Off-grid battery runtime with the Peukert effect: LiFePO4 vs AGM
The Peukert effect reduces usable capacity at higher discharge rates — but it hits AGM/lead-acid significantly while being nearly negligible for LiFePO4. So a runtime calculation that applies one Peukert correction to both chemistries will misjudge AGM and over-correct LiFePO4.
Key figures
| Parameter | Value |
|---|---|
| Peukert exponent — AGM (typical) | ~1.1 |
| Peukert exponent — lead-acid default (Victron) | 1.25 |
| Peukert exponent — flooded lead-acid (up to) | ~1.6 |
| Peukert exponent — LiFePO4 | 1.0 ideal to ~1.05 in practice |
| Practical usable depth — AGM (to preserve life) | ~50 % |
| Practical usable depth — LiFePO4 (daily use) | up to ~80 % |
| Implication | Apply Peukert to lead-acid/AGM; treat LiFePO4 as near-linear |
The core insight
The Peukert effect — usable capacity falling as discharge current rises — is real for lead-acid and AGM, but near-negligible for LiFePO4. A runtime calculation that treats both chemistries identically will be wrong in two directions at once: under-derating AGM, and over-derating LiFePO4.
Why this beats a generic runtime number
For AGM, the honest runtime is shorter than nameplate capacity suggests at higher loads, and you typically only use ~50% depth to preserve life. For LiFePO4, runtime is closer to linear with load and you can use ~80–90% depth. The chemistry, not a single formula, determines the calculation.
[Editorial pass: attach primary sources for the exponent ranges and usable-depth figures; add a worked runtime example for a fixed load (e.g. 2 kWh/day) comparing the two chemistries with the correct treatment of each.]
Frequently asked
Does the Peukert effect apply to LiFePO4?
Only marginally. LiFePO4 has a Peukert exponent very close to 1, meaning usable capacity barely changes with discharge rate within normal use. The Peukert correction that matters for lead-acid/AGM is nearly negligible for LiFePO4.
Why do generic runtime calculators get this wrong?
Many apply a single capacity or a single Peukert assumption regardless of chemistry. For AGM that ignores the real capacity loss at higher loads; for LiFePO4 it can apply a correction that does not physically occur. The two chemistries need different treatment.
What usable depth of discharge should I assume?
Conventionally around 50% for AGM/lead-acid to preserve cycle life, versus up to about 80% for LiFePO4 in daily use (per Victron's LiFePO4 datasheet, 80% DoD gives ~2,500 cycles; shallower use extends life further). This difference often matters more for real runtime than the Peukert correction itself.