Battery Peukert Calculator

Calculator

Formula used

Peukert’s law models how available runtime decreases as discharge current rises. A common form is:

This calculator finds I_ref = C / t_ref, computes the constant I_ref^k · t_ref, then estimates t = constant / I^k. Practical settings apply DoD, efficiency, and reserve margins.

How to use this calculator

1. Enter rated capacity and the hour rate from your datasheet.
2. Set the Peukert exponent (k) for your battery chemistry.
3. Choose constant current or constant power load mode.
4. Provide voltage, DoD, efficiency, health, and reserve margin.
5. Press Calculate to view usable time, Ah, and Wh above.
6. Use the download buttons to export CSV or PDF.

Rated capacity and hour-rate context

Battery labels such as 100 Ah at 20 hours assume a reference current of 5.00 A. This tool sets Iref = C/tref, so using the correct hour rate keeps comparisons consistent. A 100 Ah at 10 h rating uses 10.00 A reference, which can shift runtime estimates even before applying Peukert losses.

Selecting a realistic Peukert exponent

The exponent k describes how sharply capacity falls at higher currents. For many flooded or AGM lead-acid batteries, k often lands around 1.10 to 1.30, while high-quality cells can be closer to 1.05 under gentle discharge. If you only have two datasheet points, fit k by matching two known currents and times, then validate with a third load.

Current mode versus power mode behavior

Constant current mode is best for DC loads with stable draw. Constant power mode converts W to A using I = W/V, which is useful for inverter-fed devices when voltage is fairly steady. With 120 W at 12 V, the tool uses 10.00 A. If voltage sags to 11 V, current rises to 10.91 A and runtime drops, so treat power mode as a planning estimate.

Turning theoretical hours into usable service

Peukert provides a theoretical runtime t = (Iref^k * tref) / I^k. The calculator then applies depth of discharge, system efficiency, and reserve. In the built-in example (100 Ah @20 h, k = 1.12, 12 V, health 95%), a 20 A load yields about 4.00 h theoretical. With 80% DoD, 90% efficiency, and 10% reserve, usable time is about 2.59 h, or roughly 622 Wh. At 40 A, the same inputs give 1.84 h theoretical and 1.19 h usable, delivering about 47.7 Ah. Notice the doubling from 20 A to 40 A more than halves runtime when k exceeds 1 even before safety deratings apply.

Engineering margins and documentation outputs

Reserve margin protects against surge loads, cold weather derating, and measurement error. A practical starting point is 10% to 20% reserve for critical systems, then tune after field data. Exported CSV and PDF summaries help document assumptions for maintenance logs, project proposals, and audit trails, keeping each scenario reproducible.

FAQs

What does the Peukert exponent represent?

It measures how quickly effective capacity falls as discharge current increases. k near 1.00 means little loss, while higher k causes runtime to drop faster than linearly. Use datasheet curves when possible, or start with typical chemistry ranges.

Which hour rate should I enter?

Use the hour rate printed with the capacity rating, such as 20 h or 10 h. The tool converts that pair into a reference current. Mixing a 20-hour capacity with a 10-hour rate will misalign the baseline and skew results.

Can I use this for lithium batteries?

Peukert behavior is weaker for many lithium chemistries and may not match the simple law across the full state-of-charge range. You can still model with k close to 1.00, but expect better accuracy from manufacturer discharge curves and cutoff-voltage models.

Why do usable hours differ from theoretical hours?

Theoretical runtime comes directly from Peukert’s equation. Usable runtime then applies depth of discharge, system efficiency, and a reserve margin. These settings represent real constraints like cutoff voltage, inverter losses, and the buffer you keep for unexpected loads.

How do I estimate current from appliance wattage?

Select constant power mode and enter load watts plus nominal voltage. The calculator estimates current as W divided by V. If you expect voltage sag under load, choose a slightly lower voltage to avoid optimistic runtime estimates.

What reserve margin is reasonable?

For noncritical loads, 5% to 10% can be enough. For backups and safety systems, 10% to 20% is common to cover surge currents, cold weather, and aging. Increase reserve when load measurements are uncertain or conditions vary.