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Battery Runtime Calculator

How long a battery runs from its capacity and your load, with the energy stored and how hard you are pulling on it.

Local onlyThe math runs in JavaScript in this tab. Your figures are remembered in this browser's local storage, and nothing is uploaded.

You want to know how long your project will run before the battery quits, and that comes down to two things: how much charge the pack holds and how hard you pull on it. This gives you the runtime from your battery's capacity and your load, plus the energy it stores and how hard that load is working the pack.

Runtime is charge divided by load

A battery's capacity in amp-hours tells you how much charge it holds, and your load in amps tells you how fast you take it out. Divide one by the other and you have the hours. A 2.2 amp-hour pack feeding a 2 amp load runs about an hour, before you set anything aside. Pull twice the current and you get half the time. It really is that direct, which is why knowing your real load current is the whole game.

Do not count the charge you should not use

You rarely want to run a pack to dead flat. Draining a lithium battery all the way stresses the cells and shortens its life, so most people stop with ten to twenty percent left. That is what the usable-charge setting is for. Set it to around eighty-five percent and the runtime you get is the honest figure, the time you can run before you should stop, not the time until the pack is damaged.

Watch the C-rate

The C-rate is how hard you are pulling relative to the pack's size. Draw the pack's full amp-hour number in one hour and that is one C. Robots and drones often pull many times that, and a pack has to be rated for it or it sags, heats up and can be damaged. This shows your C-rate so you can check it against what the pack is built to deliver. If it is high, give yourself a bigger or higher-rated pack.

Why real runtime comes in a little short

The math assumes a steady draw and a pack that gives up every usable amp-hour. Real life is bumpier. A motor surges when it starts and when it works hard, so the average load is higher than the gentle number you might guess. And at a high C-rate the voltage sags and you get slightly less out. So take the result as a good estimate, then time your first real run and adjust your load figure to match what you actually saw.

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