When to use: Lead-acid and other batteries deliver less usable capacity at higher discharge currents than the nameplate Ah rating suggests. Peukert's Law models this: t = HΒ·(C/H)α΅ / Iα΅, where H is the rated discharge hours, C the rated capacity, and k the Peukert exponent (1.0 = ideal, no derating; higher k = more capacity loss under heavy load). Compare the result against the naive t = C/I estimate to see the derating effect.
This tool applies Peukert's Law to estimate the actual runtime of a battery under a real discharge current, rather than the overly optimistic runtime you get from simply dividing the rated ampere-hour capacity by the load current. It also reports the naive C/I estimate side-by-side so you can see exactly how much the Peukert effect shortens real-world battery life under heavy loads.
A battery's ampere-hour rating (e.g., 100 Ah) is only accurate at the specific discharge rate it was tested at β typically expressed as a rated discharge time H (commonly 20 hours for lead-acid batteries). Dividing capacity by load current (t = C/I) implicitly assumes the battery delivers the same total Ah regardless of how fast it's discharged, which is false: pulling current faster than the rated rate reduces the usable capacity due to internal resistance, heat, and incomplete chemical reaction at the plates.
Peukert's Law captures this with an exponent k: Iα΅ Β· t = constant. Because the constant is fixed by the rated capacity and rated hours, actual discharge time works out to t = HΒ·(C/H)α΅ / Iα΅. When k = 1.0 the battery behaves ideally and the Peukert estimate matches the naive C/I estimate exactly. Real lead-acid batteries typically have k between 1.10 and 1.40 β the higher the exponent, the more capacity is lost as discharge current increases.
The Peukert exponent depends on battery chemistry, construction, and age. Lithium (LiFePO4) cells are close to ideal (k β 1.05) because they suffer little internal-resistance penalty at moderate rates. AGM and gel lead-acid batteries typically fall in the 1.10β1.15 range. Standard flooded lead-acid batteries are commonly modeled around k β 1.20, and older or degraded flooded cells can rise to 1.30β1.40 as internal resistance increases with age and sulfation. Manufacturer datasheets sometimes publish k directly from multi-rate discharge testing β use that value when available instead of a generic preset.
Enter the battery's rated capacity C and the rated discharge hours H it was tested at (both from the nameplate or datasheet), then enter the actual load current I your application will draw. Pick a Peukert exponent preset for the battery chemistry, or fine-tune k directly if you have a manufacturer-published value. The tool reports the actual Peukert-corrected discharge time, compares it to the naive C/I estimate, and flags whether your load current exceeds the battery's rated discharge current β a common trigger for severe Peukert derating.
Peukert's Law estimates how much usable capacity a battery actually delivers when discharged at a rate different from its rated test rate. It is most commonly applied to lead-acid batteries in off-grid solar, UPS, marine, and telecom backup systems where the real load current differs significantly from the manufacturer's rated discharge rate.
Internal resistance causes voltage drop and heat generation that increase with current, and at high discharge rates the chemical reaction at the plates cannot keep pace, leaving some active material unused before the cutoff voltage is reached. Peukert's exponent k quantifies how strongly this effect scales with current for a given battery.
Use a chemistry-appropriate preset as a planning estimate: roughly 1.05 for lithium (LiFePO4), 1.10β1.15 for AGM/gel, and 1.20 for standard flooded lead-acid, rising toward 1.30β1.40 for older or degraded cells. For critical designs, derive k from two points on the manufacturer's discharge-rate curve or request it directly from the manufacturer.
No. Peukert's Law is a useful two-parameter approximation, but real discharge curves are not perfectly Peukert-shaped across their full range (especially near very high currents or very low currents). For final battery bank sizing on critical loads, cross-check against the manufacturer's published discharge-time-vs-current table.
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