would last 25 years before getting to 750 cycles — it will certainly die of something else long before this.   Of course, the advantage of using a higher depth of discharge is that the battery bank is small, lighter, and less costly.

BatteryCycleLife

Example Trojan battery Depth of Discharge vs life of battery plot. The red line is the very common Trojan T105 golf cart battery used a lot on RVs.

To help with determining battery size, I’ve put together a spreadsheet you can download and just fill in the inputs for your situation and get a battery size estimate.  It takes into account the three factors listed just above.

Download the Battery Sizing Spreadsheet…DBELBatterySize

If you don’t want to use the spreadsheet, there are some rough rules of thumb for battery size listed at the end of this section.

Here is a picture of the spreadsheet:

Estimate battery size spreadsheet

Estimate battery size spreadsheet

You can use the spreadsheet to see what the effect of adding or removing specific electrical loads make on the battery size.  Some loads like an electric fridge, or a microwave are fairly large and can make the battery size, weight and cost go up quickly.

If you want to add loads not already listed in the spreadsheet, for DC loads you need to know how much amperage they draw (often listed on the device or in the specs), and if its a device that like a fridge that runs only enough to keep a target temperature you have to measure or estimate its duty cycle (the fraction of time it will typically be on).  For AC devices, instead of amperage, you enter the power the device uses in watts — usually listed on the device.

Its interesting to look at different approaches.  For example, using an ice box for cooling rather than an electric fridge not only saves money on the fridge, but also lets you use a smaller, lighter and cheaper battery.  If you plan to add solar charging you can also use a smaller PV panel.

Some loads are seasonal, so its good to run both a summer and winter case.  The summer case will likely include lots of fan use, but little furnace use, and winter will be vice-versa.  You want to use the one that gives the largest battery.  Likewise, different types of trips may result in different battery sizes.

Especially if you use the 80% depth of discharge to determine the battery size, you want to be sure your loads cover everything for the worst case.  You don’t want to every run your batteries below 80% depth of discharge.

 

If you and spreadsheets are not getting along, here are some rules of thumb for picking a rough battery size:

Simple RV conversion with only these electrical loads:

LED lights, water pump, phone & tablet charging, some use of laptop and TV, some use of a powered vent fan (eg a Fantastic Fan for 3 hrs).

Battery:  about a 35 amp-hr, 12 volt, deep cycle battery.

Add battery capacity for these heavier loads:

Efficient electric RV fridge:  add 70 amp-hrs

Efficient furnace (to run fan and electronics): add 15 amp-hrs

A 700 watt output microwave for 10 minutes: add 30 amp-hrs

All of the above are for 1 day of service without charging.

If you want more than one day without charging, then just scale up by the number of days — that is for the simple conversion that takes a 35 amp-hr battery for 1 day, use a 70 amp-hr battery for 2 days.

For example: if you wanted to add limited use of a small microwave to the basic loads and go for up to 2 days without charging, the rough battery size would be: 35 amp-hr for basic loads + 30 amp-hr for microwave = 65 amp-hr for 1 day, so for two days the battery size would be about a 130 amp-hr, 12 volt, deep cycle battery.

The battery sizes listed above are about as low as you want to go — adding some more capacity if you have the room and budget is good.

We recorded our actual electricity use for one trip — details here…

Type of Battery

You will have to choose the type of battery to buy.  You always want to use a deep cycle battery — regular car batteries (not deep cycle) will simply not hold up to frequent deep discharges.  Among the deep cycle batteries you will have to chose between a traditional Flooded Lead Acid (FLA) battery and an Absorbed Glass Mat (AGM) battery.  While both can be good, this is a decision that depends on a number of factors — this page goes over the pros and cons of each…

Batteries last longer if you don’t discharge them deeply — for example Trojan says that their T105 six volt golf cart battery will provide 3000 discharge cycles at 20% depth of discharge, 1200 discharge cycles at 50% depth of charge, and 750 discharge cycles at 80% depth of charge.

