How to Size a Battery Bank for a Vehicle Build

Depth of discharge is the percentage of a battery's rated capacity you actually use before recharging, and it is the reason two batteries with the same number on the label are not the same battery. Sizing a bank is four steps: add up what you use in a day, decide how many days you want without charging, divide by the usable fraction for your chemistry, then check the result against weight, space and the BMS current rating. Everything else is detail.

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What does usable capacity actually mean?

A battery's rated capacity is what it holds. Its usable capacity is what you can take out without damaging it. Lead acid and AGM are conventionally used to 50 percent depth of discharge, because deeper cycling shortens their life sharply, so a 100Ah AGM gives you 50 amp-hours. LiFePO4 is safely usable to about 80 to 90 percent, so a 100Ah lithium battery gives you 80 to 90.

Read that table as cost per usable amp-hour rather than cost per battery. A lithium cell at roughly twice the purchase price of an AGM delivers about 1.7 times the usable capacity, lasts something like five times as many cycles, and weighs 40 lb less, which on a vehicle build is a payload line item as well as a lifting one. The lithium versus AGM comparison works the whole cost case through, and the battery chemistry chart puts the specifications side by side.

How do I work out my daily amp-hour consumption?

Multiply each appliance's running current by the hours it actually runs, not the hours it is switched on. That distinction is the whole exercise. A compressor fridge draws about 2.5A while running, but in mild weather it only runs about a third of the time, giving roughly 21 amp-hours a day. In 90F ambient the same fridge can run two thirds of the time, which is closer to 39. Nothing about the fridge changed; the duty cycle did.

A realistic two-person day with a ICECO VL45 45L Portable Refrigerator (Secop Compressor) , four LED fixtures, a roof fan on low, two phones and a demand water pump comes to about 35 amp-hours in mild weather and 55 to 60 in heat with a laptop added. That is the working number for most builds. Run it properly with the power consumption calculator, which lets you set the duty cycle per appliance rather than assuming one.

Better still, measure. A AstroAI Digital Clamp Meter, AC/DC Current and Voltage (4000 count) reads actual current draw on a live circuit, and a Victron SmartShunt 500A Battery Monitor (Bluetooth) counts amp-hours in and out over a whole trip. Appliance labels are peak figures and they overstate consumption badly. People who measure first almost always buy a smaller bank than people who calculate from labels.

How many days of reserve should I build in?

Reserve is how long the system runs with no charging at all: no sun, no driving, no shore power. Two days is the practical target for a travel build, because it covers a rainy layover and a day of short drives without any anxiety. Three days suits a build that parks for long stretches in poor sun. More than three is usually the wrong purchase, because charging is cheaper than capacity.

Those rated figures are daily consumption multiplied by days and divided by 0.85, the usable fraction for LiFePO4. Fifty-five amp-hours a day for two days is 110 amp-hours of consumption, which needs about 129 amp-hours of rated lithium capacity. Note what that means in practice: a single 100Ah battery does not quite cover two full days, and the honest options are either a second battery or a charging source that runs while you are parked. The battery bank calculator does this arithmetic with your own numbers and your own chemistry.

Is capacity or charging the better purchase?

Charging, in most builds, and by a wide margin. A Renogy Smart 50A DC-DC MPPT Battery Charger 12V (Dual Input) puts roughly 50 amp-hours into the bank per hour of driving, so a two hour transit covers a full day of consumption with room to spare. It costs less than 100Ah of lithium and it weighs almost nothing. Capacity buys you the ability to sit still; charging buys you everything else, and most people overbuy the first and underbuy the second.

The exception is the build that genuinely parks. If the vehicle sits at a basecamp for four days while you are away from it, no amount of charging speed helps and capacity is the only answer. Similarly, a build with a large inverter load has a peak problem rather than an energy problem, and that is solved by battery specification rather than battery size. Details of alternator charging, including cable sizing for the long run to the engine bay, are in the DC-DC charging guide.

What does the BMS rating change?

