Battery Bank Size Calculator

How many batteries do I need for my RV?

Enough to carry your daily use through the nights between recharges, at the depth your chemistry can safely give. That's three numbers, and the first one decides everything. The Power Budget's weekend couple, propane fridge and all, comes to 1,425 Wh a day; a residential fridge on its own adds 1,600 Wh through the inverter. For three nights with no solar and no generator, using the Power Budget's typical days:

Daily use is the Power Budget preset of the same name in mild weather, furnace blower included. Three nights between recharges, no solar. Usable share 80% for lithium, 50% for AGM and flooded; 12.8 V lithium, 12 V lead; golf carts are 225 Ah, 6 V, in series pairs. Weights at ~25 lb per 100 Ah lithium, ~65 AGM, ~60 flooded.

Your own first number is the one on the Power Budget: tick what you run and it saves the daily watt-hours to My Rig, where this page picks them up. Guessing it is where most battery sizing goes wrong. The fridge and the furnace blower dominate the day, and the TV and the phone chargers barely register.

How many amp-hours do I need for boondocking?

Divide the watt-hours you need to store by the battery's voltage, then by the share you can use. 4,276 Wh for three nights is 334 Ah of lithium drawn down, 418 Ah on the labels at 80%, and 713 Ah of AGM on the labels at 50%. That's why "400 Ah" means something different in each chemistry, and why the answer here comes in batteries: the label amp-hours aren't the number you camp on. Lead-acid can go deeper than half, and it pays for it in cycle life. Lithium is rated to go further than 80%, and this site plans at 80% to keep some margin in the bank.

Does solar reduce how many batteries I need?

The bank only has to carry the part of the day solar doesn't put back, so solar shrinks it, down to a floor of one full day: the panels do nothing between sunset and sunrise, and the average day's harvest isn't the gray one you're sizing for. For the weekend couple's 1,425 Wh a day over three nights, in average US sun on a flat roof:

4.5 peak-sun hours × 0.7 flat-mount derate. "Nights it lasts" counts the average day's solar, so it's longer than the three you sized for. Pick your state and season in the tool above: on NASA POWER's figures for a flat surface, December sun runs from 25% of the annual average (Washington) to 58% (Florida) across the lower 48.

The solar page asks the question from the other end: how many panels for the bank you have, and which of the two is the bottleneck. Once solar covers the day, more batteries buy cloudy-day margin and nothing else.

6 V golf-cart batteries or 12 V?

Two 6 V golf-cart batteries in series make a 12 V string, and in lead-acid they're the better buy: their thick plates take deep cycles better than a 12 V "deep-cycle" marine battery, and a Trojan T-105 is rated 225 Ah at the 20-hour rate on its data sheet, a lot of capacity per dollar. They come in pairs, though, so a bank grows two at a time, and they're heavy. For three nights at 1,425 Wh: 8 golf carts at about 540 lb, 8 AGM batteries at 520 lb, or 5 lithium at 125 lb. On a trailer whose cargo capacity is already spoken for by water, propane and gear, the difference is the reason people switch. The lithium-upgrade page prices it, and the loaded-weight page counts it against your GVWR.

Can I add a battery to my existing bank?

Batteries in parallel share the load by their internal resistance, so an old battery and a new one, or two different capacities, don't discharge or charge evenly: the newer one works harder, and the pair ages to the older one's standard. You can add one, within limits the makers set. Battle Born's parallel-wiring guide says to use one model and amp-hour rating across the bank and not to mix models, capacities or significantly different ages, and mixing chemistries puts batteries that want different charge voltages on one charger. So when your bank comes up short, the useful reading is "the bank you'd replace it with", sized from scratch, which is why the result checks the bank you have first and only then says what to buy. The opening in Battle Born's wording is a matching battery of the same model, added while the bank is still young.

Should an RV battery bank be 12 V or 24 V?

The same three nights at 1,425 Wh take 5 lithium batteries at 12 V and 6 at 24 V (they go in strings of two, so the bank grows a pair at a time), but a 3,000 W inverter pulls 260 A from the 12 V bank and 130 A from the 24 V one. The case for 24 V is current, not capacity: half the current is half the BMS rating needed, and cable of about a quarter the cross-section for the same drop. For most rigs 12 V still wins, since everything in the coach runs on it. The cost is a 24-to-12 V converter for the rig's lights, pump, fans and fridge, and a charger, solar controller and DC-DC charger that all speak 24 V. It's the right trade above about 3,000 W of inverter; below it, stay at 12.

Can a bank be too big?

Yes, in two ways. The first is refilling it: a bank you can't bring back to full behaves like a smaller one, and lead-acid left partly charged sulfates. The 5-battery lithium bank above takes about 7.2 hours of generator through a 55 A converter to replace three nights; the 8 golf carts take 17.6 hours, most of it the absorption tail the charge-time page explains. The second is paying for capacity you never reach: if your trips are two nights and a drive, a four-night bank is weight and money for a scenario that doesn't happen.

The opposite check matters just as often. A lithium bank sized for capacity can still be too small for the inverter. Every battery's BMS caps its current (Battle Born's BB10012 data sheet rates it at 100 A continuous), and 2 × 100 Ah batteries hold two nights of 800 Wh easily but can only supply 200 A, while a 3,000 W inverter at full output pulls 260 A. The result sizes for whichever is larger and says which one it was; the inverter page has the whole current check.