When the engine is off, a fully charged car battery should have a total voltage of around 12.6 to 12.8 volts. When the engine is running, the alternator increases the battery’s voltage to between 13.7 and 14.7 volts to keep it charged. Voltage is a key indicator of the battery’s state of charge.
Your battery supplies your car with power. Thus, a low battery voltage could affect your car’s performance. But, you’re probably wondering, what exactly is an adequate battery voltage? While a car is running, the battery voltage should be between 13.5 and 14.5 volts.
Although the "no load" or open circuit voltage of 12.5v seems ok, its the "under load" voltage that's more important and also the ability of the battery to deliver the current demanded by the starter motor to start the engine.
Not to mention you won't be able to start your car, especially when it's cold outside. So, how low are we talking? A typical 12-volt auto battery will have around 12.6 volts when fully charged. It only needs to drop down to around 10.5 volts to be considered fully discharged.
12V lead-acid/AGM/LiFePO4 battery voltage table chart 12.6 volts or more: A voltage reading of over 12.6 volts indicates that your battery is fully charged and in good condition, so there is nothing to worry about. 12.5 volts: A reading of 12.5 volts shows that your battery is healthy and 90% charged.
If the rest of the cells are good you can probably charge it up to 12.5 V by severely overcharging them (most chargers will happily do this), but the voltage will slowly drop back down to around 10.5 or so after being removed from the charger. – R..