Case 1
- Battery capacity
- 3000mAh
- Current draw
- 200mA
15.00h
Open with these valuesBattery Life Calculator
15.00h
Result: 15.00 hCapacity divided by draw: 3,000 mAh at 200 mA runs 15 hours, because mAh ÷ mA leaves hours. Treat it as the ceiling, not the promise — voltage sags under load, converters lose a little, and a heavy draw lowers the usable capacity by a further 10 to 20 %.
Worked examples
Case 2
- Battery capacity
- 5000mAh
- Current draw
- 500mA
10.00h
Open with these valuesCase 3
- Battery capacity
- 10000mAh
- Current draw
- 800mA
12.50h
Open with these valuesHow it's calculated
hours = capacity (mAh) ÷ draw (mA)
- StepRead the capacity in mAh from the cell or the data sheet.
- StepTake the average current the device draws, in mA.
- StepDivide capacity by draw — the milli- and the amps cancel out.
- ResultSubtract 10 to 20 % for real-world losses.
Reference table
| Capacity, draw | Typical device | Runtime (h) |
|---|---|---|
| 1500, 300 | Wireless headset | 5.00 |
| 2000, 250 | Fitness tracker charge | 8.00 |
| 3000, 200 | Phone on standby | 15.00 |
| 5000, 500 | Tablet, light use | 10.00 |
| 10000, 800 | Power bank feeding a router | 12.50 |
Questions
How do I calculate battery life from mAh?
Divide the capacity in mAh by the average draw in mA. 3,000 mAh at 200 mA gives 15 hours, since the milliamps cancel and hours remain.
Why does my device last less than the calculation says?
The formula assumes the full capacity is usable at a steady current. In practice the cell ages, the voltage drops before it is empty, and higher draws lose extra capacity through internal resistance.
Does a 5,000 mAh power bank charge a 2,500 mAh phone twice?
No — expect about 1.5 charges. The bank steps its cell voltage up to 5 V and back down again, and each conversion costs 10 to 20 %.
What if the current changes over time?
Use the average draw over a full duty cycle. For a sensor that wakes briefly every minute, that average can be a small fraction of its peak current.
Sources and last check
Information, not professional advice.