Estimating battery runtime
Battery capacity is rated in milliamp-hours (mAh): the current a battery can supply multiplied by how long it can supply it. Divide the capacity by the current your device draws and you get the runtime in hours.
runtime (h) = capacity (mAh) × usable fraction ÷ average current (mA)
A 2,500 mAh AA battery powering a 20 mA circuit gives 2500 ÷ 20 = 125 hours in theory. At 85% usable capacity that becomes about 106 hours, or 4 days 10 hours.
Why use less than the full capacity?
The rated capacity is measured under ideal conditions, usually a small current at room temperature down to a low cut-off voltage. Real devices often stop working before the battery is fully drained, high currents reduce the effective capacity (especially for alkaline cells), and cold weather cuts it further. Using 70 to 85% of the rating gives a more realistic estimate. Use a lower figure for alkaline cells under heavy load and a higher one for lithium cells at light load.
Devices that sleep
Many battery-powered gadgets, such as sensors and remotes, wake up briefly, do some work and go back to sleep. Their average current is what matters:
average = (active current × active time + sleep current × sleep time) ÷ period
A sensor that draws 20 mA for 2 seconds every minute and 50 µA the rest of the time averages about 0.72 mA. Note how the sleep current matters: at 1 mA of sleep current the same device would average about 1.6 mA and last less than half as long.
Typical capacities
| Battery | Nominal voltage | Typical capacity |
|---|---|---|
| CR2032 coin cell | 3 V | 210–240 mAh |
| AAA alkaline | 1.5 V | 1,000–1,200 mAh |
| AA alkaline | 1.5 V | 2,000–3,000 mAh |
| AA NiMH rechargeable | 1.2 V | 1,900–2,500 mAh |
| 9 V alkaline | 9 V | 400–600 mAh |
| 18650 Li-ion | 3.6–3.7 V | 2,500–3,500 mAh |
| Smartphone | 3.85 V | 3,000–5,000 mAh |
Batteries in series and parallel
Cells in series add their voltages but keep the same mAh. Cells in parallel keep the voltage and add their capacities. If your device uses a voltage regulator or converter, the current it pulls from the battery can be different from the current the circuit uses, so measure at the battery if you can.
