Battery Life Calculator
Estimate how long a battery lasts for a given load, with a realistic derating factor.
About 8h 0m of real use — ideal (no derating) would be 10 h (10h 0m). This is a rough estimate.
Estimated battery life ≈ capacity ÷ load current × derating. For a 2000 mAh battery driving a 200 mA load, the ideal runtime is 2000 ÷ 200 = 10 h; applying a 0.8 derating factor gives a realistic 8 h (8h 0m). It is only an estimate — real runtime is usually a bit lower.
How the estimate works
Divide the battery’s charge capacity (in mAh) by the average current your device draws (in mA) and you get the ideal runtime in hours. That figure assumes a perfect battery that delivers every last milliamp-hour at full voltage — which never happens. A derating factor between 0 and 1 shaves the ideal number down to something closer to reality, accounting for converter losses, self-discharge, and the fact that a device cuts off before the cell is truly empty. A common starting point is 0.8, meaning you expect to use about 80% of the nameplate capacity. Because the derating is a judgement call, treat the output as a ballpark, not a guarantee.
capacity = battery charge (mAh), load current = average draw (mA), derating = efficiency factor between 0 and 1
Worked example
Estimate how long a 2000 mAh battery lasts driving a steady 200 mA load, using a 0.8 derating factor:
- 1 Match your units. Put capacity in mAh and load current in mA so they cancel to hours. A 2 Ah battery is 2000 mAh.
- 2 Divide capacity by current. 2000 mAh ÷ 200 mA = 10 h. This is the ideal, best-case runtime.
- 3 Apply a derating factor. Multiply by your efficiency estimate: 10 h × 0.8 = 8 h of realistic runtime.
- 4 Convert to a friendly duration. 8 h reads as 8h 0m; a 30 h estimate would show as 1d 6h 0m.
Ideal runtime for a 2000 mAh battery
Capacity ÷ current before any derating. Multiply by your derating factor (e.g. × 0.8) for a realistic figure.
| Load current | Ideal runtime (2000 ÷ mA) | At 0.8 derating |
|---|---|---|
| 20 mA | 100 h | 80 h |
| 50 mA | 40 h | 32 h |
| 100 mA | 20 h | 16 h |
| 200 mA | 10 h | 8 h |
| 500 mA | 4 h | 3.2 h |
Why real battery life is shorter
The nameplate capacity is a lab number. Manufacturers rate cells at a gentle discharge current and a friendly temperature. Draw more current and the usable capacity falls — the Peukert effect means a high, sustained load empties a battery faster than the simple ratio predicts. Cold temperatures raise internal resistance and cut capacity further, while heat accelerates self-discharge.
Devices also quit early. Most electronics stop working at a cutoff voltage well above the battery’s absolute floor, leaving usable charge stranded. Add DC-DC converter losses and a load that spikes rather than staying constant, and you can see why a derating factor is essential. For anything critical, measure the real current draw and treat this calculator as a rough first estimate.