Battery Discharge Time Calculator
Estimate how long a battery will last from its capacity and the current your device draws. Useful for power banks, LED lights, RC models, IoT sensors, and DIY electronics — anything that runs on a cell. Enter capacity in mAh and current draw in mA, and the tool shows runtime in hours and minutes. A reverse mode also answers the opposite question: how many mAh do you need to run something for a target time?
💡 About this tool
The core relationship is simple: runtime = capacity ÷ current draw. Real batteries, however, never deliver their full rated capacity — conversion losses and voltage sag eat into it. That is why there is a discharge-efficiency slider (50–100%): the tool computes effective capacity = capacity × efficiency. Set it lower for lossy circuits such as linear regulators, and higher for switching supplies or direct loads.
The energy field (Wh) is a reference figure that converts capacity to energy, assuming a single 3.7 V nominal Li-ion cell. Multi-cell packs and other chemistries (NiMH at 1.2 V, lead-acid at 2 V) run at different voltages, so treat the Wh number as a rough guide rather than an exact value.
Reverse mode is built for design work. Start from a requirement — "this sensor must run 24 hours" or "this lamp should last overnight" — and the tool works backward to the battery capacity you need.
🧐 FAQ
Why is my real runtime shorter than the calculation?
Usually because the efficiency estimate was too generous, the ambient temperature is low, the cell has aged, or the current draw is not constant (it spikes under load). Lower the efficiency slider and recalculate to get closer to reality.
What if I do not know the current draw?
Estimate it from the device spec sheet or the AC adapter rating (for example 5 V 1 A). For USB gear, current is roughly power (W) ÷ voltage (V).
Should I compare in mAh or Wh?
mAh works when devices share the same voltage, but for batteries at different voltages, Wh (energy) is the fair comparison. This is exactly why power-bank capacity labels can be misleading.
📚 Fun Facts
A milliamp-hour (mAh) measures charge, not energy: it tells you how many milliamps a cell can supply for one hour. That is why a 10,000 mAh power bank delivers far less than 10,000 mAh at its 5 V USB output — the internal 3.7 V charge has to be boosted, and the conversion typically runs at 60–90% efficiency. The honest number to compare across products is watt-hours, not the headline mAh figure.