Electronics & Engineering

Battery Life Calculator

Work out battery runtime from capacity and current draw.

By TheCalcTools Updated Formula checked and tested

Your numbers

Batteries rarely deliver their full rated capacity. 70–90% is typical.

Your results

Runtime (hours)
Runtime (hours)
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Runtime (days)
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Runtime
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Battery Life Calculator by TheCalcTools

How the Battery Life Calculator works

Enter the battery’s rated capacity, the average current the device draws and how much of the capacity is usable.

Runtime is the usable capacity divided by the load current.

For devices with variable load, use a time-weighted average current.

The formula

runtime (h) = capacity (mAh) × usable fraction ÷ load (mA)

Worked example

Using the default values shown in the calculator:

Battery capacity (mAh)2000
Average load (mA)100
Usable capacity (%)85
  1. Usable capacity = 2,000 × 0.85 = 1,700 mAh.
  2. Runtime = 1,700 ÷ 100 = 17.0 hours.
  3. That is 17h 0m, or 0.71 days.

Result:

Runtime (hours)17.00
Runtime (days)0.71
Runtime17h 0m

Frequently asked questions

Why does real life differ from the estimate?

High current draw, cold temperatures, ageing and cut-off voltage all reduce the usable capacity.

How do I use a duty cycle?

Multiply the active current by the fraction of time active and add the sleep current times the fraction asleep to get the average.

Sources and further reading

UPS and Battery Backup Runtime CalculatorHow long will a battery bank run your load? Ohm's Law CalculatorPick the two values you know and get the other two Transformer Turns Ratio CalculatorFind the secondary side of an ideal transformer from the primary values

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This calculator gives estimates for information only. Check important results independently before you rely on them. See our disclaimer.