Voltage at 25 °C, moderate C-rate
| Chemistry | Full V | 50% V | Empty (nominal) | Curve shape |
|---|---|---|---|---|
| Li-ion (NMC) | 4.20 V | 3.80 V | 3.00 V | Gradual drop, knee at ~3.3 V |
| LiFePO₄ | 3.65 V | 3.30 V | 2.50 V | Flat plateau — poor SoC from voltage alone |
| Alkaline (AA) | 1.55 V | 1.25 V | 0.90 V | Sloping |
| NiMH (AA) | 1.40 V | 1.25 V | 1.00 V | Very flat, knee at end |
| Lead-acid 12 V | 12.7 V | 12.2 V | 11.8 V (50% DoD) | Sloping, depends on rest |
| Lithium primary | 3.60 V | 3.30 V | 2.00 V | Long plateau |
Observations
- Flat curves (LiFePO₄, NiMH): voltage is a poor SoC indicator — use coulomb counting.
- Sloped curves (Li-ion NMC, lead-acid): voltage reading with load removed gives decent SoC.
- Under load: terminal voltage drops due to internal resistance — wait for it to rest for an "open-circuit" reading.
- Temperature effect: cold batteries show lower voltage and less usable capacity.
- High C-rate: steeper voltage sag; rated capacity is at specified (often low) discharge rate.
Peukert effect
| Peukert law | C = I^k · t, where k ≈ 1.1–1.3 for lead-acid, ≈1.05 for Li-ion |
|---|---|
| Implication | Lead-acid delivers much less than rated capacity at high current |
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