If the battery state of charge is 25%, the open circuit voltage is about:

• A. 12.00 volts
• B. 12.06 volts
• C. 12.60 volts
• D. 11.75 volts

Answer: (B)

Open circuit voltage is the voltage you measure when no current is flowing, so it reflects how much charge remains in a lead-acid battery. For a typical 12-volt automotive battery, the resting voltage increases as the state of charge increases. Full charge is about 12.6–12.7 V, and around 25% remaining charge corresponds to roughly 12.06 V. That’s why 12.06 volts is the best match. The other numbers correspond to higher or lower charge levels: 12.60 V is near full, 12.00 V is lower than the 25% level, and 11.75 V indicates a much deeper discharge. Remember to measure after the battery has rested to avoid surface-charge effects.