Battery Life Calculator

How the Battery Life Calculator works

The formula. Run time in hours = capacity in mAh × efficiency × (1 − reserve) ÷ load in mA. For example, a 2,000 mAh cell at 50 mA with 85% efficiency and a 10% reserve gives 2,000 × 0.85 × 0.9 ÷ 50 = 30.6 hours. A second example: a 100 Ah leisure battery feeding a 5 A load at 90% efficiency with a 50% reserve lasts 100 × 0.9 × 0.5 ÷ 5 = 9 hours. The load is the figure that decides how many hours or days a device will last.

Battery run time is capacity divided by current: a 2,000 mAh cell supplying 150 mA lasts about 13 hours in theory. The theory rarely survives contact with a real device, which is why the calculator has efficiency and reserve fields. Use both honestly and the estimate will be close; leave them at 100 and 0 and it will be optimistic by a third or more.

Capacity units. Milliamp-hours and amp-hours describe charge, and only translate to energy when multiplied by voltage. That is why the voltage field matters: 2,000 mAh at 3.7 V is 7.4 Wh, while 2,000 mAh at 12 V is 24 Wh. Power banks quote the mAh of their 3.7 V cells, which is why a "10,000 mAh" bank charges a 3,000 mAh phone only two or three times once the 5 V conversion and losses are accounted for. If the label gives watt-hours, choose Wh and enter the voltage anyway so loads in milliamps can be converted.

Loads in watts. Many devices are rated in watts rather than amps. The calculator divides watts by the battery voltage to get the current the battery actually supplies, so a 5 W device on a 12 V battery draws about 420 mA from it. For a device that uses a different voltage through a converter, enter the device's watts and the battery's voltage, and let the efficiency field cover the converter loss.

Efficiency. Voltage regulators, inverters and boost converters waste some of the energy as heat. Small switching converters manage 85 to 92 percent, linear regulators far less, and inverters from 12 V to mains around 85 to 90 percent at a fair load and much worse at a light one. Peukert's effect also reduces the effective capacity of lead-acid batteries at high discharge rates, which you can fold into this figure.

Reserve. Most batteries should not be run flat. Lithium cells cut out around 3.0 V with some capacity notionally left; lead-acid batteries are damaged by going below 50 percent; and a device will usually shut down before the battery is truly empty. Set the reserve to the share you do not expect to use: 10 percent for a lithium gadget, 50 percent for a lead-acid leisure battery.

Average, not peak. A device that sleeps at 20 µA and wakes for a second at 100 mA every minute averages about 1.7 mA, and it is the average that determines run time. For duty-cycled equipment work out the time-weighted average first and enter that. The table of run times at other loads shows how strongly run time depends on this figure, which is why sleep modes matter far more than battery size for sensors and remotes.

C-rate. The discharge rate line expresses the load as a fraction of capacity per hour: 0.075C means the battery would take just over 13 hours to empty. Most cells are rated for 0.2C to 1C continuous; drawing several C shortens life and reduces the capacity you get. For sizing a battery to a known load rather than the reverse, use the battery sizing calculator.

Battery life calculator by load current

A 2,000 mAh, 3.7 V cell at 85% efficiency with a 10% reserve.

Run time and discharge rate for a 2,000 mAh battery by load

Sources: Ampere hour.

Checked against: 2,000 mAh at 100 mA = 20 hours before losses

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