Peukert Effect

Formula

Description

Peukert's law describes how the effective capacity of a battery decreases at higher discharge rates. The exponent k (Peukert constant) is always greater than 1 for real batteries, meaning that doubling the discharge current more than halves the runtime. For an ideal battery k = 1, but real batteries have internal resistance and chemical kinetics that reduce available capacity under heavy load. Lead-acid batteries typically have k = 1.1-1.3, while lithium-ion batteries have k closer to 1.02-1.10, making them much better at high discharge rates.

Variables

• t — Effective runtime in hours (h)
• C — Rated capacity at 1A discharge rate (Ah)
• I — Actual discharge current (A)
• k — Peukert exponent (dimensionless, >1)

Practical Notes

The Peukert constant is determined experimentally by discharging batteries at different rates and fitting the results. It is most significant for lead-acid and NiCd batteries. For lithium-ion cells, the effect is small enough that simple capacity derating at high C-rates is often sufficient. Temperature also affects the effective Peukert constant.