A battery management system protects the cells and enforces limits, and its continuous discharge rating is a hard ceiling on what the battery will deliver. A 100A BMS will not supply an inverter that wants 185A, no matter how many amp-hours are inside the case. DC amps equal AC watts divided by 12 times efficiency, so a Renogy Pro 2000W Pure Sine Wave Inverter 12V to 120V running at its full 2000W output at 90 percent efficiency pulls roughly 185A from a 12V bank. Against a 100A BMS, that inverter simply trips the battery offline under load.

There are three ways out. Buy a battery with a higher continuous rating. Put two batteries in parallel so the BMS current shares between them, which is one of the better arguments for two 100Ah packs rather than one. Or size the inverter to the battery you have, which is usually the cheapest and most honest answer: a Renogy Pro 1000W Pure Sine Wave Inverter 12V to 120V at 1000W pulls closer to 93A and lives comfortably behind a 100A BMS. The inverter sizing calculator shows the DC current for any AC load.

How does cold change the sizing?

In two ways, and the second is the one that ruins trips. Discharge capacity falls modestly in cold, on the order of 10 to 20 percent near freezing for LiFePO4, and it comes back as the cell warms. That is a planning nuisance, not a failure.

The real limit is that LiFePO4 cannot safely accept a charge below freezing. It will happily discharge at sub-freezing temperatures and power your fridge all night, and then refuse every amp the solar array and the alternator offer it the next morning. A good BMS blocks the charge to protect the cells, which is correct behaviour and looks exactly like a fault. The fix is a self-heating battery such as the Renogy 12V 100Ah Self-Heating LiFePO4 Battery (DuoHeat, Mini) or the Litime 12V 100Ah RV Self-Heating LiFePO4 Battery , where the heater draws from the charge source rather than the stored capacity, so it costs you charging time rather than amp-hours. The winter camping power guide covers this properly.

Where do weight and space enter the decision?

Sooner than most people expect. A 100Ah LiFePO4 battery is about 25 lb and drops into a standard Group 24 tray. Three of them is 75 lb plus cable and hardware, which is a real fraction of a mid-size SUV's payload once water, a rooftop tent and passengers are counted. The Litime 12V 320Ah Mini LiFePO4 Battery puts roughly 4kWh in one case, which removes the parallel wiring problem but concentrates a lot of mass in one place that must be properly secured.

Whatever you buy, it gets strapped down. An unsecured battery is a projectile in a crash and a short waiting to happen on a washboard road, and a NOCO BT31S Group 31 Heavy Duty Battery Tray and Hold Down costs less than a tank of fuel. Check the total against the payload calculator before you order, because battery weight is the line item most often left out of a build's weight budget.

What if I want to grow the bank later?

Plan for it now and it is easy; retrofit it later and it is expensive. Batteries added in parallel should be the same chemistry, same capacity, same age and ideally the same model, connected with equal-length cables to a pair of Joinfworld 12V 250A Busbar Power Distribution Block (4 x 3/8 in studs) rather than daisy-chained post to post. Unequal cable lengths mean unequal internal resistance, which means one battery does more work than the other and ages faster.

Size the trunk cable, the main fuse and the busbars for the bank you expect to end up with, not the one you are starting with. Cable is the part that is genuinely painful to change, because it is routed through the structure of the build. A 4 AWG Marine Grade Battery Cable, 20 ft Red and 20 ft Black run in 4 AWG carries 160A under ABYC ratings outside engine spaces, which covers a great deal of growth, and a RED WOLF 4 Way ANL Fuse Holder and Distribution Block (12V) at the battery gives you fused distribution that scales.

Once the bank is in, have the installation inspected by a qualified installer before it carries load, with particular attention to fuse position, fuse rating, terminal torque and cable support. The numbers on this page are researched guidance drawn from published standards and manufacturer documentation. They are arithmetic you can check, not an electrical certification. If you want the field sorted by tier before you decide, the lithium battery roundup compares cases, BMS ratings and warranties.

The same arithmetic covers a house. If you are sizing a bank to carry a fridge and a furnace fan through an outage rather than to run a fridge in a van, the load side of the calculation changes but the method does not, and DisasterPrepCalc works it for a home backup system including the runtime figures for common household loads.