Peukert's Law Runtime

Estimate battery runtime under real loads using Peukert exponent k. Great for lead-acid and other chemistries where capacity changes with discharge rate.

How to Use

  1. Enter the battery rating (Ah at a rated-hours test, or specify the reference current).
  2. Enter the Peukert exponent k (typical: ~1.05–1.30 depending on chemistry/quality).
  3. Enter your load current.
  4. Optional: add efficiency/derating to approximate real-world runtime.
  5. Open “Show Work” to see the formulas and intermediate steps.
Battery Lab View
Quick readout: runtime, effective capacity, and discharge severity.
Runtime
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Eff. Ah
—
Ref I
—
Load I
—
Severity: —
Estimated runtime
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Tip: Peukert estimates vary by battery condition, temperature, and cutoff voltage. Use this as a planning tool, not a guarantee.
Inputs & Settings
Enter a rating method + your load. Results update instantly (no URL changes while typing).
Example: 100Ah (often rated at 20-hour discharge)
Typical: 1.05–1.30 (lower is better under high load)
Common test rates: 5h, 10h, 20h
Your real draw (steady). For variable loads, use average current.
Use to approximate inverter/heat losses. Leave blank for ideal (100%).
Show Work (step-by-step)
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Work is shown using consistent base units (Ah, A, hours). Current inputs are normalized to amps internally.

Peukert’s Law (Reference)

Peukert’s Law models how effective battery capacity drops as discharge current increases.

  • Rated reference current (if rated-hours is known): Iref = Crated / Hrated
  • Peukert constant: Cp = Irefk × Hrated
  • Runtime at load current: t = Cp / Iloadk
  • Effective capacity at load: Ceff = Iload × t
Where k is the Peukert exponent (unitless), t is hours, and currents are in amps.

FAQ

What does the Peukert exponent (k) mean?

k describes how strongly capacity falls as current increases. A lower k keeps runtime closer to “ideal” at high loads.

Why does my “100Ah” battery not last 100Ah at high load?

Many batteries are rated at a slow discharge (often 20 hours). Pulling high current increases losses (internal resistance, chemistry limits), reducing usable capacity.

Should I use this for lithium batteries?

Lithium chemistries often have much less Peukert effect than lead-acid, but conditions still matter. If you have a known k, you can use it—otherwise treat results as approximate.

Does this include cutoff voltage and temperature?

Not directly. Add efficiency/derating to approximate real-world conditions. For precise planning, measure runtime under your real load.

Tool Info

Last updated:

Updates may include unit support, improved edge-case handling, and better real-world guidance.