Battery runtime estimates how long a battery powers a load from its amp-hour capacity.
Where
| Capacity | Battery rating in amp-hours (Ah) |
| Load current | Draw of the load in amps (A) |
Worked example
A 100 Ah battery feeding a 5 A load lasts 100 ÷ 5 = 20 hours in the ideal case.
FAQ
Why does my actual battery runtime come in shorter than calculated? The simple formula assumes 100 percent usable capacity and constant current draw, but real batteries lose some capacity to internal inefficiency, and most electronics don’t draw a perfectly constant current, both of which reduce actual runtime.
Does battery age affect this calculation? Yes. A battery’s usable capacity declines with age and charge cycles, so plugging in a fresh, rated capacity value for an older battery will overestimate real-world runtime.
FAQ
Why does my actual battery runtime come in shorter than calculated? The simple formula assumes 100 percent usable capacity and constant current draw, but real batteries lose some capacity to internal inefficiency, and most electronics don’t draw a perfectly constant current, both of which reduce actual runtime.
Use the Battery Runtime calculator in the automotive calculators.