Terminal Voltage
The open circuit voltage, is measured when the engine is off and no loads are connected. It can be approximately related to the charge of the battery by:
| Open circuit voltage | Approximate charge |
Relative acid density |
|
|---|---|---|---|
| 12 V | 6 V | ||
| 12.60 V | 6.32 V | 100% | 1.265 g/cm3 |
| 12.35 V | 6.22 V | 75% | 1.225 g/cm3 |
| 12.10 V | 6.12 V | 50% | 1.190 g/cm3 |
| 11.95 V | 6.03 V | 25% | 1.155 g/cm3 |
| 11.70 V | 6.00 V | 0% | 1.120 g/cm3 |
Open circuit voltage is also affected by temperature, and the specific gravity of the electrolyte at full charge.
The following is common for a six-cell automotive lead-acid battery at room temperature:
- Quiescent (open-circuit) voltage at full charge: 12.6 V
- Fully discharged: 11.8 V
- Charge with 13.2–14.4 V
- Gassing voltage: 14.4 V
- Continuous-preservation charge with max. 13.2 V
- After full charge the terminal voltage will drop quickly to 13.2 V and then slowly to 12.6 V
- Open circuit voltage is measured 12 hours after charging to allow surface charge to dissipate and enable a more accurate reading.
- All voltages are at 20 °C, and must be adjusted -0.022V/°C for temperature changes (negative temperature coefficient - lower voltage at higher temperature).
Read more about this topic: Automotive Battery
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