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The simple estimate — runtime equals capacity divided by load current — is optimistic, often substantially. Three effects reduce the result.
Higher current means more of the stored energy is lost as internal heating rather than delivered to the load. In lead-acid this is pronounced enough to carry its own name: a battery rated 100 Ah at the 20-hour rate may deliver only 55–65 Ah when discharged in one hour. Lithium chemistries hold up far better but still lose a few percent at high rates.
This is why a capacity figure means nothing without the rate it was measured at — and why comparing a cell specified at 0.05C against one specified at 1C flatters the first considerably.
For a constant-power load such as an inverter or a switching regulator, falling battery voltage means rising current to maintain the same output. The discharge therefore accelerates toward the end, and the simple ampere-hour division understates how quickly the last part disappears.
Cut-off voltage matters: equipment stops working when the supply falls below its minimum, which may happen while useful charge remains. Lithium systems also reserve capacity at both ends to protect cycle life, so usable energy is commonly 80–90% of nominal, and every pack loses capacity with age.
Calculate the ideal runtime, then apply a realistic derating — commonly 10–20% for lithium at moderate rates, and considerably more for lead-acid at high rates. Where runtime genuinely matters, measure it with the real load rather than trusting any calculation, since the datasheet discharge curve was produced under conditions that rarely match the application.
Divide usable capacity by load current for a first estimate, then derate. Real runtime is shorter because capacity falls at higher discharge rates and because usable capacity is less than nominal.
Chiefly because capacity falls with discharge rate, usable capacity is less than nominal, and equipment cut-off voltage leaves some charge unused. Ageing reduces it further.
The reduction in delivered capacity at higher discharge currents, most pronounced in lead-acid. A 100 Ah battery rated at the 20-hour rate may give only 55–65 Ah if discharged in an hour.
Because as battery voltage falls, current must rise to maintain the same output power. Inverters and switching regulators behave this way, so the final part of the discharge happens faster than a simple calculation suggests.
The minimum voltage at which the equipment still operates or the battery management system disconnects the pack. Charge remaining below that point is not usable in practice.
Commonly 10–20% for lithium at moderate rates, and considerably more for lead-acid at high rates. Where runtime is critical, measure with the real load rather than relying on any calculation.
Written and reviewed by
Lead Electronic Engineer (R&D) · Power Electronics Specialist · M.E. Power Electronics
Founder of Power4All. Works in power-electronics R&D and writes the converter guides, calculators and simulators on this site.
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