The formula
t = H × ( C ÷ (I × H) ) ^ k
Effective capacity = I × t
Here t is run time in hours, H is the rated discharge time (20 h), C is rated capacity in Ah, I is the actual current, and k is the Peukert exponent. A perfect battery has k = 1.
Worked example
A 100 Ah (C20) flooded battery with k = 1.25 supplying 20 A: t = 20 × (100 ÷ 400)1.25 = 20 × 0.177 = 3.5 hours. That's only 71 Ah of effective capacity, compared with the 5 hours you'd expect from 100 ÷ 20.
Limitations
- Peukert's law is an approximation and works best between about the 1-hour and 100-hour rates.
- It doesn't include temperature or battery age, which also reduce capacity.
- Lithium batteries have an exponent close to 1, so the effect is small.
Frequently asked questions
What is a typical Peukert exponent?
About 1.2–1.3 for flooded lead-acid, 1.1–1.15 for AGM, and 1.03–1.05 for LiFePO4.
Does Peukert matter for lithium batteries?
Much less. LiFePO4 batteries deliver close to their rated capacity even at high currents.