This is an argument for picking a larger battery so that it won’t be discharged as deeply and will last longer.  But, for typical Camper Van or RV use the battery won’t actually see that many charge cycles — for example it you use it 30 nights a year, it would take 25 years to reach the 750 cycles that Trojan gives for an 80% depth of discharge.  Your battery will die of something else long before 25 years and 750 cycles rolls around.  And, if you design for 80% depth of discharge under the worst conditions (you never want to go below 80% depth of discharge), then for most cases (most nights) your depth of discharge will likely be less than 80% anyway.

What I did was to conservatively estimate my electrical loads, and then size the battery for an 80% discharge — this resulted in a smaller, lighter, and cheaper battery than if I had designed for 50% depth of discharge.  If you use your RV every day, than designing for 50% depth of discharge probably makes more sense.

There are a lot of ways to kill a battery long before it wears out from the number of discharges — you can chronically over charge it, chronically under charge it, not water it, freeze it, …    If you can actually keep your batteries alive until they die from too many cycles, you are a much better than average battery maintainer.

Sizing Your Solar Charging System

 

To be added soon…

Sizing Your Shore Power Charger

Most camper vans include a battery charger that can charge the battery when you are hooked up to shore power.  For regular flooded lead acid batteries, most of the manufacturers recommend a charging current about equal to 10% of the 20 hour amp-hr rating of the battery.  So, as an example, our van uses two 6 volt batteries in series, each rated at 220 amp-hrs — so, a charger providing (0.1)(220AH) = 22 amps charging current is about right.  Smaller is probably OK, it just takes longer to charge the battery. A bit larger is probably also OK as well, but excessive charging currents result in more gassing and shorter battery life.

For AGM batteries, the charging current might be able to be higher, but at least some of the AGM manufactures still recommend 10% of the C20 battery rating.  I have seen some that recommend up to 20% of the C20 battery rating — this would 44 amps for a 220 amp-hr rating.

It is best to contact the manufacturer of your battery to get their recommendation on maximum charging current.

Sizing Your Inverter

The inverter supplies power to run 120 VAC to things that require regular household power.  It should have a rated output at least as large as all of the AC power devices you plan to run at the same time.  Most loads have a startup surge that is larger than their nominal power rating, and most inverters have a start up surge rating that can handle this, but some loads with heavy startup surges may require going to an inverter with a higher power rating than the nominal rating of the load.

One thing to note is that microwaves are rated at output power, so a 1000 watt microwave actually draws more like 1200 watts.

Inverters come in “pure sine wave” or in  “modified sine wave” flavors.  The pure sine wave inverters provide cleaner power but are more expensive.  Some sensitive devices won’t run on a modified sine wave inverter.  The price difference between pure sine wave and modified sine wave inverters is decreasing, so if you are in doubt about whether your devices will run on the modified sine wave, it is probably worth springing for the pure sine wave.

Most people find that its better to run things that can either be run on 120 VAC or 12 VDC to run on the 12 VDC.  The advantage of running on 12 VDC is that the inverter does not have to be turned on as much (it draws power even when there is no load on it), and you eliminate the inefficiency of the inverter, and, if the inverter has a cooling fan, you don’t have to listen to it as much.

Sizing Wire Runs and Fuses/Breakers

DC Circuits

For most camper van conversions, most of the loads are 12 volts.  This includes things like the LED lights, water pumps, charging phones and laptops, etc.  This can make for a lot of 12 volt circuits, so try to  use a DC distribution panel with room for more circuits that you think you will need.

The maximum current carrying capacity of the wires (the ampacity) must be at least as much as the actual current draw of the devices on a circuit.  Most camper van circuits use #14 with a maximum current of 15 amps or #12 with a maximum current of 20 amps.  But, some circuits with a heavier demand (eg the inverter) will require larger gauge wires.  The calculator below is probably the easiest way to get to the right wire size.

