Battery Life Calculator

This calculator estimates how long a battery will run a device, or how big a battery you need for a target runtime. Enter the battery's rated capacity in mAh, Ah or Wh, its voltage, and the device's current draw in mA or its power in watts. For devices that sleep most of the time, such as sensors and remotes, add the share of time they are active and their idle current, and it works out the average draw.

Choose the battery chemistry to start from a sensible usable capacity. Lead-acid batteries also get a Peukert correction, because their effective capacity drops at higher currents. The chart shows how runtime changes if the load goes up or down.

Runtime (h) = rated capacity (Ah) × usable share ÷ average current (A)

Average current = I_active × duty + I_idle × (1 − duty)

Peukert: t = H × (C ÷ (I × H))^k

Watt-hours are converted to amp-hours by dividing by the battery voltage, and watts to amps the same way. In Peukert's law C is the capacity at the rated discharge time H, usually 20 hours for lead-acid batteries, and k is the Peukert exponent, about 1.1 to 1.3 for lead-acid and close to 1 for lithium cells. Real runtime also depends on temperature, age, cutoff voltage and the efficiency of any voltage converter, which the usable share is meant to cover.

A 3,000 mAh lithium cell at 250 mA. With 90% usable, it runs about 10.8 hours.

A coin cell sensor. A 220 mAh coin cell powering a sensor that wakes 1% of the time at 20 mA and sleeps at 10 µA averages 0.21 mA and lasts about 39.3 days. Cutting the active time matters far more than the sleep current here.

A 100 Ah lead-acid battery at 10 A. Using only half the capacity to protect it and k = 1.25, it runs 3.54 hours, less than the 5 hours a simple division suggests.

Sizing a power bank. To run a 150 mA device for 2 days with 85% usable, you need 8,471 mAh at the cell voltage.

Milliamp-hours measure charge, and only compare batteries of the same voltage. Watt-hours measure energy and allow fair comparisons across voltages. A 10,000 mAh power bank rated at its 3.7 V cells holds 37 Wh, but delivers less at its 5 V USB output, typically 60 to 70% of the rating once the boost converter's losses are counted. Airlines limit carry-on lithium batteries by watt-hours, usually 100 Wh without approval, for the same reason.

Lower the average current: reduce screen brightness, transmit less often, or let a microcontroller sleep between readings. Keep batteries near room temperature, since cold weather can cut available capacity by a third or more. For rechargeable lithium cells, partial charges between about 20% and 80% extend cycle life, and storing them half charged slows aging. For lead-acid batteries, avoid deep discharges, which shorten their life sharply. Check the device's real current with a USB power meter or a multimeter in series, because datasheet figures are often typical rather than worst case and radios can draw short peaks many times the average.