๐ Battery Life Calculator
Leave efficiency at 100% for a rough estimate if you don't know it.
Estimate how long a battery will last, given its capacity, the device's current draw, and the power conversion efficiency. Useful for estimating runtime for power banks or battery-powered devices.
How to use
- Enter the battery capacity (mAh).
- Enter the device's current draw (mA).
- Enter the power conversion efficiency (%) โ leave at 100% if unknown.
- The runtime is calculated automatically.
How the calculation works
Estimate how long a battery will last from its capacity (mAh) and the current the device draws (mA). Runtime (hours) = capacity (mAh) ร efficiency รท current draw (mA) Efficiency accounts for losses in circuits that convert the voltage. When a power bank charges a phone, converting from the cell voltage (3.7 V) to USB (5 V) loses energy, so often only about 60โ70% of the rated capacity is usable.
Worked example
A 10,000 mAh battery, 500 mA draw, 85% efficiency 10,000 ร 0.85 รท 500 = 17 hours At 100% efficiency it would be 20 hours. With losses, a 10,000 mAh power bank charges a phone with a 4,000 mAh battery about 1.5โ1.7 times.
Things to be aware of
- Current draw varies a lot with screen brightness and network use; an average figure gives a better estimate.
- Batteries perform worse in the cold, shortening runtime.
- Batteries lose capacity with age, so a well-used battery runs for less time than calculated.
FAQ
What is conversion efficiency?
It represents power lost when the battery's output is converted for the device, e.g. via a DC-DC converter โ typically below 100%.
What formula is used?
Runtime (h) = battery capacity (mAh) ร (efficiency% รท 100) รท current draw (mA).
Why might the real runtime differ from this estimate?
Actual runtime varies with the battery's discharge curve, temperature, and load changes, so treat this as an estimate only.