Battery Runtime Calculator
A battery's amp-hour rating says how much current it can supply over one hour. Dividing that by the load gives a rough runtime. This page does the division and allows for the share of the capacity you can actually use.
- Free
- Works offline after your first visit
- Saves on this device
- No sign-up
- Updated
How it works
Runtime in hours is the amp-hour rating times the usable share, divided by the load in amps. The result is then shown as hours, minutes and seconds.
Real batteries deliver less at high loads, and their capacity falls as they age or get cold. Treat the figure as a planning estimate, not a promise.
Worked example
A battery rated 100 amp-hours, with 90% of that usable, feeding a steady 5 amps, lasts about 18 h.
Questions
What is the usable share?
Many batteries are meant to be drained only part of the way, so the usable share is less than the full rating. The maker's guide gives the figure.
Why can a heavy load shorten runtime more than the math says?
At high currents many batteries lose some of their capacity, so the real runtime comes out shorter than a simple division gives.
Does this work for any battery?
It is a rough guide for any battery with a stated amp-hour rating and a steady load. A changing load needs a closer look.
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