Car Battery Capacity Calculator
Find out how many hours your car battery can power an electrical load before draining. Use this when running accessories, winches, or camping equipment off your vehicle's battery.
About this calculator
A battery's amp-hour (Ah) rating tells you how much total electrical charge it can deliver. When you divide that capacity by the current draw of your load, you get the theoretical runtime. The formula is: Runtime (hours) = Amp Hours (Ah) / Load (Amps). For example, a 60 Ah battery powering a 10-amp load will last approximately 6 hours. In practice, actual runtime is shorter because battery efficiency drops under high discharge rates (Peukert's effect) and in cold temperatures. This calculator gives you a baseline estimate that's accurate for moderate, steady loads — always build in a safety margin to avoid deep-discharging your battery, which shortens its lifespan.
How to use
Say your car battery is rated at 75 Ah and you're running a 12V air compressor that draws 15 amps. Apply the formula: Runtime = 75 Ah / 15 A = 5 hours. That means you have roughly 5 hours of runtime before the battery is fully drained. In practice, you should stop at around 50% discharge (2.5 hours) to preserve battery health. If you add a second device drawing 5 amps, your total load becomes 20 amps and runtime drops to 75 / 20 = 3.75 hours.
Frequently asked questions
What does amp-hour rating mean on a car battery?
The amp-hour (Ah) rating describes the total electrical charge a battery can deliver at a steady rate over a set period, typically 20 hours. A 60 Ah battery can theoretically supply 3 amps continuously for 20 hours before reaching full discharge. Higher Ah ratings mean more capacity and longer runtimes for the same load. When comparing batteries, also check the cold cranking amps (CCA) rating, which measures starting performance in cold conditions — a different but equally important metric.
How do I calculate how long a car battery will last running accessories?
Divide the battery's amp-hour rating by the total current draw of all connected accessories in amps. First, list every device and find its current draw (often labeled in watts — divide watts by 12 to get amps for a 12V system). Sum all the amps, then divide the battery Ah by that total. Remember that this gives a theoretical maximum; real-world runtime is about 50–80% of this figure due to efficiency losses and the need to protect battery longevity by avoiding full discharge.
Why should I avoid fully discharging a car battery when running accessories?
Lead-acid batteries, which are standard in most cars, suffer permanent capacity loss when discharged below 50% repeatedly — a process called sulfation. Deep discharges can reduce the battery's total lifespan from several years to just a few months. Most experts recommend stopping use at 50% depth of discharge (DOD) to preserve cycle life. If you regularly need deep discharge capability, consider a dedicated auxiliary battery or a lithium iron phosphate (LiFePO4) battery, which tolerates deeper cycles much better.