Formulas
| Basic form | C_p = I^k · t |
|---|---|
| Runtime | t = C / I^k × (C / C_rated)^(k−1) |
| Peukert exponent k | 1.1 – 1.25 for flooded lead-acid; ~1.05 AGM; ~1.05 Li-ion |
Typical k values
| Chemistry | k |
|---|---|
| Lead-acid (flooded) | 1.20 – 1.30 |
| Lead-acid (AGM) | 1.05 – 1.15 |
| Lead-acid (gel) | 1.10 – 1.20 |
| NiMH | 1.05 – 1.10 |
| Li-ion / LiFePO₄ | ~1.05 |
Example (100 Ah AGM, k=1.1)
| Discharge rate | Available capacity |
|---|---|
| C/20 (5 A, rated) | 100 Ah |
| C/10 (10 A) | ~89 Ah |
| C/5 (20 A) | ~79 Ah |
| C/2 (50 A) | ~65 Ah |
| 1C (100 A) | ~58 Ah |
Notes
- Manufacturer capacity is specified at a reference discharge rate (often C/20 for lead-acid, C/5 for NiMH, 0.5C for Li-ion).
- Peukert matters most for engine-start (high I) vs house loads (low I) comparisons.
- Low-k chemistries (Li-ion) are robust to discharge rate — one reason Li-ion dominates modern portable.
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