Battery Life Calculator
Real runtime from capacity and load, using Peukert's law, depth of discharge and temperature — not the optimistic capacity-over-current figure.
Capacity ÷ current is optimistic every single time
A battery's rated capacity is measured at one specific, gentle discharge rate — typically twenty hours for lead-acid, five for lithium, and something very slow indeed for alkaline. Draw it faster and you get less out, and not by a little. A 100 Ah lead-acid battery discharged over five hours instead of twenty delivers around 70 Ah, not 100. That relationship is Peukert's law, and it is the difference between a runtime figure you can plan around and one that leaves you in the dark.
- Chemistry decides how badly it bites. LiFePO4 barely notices high rates. Flooded lead-acid is severe. Alkaline is the worst of the lot, which is why a AA rated 2500 mAh at a trickle gives well under half that into a motor.
- Depth of discharge is a lifespan choice. Lead-acid taken below half its charge wears out fast, so half the nameplate is the honest usable figure. Lithium will run flat happily once and repay you with markedly fewer cycles.
- The cold does not destroy charge, it hides it. Capacity falls because the chemistry slows down, and it comes back when the battery warms up. Design for the coldest temperature it will actually work at.
- Series adds voltage, parallel adds capacity. Never both from the same cells. The energy in watt-hours comes out the same either way, but the current at a given power does not — and it is current that heats wires and connectors.
- The C-rating is a safety limit, not a suggestion. Exceeding a cell's rated discharge current overheats the pack, and on lithium that is a fire rather than a fault.
How to use
- Pick the chemistry — it decides how badly a fast discharge costs you.
- Enter the cell capacity in mAh and the steady load in mA.
- Set the pack arrangement, temperature and whether you will run it flat.
- Compare the real runtime against the naive capacity-over-current figure.
Frequently asked questions
Why does my battery not last as long as the arithmetic says?
Because rated capacity is measured at one specific, gentle discharge rate — typically twenty hours for lead-acid and five for lithium. Draw it faster and you get less out. A 100 amp-hour lead-acid battery discharged over five hours instead of twenty delivers around 70 amp-hours, not 100. That relationship is Peukert's law and it is exactly what capacity divided by current ignores.
Which chemistries suffer most?
Alkaline is the worst by a distance — a AA rated 2500 mAh at a trickle gives well under half that into a motor or a transmitter. Flooded lead-acid is next, which is why off-grid systems are sized so carefully. LiFePO4 barely notices, and ordinary lithium-ion is close behind it. If your load is heavy and intermittent, the chemistry choice matters more than the capacity number.
What is depth of discharge and why does it reduce my runtime?
It is how far down you are willing to take the battery, and it is a lifespan decision rather than a physical limit. Lead-acid taken below half its charge wears out dramatically faster, so half the nameplate is the honest usable figure. Lithium will run flat happily once, and repay you with markedly fewer cycles over its life. The tool counts the sensible fraction by default and will show you the full figure if you ask.
How much does cold weather cost?
Roughly a quarter of capacity at freezing and about half at minus twenty, though it varies by chemistry and cell. The important thing is that the charge has not gone anywhere — the reaction is simply slower — so it returns when the battery warms up. Design around the coldest temperature the thing will actually be used at, not the temperature of the room you tested it in.
Does wiring cells in series give me more capacity?
No. Series adds voltage and leaves capacity alone; parallel adds capacity and leaves voltage alone. The stored energy in watt-hours works out the same either way, but the current needed for a given power does not — and current is what heats wires, connectors and cells. That is the main practical argument for higher-voltage packs.
What does the C-rating mean?
It is the discharge current expressed as a multiple of capacity, so a 3000 mAh cell at 2C means 6 amps. It is a safety limit rather than a performance suggestion: exceeding a cell's rated discharge current overheats it, and on lithium chemistry that is a fire rather than a fault. Parallel strings raise the pack ceiling proportionally, which is often why they are there.
How accurate is this?
It is a planning estimate, not a measurement. Peukert exponents vary between cells of the same chemistry, capacity drifts down over a battery's life, and a load that pulses is not the steady current this assumes. Treat the answer as a floor to design against rather than a promise, and remember that it is deliberately capped so it never predicts getting more out than the battery holds.
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