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Battery Life Calculator

Calculators · Added

Divide capacity by current and you get a runtime — but it is always longer than the real one, because a cell never gives back every milliamp-hour on its label. This works the division both ways: the theoretical figure, and the figure once you have set how much of the pack you actually expect to use. It also reports the C-rate, which is the number that tells you whether the whole calculation is even close.

mAh

Printed on the cell or in its datasheet

mA

Average, not peak — see the note below

%

85% is a realistic default; 100% gives the theoretical maximum

V

Adds watt-hours and average power

Common cells:

How to use the battery life calculator

  1. 1Enter the battery capacity in milliamp-hours, or pick one of the common cells.
  2. 2Enter the average current the circuit draws — the average over a whole duty cycle, not the peak.
  3. 3Set the usable-capacity percentage. 85% is a reasonable default; 100% gives the theoretical maximum.
  4. 4Add the pack voltage if you also want the energy in watt-hours and the average power.
  5. 5Read the runtime, and check the C-rate line before trusting a high-current result.

Examples

A sensor on a AA pair

Input
2500 mAh, 5 mA average, 85% usable
Result
425 hours — about 17 days

The kind of low, steady draw where this arithmetic is at its most reliable.

A phone battery under load

Input
4500 mAh, 600 mA average, 85% usable, 3.85 V
Result
6 h 22 min, and 17.3 Wh of stored energy

0.13C is a gentle rate for a lithium cell, so the estimate holds up well.

A high draw that breaks the assumption

Input
1000 mAh, 2000 mA average
Result
30 min at 100% usable, but flagged at 2C

At twice the pack's own capacity per hour, the cell delivers noticeably less than rated and the answer is optimistic.

About the battery life calculator

What a capacity rating actually promises

A milliamp-hour is a quantity of charge: one milliamp flowing for one hour. A 2000 mAh cell should therefore supply 2000 mA for an hour, or 20 mA for a hundred hours, and to a first approximation it does. That first approximation is the calculation on this page, and for gentle, steady loads it is accurate enough to plan around.

The rating is measured under specific conditions, which the datasheet states and marketing does not. Typically that means a constant current discharge at a low rate, at around 20 °C, down to a defined cut-off voltage. Change any of those and the number changes. Discharge faster and internal resistance wastes energy as heat inside the cell rather than delivering it to your circuit. Run it cold and the chemistry slows down. Stop at a higher cut-off voltage — which your regulator decides, not the cell — and some of the charge is simply left behind.

None of this makes the rating dishonest, but it does make it a ceiling rather than a forecast. The usable-capacity percentage on this page exists so the forecast can be separated from the ceiling, and so the assumption is visible instead of buried in the arithmetic.

Getting a useful average out of a bursty circuit

Most battery-powered designs are deliberately bursty, because sleeping is cheap and working is expensive. The way to handle that is to work in charge rather than current: for each phase of the cycle, multiply its current by its duration, add the products, and divide by the total cycle time. A device drawing 100 mA for 2 seconds and 0.05 mA for 598 seconds uses 200 mA-seconds plus 29.9 mA-seconds over 600 seconds, which averages 0.38 mA.

That figure is what goes in the box above, and the result usually surprises people — the sleep current, which looks negligible, often dominates the total once it is multiplied by the time spent in it. Halving the transmit power of a device that sleeps 99% of the time changes almost nothing. Halving the sleep current can double the battery life.

The one thing an average cannot capture is whether the cell can supply the peak at all. A coin cell has a high internal resistance, so a brief 30 mA demand may collapse its voltage enough to reset the circuit even though the average draw is microamps. That is a question about the cell's internal resistance rather than its capacity, and it needs the datasheet rather than this page.

Frequently asked questions

Why does the calculator want an average current rather than the peak?
Because most circuits spend their time doing very different things. A sensor node might sit at 20 microamps for fifty-nine seconds and then draw 120 milliamps for one second while it transmits. Using the peak would predict hours when the truth is months. Measure or estimate the total charge used over a complete cycle, divide by the cycle length, and use that.
What is a C-rate and why does it matter here?
It expresses the load as a multiple of the pack's own capacity per hour: drawing 500 mA from a 1000 mAh cell is 0.5C. It matters because a cell's usable capacity falls as the rate rises — an effect described by Peukert's equation, and pronounced in lead-acid and alkaline chemistries. Below about 0.2C the loss is small enough to ignore. Above 1C, expect meaningfully less than the label promises, and check the cell's continuous discharge rating too.
Why can I not use 100% of the capacity?
Several reasons stack up. Most circuits stop working before the cell is truly empty, because a regulator drops out or a microcontroller hits its brown-out threshold with charge still left. The rated capacity itself was measured at a gentle discharge rate and a comfortable temperature, neither of which matches a real device in a cold shed. And cells lose capacity as they age. The percentage field is where all of that goes.
Does this apply to rechargeable and disposable cells alike?
The arithmetic does, since capacity divided by current is capacity divided by current. What differs is how much of the rating each chemistry actually delivers under load. Lithium-ion holds its voltage well and gives up most of its capacity even at moderate rates. Alkaline sags steeply and does much worse at high current than its headline figure suggests, which is why an alkaline pack in a high-drain device disappoints so reliably.
How do I convert milliamp-hours to watt-hours?
Multiply by the pack voltage and divide by a thousand, which is what the optional voltage box here does. The reason it matters is that milliamp-hours are only comparable between packs at the same voltage — a 3000 mAh cell at 3.7 V stores nearly two and a half times the energy of a 3000 mAh cell at 1.5 V. Watt-hours is the figure that compares honestly, and the one airlines use for their limits.

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