Peukert's law, presented by the German scientist Wilhelm Peukert [de] in 1897, expresses approximately the change in capacity of rechargeable lead–acid batteries at different rates of discharge.
As the rate of discharge increases, the battery's available capacity decreases, approximately according to Peukert's law.
, the exponent, were equal to unity, the delivered capacity would be independent of the current.
For a real battery the exponent is greater than unity, and capacity decreases as discharge rate increases.
Application at low discharge rates must take into account the battery self-discharge current.
At very high currents, practical batteries will give less capacity than predicted with a fixed exponent.
: [1] Peukert's law becomes a key issue in a battery electric vehicle, where batteries rated, for example, at a 20-hour discharge time are used at a much shorter discharge time of about 1 hour.
[1] It is a common misunderstanding[2] that the energy not delivered by the battery due to Peukert's law is "lost" (as heat for example).
This is because the law applies specifically to batteries discharged at constant current down to the cut-off voltage.
If this battery is discharged at 10 A, it will last 20 hours, giving the rated capacity of 200 Ah.
This means that it will therefore also be (nearly) fully charged again after recharging 100 Ah – while the same battery which was previously discharged with I20 = 10 A and lasted 20 hours will be nearly fully charged after recharging 200 Ah.
Peukert's law brings a certain degree of fire-safety to many battery designs.
The primary fire hazard with lead–acid batteries occurs during over-charging when hydrogen gas is produced.
Discharging batteries at extreme rates can cause thermal runaway.
In particular, if the cell develops an internal short, it tends to overheat, release electrolyte, and catch fire.