In addition to choosing a wire with a high enough maximum current carrying capacity, you also need to check that the voltage drop on the circuit is not to high — this gets to be more of a problem as the wire runs get longer.  You should keep the voltage drop to 2% or less.    The calculator below is probably the easiest way to get to the right wire size.

In some cases, 12 volt circuits that power large loads will require heavy gauge wires.  For example, a 1000 watt inverter will draw about (1000 watts/12 volts) /  (0.9) = 93 amps (the 0.9 is the inverter efficiency) in the wires from the house battery to the inverter.  This would require at least #6 wire, and if the run is longer than 3 ft, it would require a larger gauge to avoid going over 2% voltage drop.

BlueSea (a seller of electrical supplies for boats) has a good wire sizing calculator that takes into account both the ampacity of the wire and voltage drop.  The calculator is intended for sizing wires for boats, which seems like a good fit for RVs. Its called the Circuit Wizard and it looks like this:

The current should be the maximum you expect the circuit to draw.

Select the temperature rating for the insulation on the wire you are  using.  The temperature rating of the wire will be printed on the wire (the most common is 90C).

The length you enter is the total length of the supply plus the return wire.  That is, if the positive wire going to the load is 2 ft and the negative wire coming back is also 2ft, then enter 4 ft.

Click the calculate button and it will give you the required minimum wire gage.  Clicking the Explain Results will show what gage is required for ampacity reasons and what gage is required for voltage drop reasons .

The calculator is available for the PC and also as an app for Android and IOS smart phones.

The fuses on 12 volt circuits are there to protect the wiring, so they should be no larger than the ampacity rating of the wire used on the circuit.  The fuse or breaker should placed as close to the power source (battery) as possible.

Loads that are continuous need special attention.  As a rough guide:  for continuous loads (30 minutes or more) bump up the load current by 25% when selecting wire size, but use a fuse/breaker rated for the nominal load size.

Don’t bury wires inside insulation as it will cause them to overheat.

Circuit breakers or fuses should be placed as close to the current source (battery) as possible.  If a short to ground occurs in the wire between the battery and the fuse, the fuse does not provide any protection because it does not see the short circuit current.

Some useful references:

Maximum current for wire gages…
From BlueSea and based on ABYC standards.

Combined current and voltage drop wire sizing table…
F
rom BlueSea and based on ABYC standards.

The ABYC is the American Boat and Yacht Council — they publish safety standards that are widely used in boat design.

Sizing the Battery Isolator

The battery isolator is a relay that only connects the van battery to the house battery when the engine is running.  It keeps electrical loads from the furnace etc. from discharging the van battery overnight and leaving you with no juice to start the van in the morning.  A VSR (Voltage Sensing Relay) is similar to a battery isolator, but just uses the alternator voltage to determine when to close and charge the house battery.

The isolator  should be rated to carry a current at least equal to the maximum the alternator will supply to the house battery with the engine running.  Normally this can be substantially lower than the maximum rating of the alternator.  In our case, the alternator is rated at 180 amps, but the charge current to our house battery has never exceeded 60 amps, and that only briefly.  The maximum charge current may be somewhat greater for AGM batteries with their lower internal resistance.

Gasoline Generators

Gas generators are just about universally disliked in campgrounds because of the noise and fumes, so if you can possibly work out a way to not require a gasoline generator, that would be great — your neighbors will be grateful.

About the only load large enough to require a gas generator on a van conversion is an air conditioner.

 

Gary
August 15, 2015

March 23, 2017

 

Comments, Suggestions, Questions, Ideas?

 

15 Comments

  1. Gary,

    I’m just starting the planning process for my soon-to-be conversion van and this website has been *invaluable* in helping me decipher some of the aspects of the build that were totally foreign to me beforehand. THANK YOU so much for taking the time to document your experience and share your knowledge. You’re the man!

