Battery Runtime Calculator
How long a battery lasts at a given load, ideally and realistically.
What is Battery Runtime?
Capacity divided by load current is the answer everyone quotes, and it is optimistic. A lead-acid deep-cycle pack is routinely run to 50% of its nameplate capacity to preserve cycle life, an inverter wastes 10–20% of what it passes through, and both have to be taken off the ideal figure before the estimate means anything. This page reports both the ideal and the realistic runtime so the gap is visible rather than surprising.
How it works
The nameplate capacity is entered in mAh or Ah and multiplied by the depth of discharge and the converter efficiency to give usable amp-hours, which are then divided by the load current in mA or A. Everything is converted to amp-hours and amps at the edge, so a 2000 mAh pack at 400 mA cannot be mixed with an Ah figure. The depth-of-discharge presets cover lithium, AGM and flooded lead-acid, including the case where a car starter battery is being asked to do a deep-cycle job it was never designed for.
- Enter your input. Type or paste your input into the field above — the tool responds instantly.
- Review the result. Output updates live as you type. Everything runs entirely in your browser.
- Copy or download. Use the Copy button to grab the result, or Download to save it as a file.
Examples
Realistic sample
Type or paste any realistic input — the tool computes the result locally and instantly.
Common mistakes
Expecting a server round-trip
Nothing is uploaded — every tool in this category runs entirely in your browser.