Battery Life Calculator
Capacity divided by draw, then trimmed by reality. Find out whether your sensor runs for a weekend or a winter.
Why the efficiency factor
Capacity ÷ draw gives the textbook answer: 2000 mAh at 80 mA is 25 hours. Real batteries deliver less. Ratings are measured at gentle currents and room temperature, converters waste a slice, and cells age. Derating to 85% is honest for fresh alkalines at modest draw; drop toward 70% for cold weather, high current, or cells of mysterious provenance.
Average draw is the hard part
A microcontroller that sleeps at 10 µA and wakes to 80 mA for two seconds each minute averages under 3 mA. Weight the on and off currents by their share of the cycle and use that number here: it's the difference between a battery estimate in days and one in months.
Why capacity ratings disagree
Two AA cells can both claim 2,500 mAh and deliver very different runtimes, because the rating depends entirely on the test. Alkaline capacity is measured at a small fraction of the current a project usually draws, and the figure collapses under load: the same cell that gives 2,500 mAh at 25 mA might give 1,200 mAh at 500 mA. Lithium cells hold up far better under current, which is why an 18650 rated at 3,000 mAh often outlasts alkalines with a larger number on the label.
The cut-off voltage decides the answer
A battery is empty when your circuit says it is. A 3.3 V regulator with a 200 mV dropout stops working at about 3.5 V, so an alkaline cell that still has energy left between 3.5 V and 3.0 V simply cannot be used. Choosing a lower-dropout regulator, or a boost converter that runs down to 1 V, can add a large fraction to the runtime without changing the battery at all.