  2. Gary, I’d like to thank you for providing so much helpful information; you are a key resource for me as I begin my own camper van conversion. To that end, I have a nagging question about electrical systems I hope you can answer for me.

    In your diagram (and most others I’ve looked at), multiple sources of 12v current (charger, solar & alternator) simply join together at the coach battery. But these can all have different charging patterns (1 stage, 3 stages, etc.). What is it that makes them play nice together? E.g., if two or more are active at one time, what keeps the combination from overcharging the house battery. And if one is able to switch over to topping or floating and the other is not, what is the result?

    I could just build the system according to the diagrams, but I hate to build something I don’t understand.

    Thanks,

    David

    • Hi David,
      Sorry about slow response — we are in the Yukon for next couple weeks with limited internet.

      This is a concern that comes up quite a bit, and I’m not sure there is a really good answer except that people have been doing it for a long time and it does not seem to cause serious problems.

      I guess to add to the discomfort level, when the engine is running and the alternator (and maybe solar) are hooked to both the house and van batteries in parallel, so they see some mix of the two batteries.

      There are some “battery to battery” chargers that (at least on paper) do a better job of handing this. I believe that Ctek is one of these brands. At least one of the products in this area offers both charging from the van battery and also has a solar controller built in, and claims to do all this correctly.

      You might find some details on the ProMaster forum search for “battery to battery” or “Ctek”, or maybe “sterling”.

      Gary

      • Thanks, Gary. Wish I was in the Yukon too, but there is the small matter of building out my van to deal with first. ;-D

        As to my question, I’ve been doing some further research, and as I understand it the various charging sources are doing a rather intricate dance together. They all track the state of the battery being charged and turn their own charge down or off depending on that state. They will back off their charging if the battery has a reading above the charger’s threshold, even if that reading is due to another charging source raising the battery’s reading even though the battery hasn’t actually reached that level of charge. (I hope that makes sense; I don’t think I said it well.) So as long as they all have a charging profile that matches the battery type (and proper diodes, of course), the battery absorbs its charge as fast as it can handle it — and no faster — regardless of where that charge is coming from.

  3. Hi Gary,
    While the “Download the Battery Sizing Spreadsheet DBELLoadDetermination” file is helpful it is not the same as …. “Here is a picture of the spreadsheet:” titled Spreadsheet to Estimate Size of RV House Battery.
    I understand how continuous improvement can result in multiple versions.
    Would it be possible to get a copy of the “Spreadsheet to Estimate Size of RV House Battery” file ?
    Thanks for the inspiration !
    Jim

  4. In terms of sizing the isolator, the alternator is rated at 180 amps, but only puts out a maximum of 60, so what size isolator did you use? Thanks!

    • Hi Mark,
      I suppose one sensible way to do this would be to 1) find out the maximum recommended charging current for your batteries from the manufacturere, 2) put in a breaker or fuse in the line from the van battery to the house battery that is a bit above the maximum recommended charging current, 3) choose an isolator with a rating that is somewhat above the fuse rating.

      The logic being that you don’t want to charge the battery any harder than the battery manufacturer recommends, so insure this by putting the fuse in the charging line — if the fuse blows, you are charging at too high a current, and you have figure out a way to prevent this — maybe add some resistance. Since the fuse is in the line, there is no need to have an isolator that is rated from much more than the fuse, since the fuse protects it from higher currents.

      In my case, the max charging rate I have measured is around 38 amps, which is close to what Trojan batteries recommends as a maximum, and I have a 50 amp breaker in the chargiing line to tell me if the charging rate gets to high (which is never has).

      Maybe others have alternative ideas on this?

      Gary

  5. Greetings,

    I did not see an active link to download the spreadsheet. Any fix you can provide please?

    Thanks for all the info!
    Cheers,
    Al

  6. Hi,

    I’m in the design stage of building my 2015 Sprinter 4x and found your info on electrical systems great, with several useful links.

    Thank you for documenting this for the benefit of others. I really appreciate your help.

    Take Care…Jeff

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