Battery Discharge Time Calculator

Calculate how long a battery lasts at a given load, allowing for capacity derating.

About Battery Discharge Time Calculator

Lead-acid capacity falls as discharge current rises, described by the Peukert exponent. Lithium has an exponent close to 1.0, so its capacity is nearly independent of discharge rate. Rating a lead-acid bank at the 20-hour rate then discharging it in two hours can lose a fifth of the expected capacity.

Formula

Battery Discharge Time

Runtime = usable Ah ÷ load, corrected by (rated current ÷ load)^(k−1) for the Peukert effect

Peukert exponent: 1.0 for lithium, 1.1 to 1.3 for lead-acid

Worked example

With the default values below, Battery Discharge Time Calculator returns Runtime of 7.210. Change any input above and every figure here is recalculated in your browser.

Inputs used

Rated capacityminimum 0 Ah: 100 Ah
Load currentminimum 0.01 A: 10 A
Usable depth of dischargeaccepts 0–100 %: 80 %
Peukert exponentaccepts 1–1.5: 1.15

Results

Runtime: 7.210 h
Runtime Ignoring the Peukert Effect: 8.000 h
Effective Capacity at That Load: 72.10 Ah
Discharge C-Rate: 0.1000 C
Capacity Lost to the Discharge Rate: 9.88%

Step by step

1Values used
2Battery Discharge Time
3Runtime= 7.210 h
4Runtime Ignoring the Peukert Effect= 8.000 h
5Effective Capacity at That Load= 72.10 Ah
6Discharge C-Rate= 0.1000 C
7Capacity Lost to the Discharge Rate= 9.88

Assumptions and limits

The result is derived only from the 4 inputs listed above. Anything not among them is not part of the calculation.
In particular, this calculator does not take inflation, tax or compounding frequency into account.
Figures are rounded for display to 4 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Frequently Asked Questions

How is Battery Discharge Time calculated?

Runtime = usable Ah ÷ load, corrected by (rated current ÷ load)^(k−1) for the Peukert effect Lead-acid capacity falls as discharge current rises, described by the Peukert exponent. Lithium has an exponent close to 1.0, so its capacity is nearly independent of discharge rate.

Why does Battery Discharge Time matter?

Rating a lead-acid bank at the 20-hour rate then discharging it in two hours can lose a fifth of the expected capacity.

What values do I need to enter?

This calculator takes 4 inputs: Rated capacity, Load current, Usable depth of discharge, Peukert exponent. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.