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What does terminal voltage tell?

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 (12V) Open Circuit Voltage (6V) Approximate charge Relative acid density
12.65 V 6.3 V 100% 1.265 g/cm3
12.45 V 6.2 V 75% 1.225 g/cm3
12.24 V 6.1 V 50% 1.190 g/cm3
12.06 V 6.0 V 25% 1.155 g/cm3
11.89 V 6.0 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
  • Unloading-end: 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 12.6V.


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