On this page
When a battery box on a camper van will not take the bank you planned, lithium battery size is the question to settle before you buy: the case must fit the box, the capacity must cover the loads, and the weight must suit the rig. The steps below show how to measure the box, work out the capacity from the loads, read the group size on the label and check the weight before the battery ships. Anything past the battery toward home wiring is work for a licensed electrician, and this guide does not cover it.
SolarKitHQ earns a commission when you buy through links on this page, at no extra cost to you. As an Amazon Associate I earn from qualifying purchases. Affiliate disclosure.
The quick answer: lithium battery size
Lithium battery size means the physical size of the case and the capacity of the cells inside. Measure the length, width and height of the box the battery must sit in, then work out the amp-hours the loads need over the hours you plan to run them. Match the group size on the label to the box, match the amp-hours to the loads, and check the weight against what the shelf and the rig can carry. A battery that fits the box but cannot cover the loads is the wrong size, and a battery with ample capacity that will not fit the box is no better.
Measure the box
Start with a tape measure at the battery box: length, width and height, with room for cables, a hold-down and air around the case. Note the lid clearance and the cable entry, since a battery that fits the floor but fouls the lid is no fit at all. Measure twice and write the figures down, then compare them with the dimensions on the battery label before you buy. If the box is tight, a shorter case in the same group size may fit where a taller case will not, so check the height first.
| Size | What to measure | What to match |
|---|---|---|
| Case | Length, width, height | Battery box |
| Capacity | Amp-hours on the label | Loads over the hours you run them |
| Weight | Pounds on the label | Shelf and rig limits |
The table above is the whole rule: fit the case to the box, fit the capacity to the loads, and fit the weight to the shelf. A battery that passes every check is the right size for the job.
Work out the capacity
List the loads the battery must run and the hours each runs, then add the amp-hours together from the load labels. Add a margin for cloudy days and for loads you forgot, since a bank sized to the exact sum runs dry the first time a load runs long. Check the depth of discharge the maker states for the chemistry, since lithium cells give more of their capacity than lead-acid cells do. If the sum points to a bank larger than the box can hold, split the loads or add another battery in parallel with matching figures.
Read the label
The label on the case states the figures that decide the fit: the group size, the amp-hours, the voltage and the weight. Read the group size first, since it fixes the case dimensions, then the amp-hours for capacity and the weight for the shelf. Check the terminal layout as well, since a battery with posts on the wrong side may not reach the cables. If the label is worn or missing, do not guess the figures: use a battery with a readable label instead.
Check the weight and the terminals
Weigh the choice as well as the case: lithium cells are lighter than lead-acid cells for the same capacity, but a big bank is still heavy, so check the shelf, the floor and the rig limits before you buy. Check the terminal posts for size and side, and check the cable lugs fit the posts with room for a wrench. Keep the hold-down snug and the terminals covered, since a loose battery in a moving rig is a hazard.
Ways to keep the bank healthy
- Keep the bank in the temperature range the maker states, out of freezing cold and fierce heat.
- Charge with a charger made for lithium cells, and never mix lithium and lead-acid cells in a bank.
- Check the hold-down and the terminals at the start of each season, and re-seat any connection that feels loose.
- Keep the case clean and dry, with air around it and cables clear of sharp edges.
- Size the bank for the loads you actually run, not the loads you might run someday.
FAQ: power away from the grid
What does group size mean on a lithium battery?
Group size fixes the case dimensions and the terminal layout, so a battery of the same group size fits the same box. Check the group size on the label against the box measurements before you buy, and mind the height as well as the length and width.
How many amp-hours do I need?
It depends on the loads: add the amp-hours for each load over the hours it runs, then add a margin for cloudy days and forgotten loads. A bank sized to the exact sum runs dry when a load runs long, so size with room to spare.
Can I mix lithium and lead-acid batteries in a bank?
No: keep the chemistries apart, since they charge at different voltages and age at different rates. Use lithium cells with lithium cells, and lead-acid with lead-acid, with a charger made for the chemistry in the bank.
Does a bigger battery always last longer?
A bigger bank holds more amp-hours, so it runs the same loads for longer, but only where the box, the shelf and the charger suit it. A bank too big for the box is no fit, and a bank too big for the charger takes too long to fill. Size to the loads, the box and the charger together.
Related solar guides
See the lithium vs lead acid battery for travel trailer, the gel vs lithium battery, the best RV battery for weekend camping and the lithium vs lead acid charger.
