Battery Atlas

Sizing & runtime

Battery Discharge Calculator (Peukert)

See how much of a lead-acid battery's rated capacity is available at a given discharge current, and how long it lasts to full discharge, using Peukert's law.

Results are mathematical estimates from the values you enter — not measurements and not data about a specific product or vehicle.

Ah

h

A

How does the Battery Discharge Calculator (Peukert) work?

Available capacity at current I is C × (C ÷ (I × H))^(k−1), where C is the rated capacity at the H-hour rate and k the Peukert exponent. Discharging faster than the rated current (I > C ÷ H) delivers less than the rated amp-hours when k > 1.

Formula

I(rated) = C ÷ H
t = H × (C ÷ (I × H))^k
C(available) = I × t = C × (C ÷ (I × H))^(k−1)

Worked example

100 Ah @ C20, k = 1.2, 20 A → I(rated) = 5 A → t = 20 × (100 ÷ 400)^1.2 ≈ 3.79 h → 75.8 Ah available (75.8%).

Limitations and warnings

  • Peukert's law fits lead-acid batteries at constant current and room temperature. Lithium (LiFePO4) batteries have exponents close to 1.
  • 100% discharge damages lead-acid batteries; use the runtime calculator with a depth-of-discharge limit for planning.

Four values that are often confused

Nominal capacity (Ah)
Charge a battery delivers under rated conditions, usually over 20 hours (C20) at 25 °C.
Usable capacity
The part of the nominal capacity you can use without damaging the battery — about 50% for lead-acid, 80–90% for LiFePO4.
Cranking current (CCA)
Current deliverable for a short time in the cold to start an engine. It measures power, not stored charge, and cannot be converted to Ah.
Stored energy (Wh)
Ah × voltage. The right unit to compare batteries of different voltages.