To help with determining battery size, I’ve put together a spreadsheet you can download and just fill in the inputs for your particular electrical loads and get a battery size estimate. It takes into account the three factors listed just below.
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.
Download the Battery Sizing Spreadsheet…DBELBatterySize
If you don’t want to use the spreadsheet, there is a lot more information and advise on estimating electrical loads on this page…
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.
Here is a picture of the spreadsheet: