Battery runtime estimates how long a battery powers a load from its amp-hour capacity.
Runtime (h) = Capacity (Ah) ÷ Load Current (A)
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.
Real runtime is shorter. Lead-acid should only be drawn to about 50% depth, and high currents cut effective capacity (the Peukert effect). Plan for roughly half the ideal number.
Use the Battery Runtime calculator in the automotive calculators.