Battery Life Calculator

How to use the battery life calculator

Enter the battery’s amp-hour capacity and the steady current it’s supplying, set the usable depth of discharge under Advanced, and you get the runtime in hours. Runtime is just usable amp-hours divided by the load. There’s no rate here — this is how long the battery lasts, not what it costs.

The key idea is that you never use a battery’s full rated capacity. Lead-acid batteries last far longer if you only draw the top 50% before recharging; lithium (LiFePO4) tolerates about 80%; 100% is a theoretical figure that shortens any battery’s life and is rarely reached in practice. The Advanced depth-of-discharge setting applies this, defaulting to a conservative 50% for lead-acid.

The math is two short steps: usable amp-hours = capacity × depth of discharge, then hours = usable amp-hours ÷ load. A 100 Ah battery at 50% gives 50 usable Ah; at a 10 A load that’s 5 hours of runtime. Halve the load and you double the time; switch to an 80% lithium battery and the same 100 Ah runs 8 hours.

One real-world caveat the simple math leaves out is Peukert’s effect: batteries deliver less than their rating when discharged quickly, so a 100 Ah battery drained hard might yield only 80 Ah of usable energy. This calculator uses the straightforward linear model, which is accurate for moderate loads; for heavy loads relative to capacity, treat the runtime as an optimistic ceiling.

Make the load realistic. It’s the steady current of everything running at once — add up the amps directly, or convert each device’s watts at the system voltage with the volts-to-watts tool. For a load that fluctuates, use the average. To compare batteries of different voltages before you start, the watt-hours calculator puts them on equal footing.

The formula

Runtime is the usable capacity divided by the load; usable capacity is the rating times how deeply you safely discharge it:

usable Ah = capacity × depth of discharge
hours = usable Ah ÷ load

Worked example with the defaults — a 100 Ah battery at 50% usable with a 10 A load: usable = 100 × 0.50 = 50 Ah; runtime = 50 ÷ 10 = 5 hours. At 80% (lithium) it would be 8 hours; at a 5 A load, 10 hours.

Peukert’s effect: heavy discharge yields less than the rated capacity, so at high loads treat this as an upper bound. The linear model here is accurate for moderate draws.

Frequently asked questions