Car Battery Group Size Runtime Calculator
Estimate how long a car battery of a given group size will run an accessory load — camping fridge, lights or audio — before it hits a safe depth of discharge.
How it works
Usable Ah = capacity × depth of discharge% · Runtime = usable Ah ÷ load amps
Runtime is simply the usable amp-hours divided by the current draw. The catch is "usable": a standard starter battery should not be taken below about 50% of its capacity or its life collapses, so a 70 Ah battery only offers around 35 Ah for accessories. Real capacity also falls under heavy loads and in the cold, so treat the figure as an optimistic ceiling and leave headroom to still start the engine.
Worked example
A Group 48 battery is about 70 Ah. Limiting discharge to 50% leaves 35 Ah usable; running a 5 A cooler draws that down in 35 ÷ 5 = 7 hours.
Frequently asked questions
How long will my car battery run a fridge or lights?
Divide the usable amp-hours by the device’s current draw. A 70 Ah battery at a safe 50% depth gives ~35 Ah, so a 5 A cooler runs about 7 hours — but only if you keep enough charge to restart the engine.
Why can’t I use the full amp-hour rating?
Deeply discharging a starter (SLI) battery damages it, so you keep to roughly half its capacity. Deep-cycle batteries tolerate 70–80% depth and are the right choice for sustained accessory loads.
Does temperature affect this?
Yes. Cold weather can cut effective capacity substantially, and high loads reduce the delivered amp-hours too. The estimate is best-case — plan for less in winter or under heavy draw.
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This calculator is for educational purposes. Double-check important results before acting on them.