Battery Life Calculator
Battery runtime from mAh and load, duty-cycle averaging and power bank charges.
About this tool
In runtime mode, enter the battery capacity, the load in matching units (mAh and mA, or Wh and W) and the share of capacity you can really use. For a device that wakes and sleeps, tick the duty-cycle box and enter its active current, sleep current and the share of time it is active. In power bank mode, enter the bank's capacity and cell voltage, the phone's battery and voltage, and the conversion efficiency.
How long a battery lasts
Runtime is capacity รท load. A 2,000 mAh battery powering a 100 mA load lasts 20 hours in theory, or about 17 hours if only 85% of the capacity is usable. Real batteries deliver less than their label at high currents, in the cold and as they age, and many devices shut down before the battery is empty. Devices that sleep most of the time are dominated by their sleep current. A sensor drawing 20 mA for 5% of the time and 0.05 mA asleep averages about 1.05 mA, so the same battery lasts about 68 days. Power banks need an energy comparison, not a mAh one. Their cells are rated at 3.7 V and lose roughly 15% converting to 5 V USB, so a 10,000 mAh bank gives about two full charges of a 4,000 mAh phone. Honest limits: these are estimates, and the device's datasheet or a real measurement of its current is always better. The Ohm's law calculator converts between current, voltage and power, the solar panel calculator sizes charging systems and the electricity cost calculator prices mains use.
Frequently asked questions
How do I calculate battery life?
Battery life in hours = capacity (mAh) รท load (mA) ร the usable share. 2,000 mAh at 100 mA with 85% usable is about 17 hours.
How many times will a power bank charge my phone?
Roughly bank mAh ร 3.7 ร efficiency รท (phone mAh ร 3.85). A 10,000 mAh bank charges a 4,000 mAh phone about twice.
Why does my power bank give fewer charges than its mAh suggests?
Its rating is at 3.7 V, but it outputs 5 V or more through a converter that loses energy as heat.
Is my data saved?
No. The calculation runs in your browser and nothing is stored or sent.