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Battery Voltages : What they mean

AUTOMOTIVE  LEAD/ACID BATTERY  VOLTAGE  STATES 

Voltage

Engine off.

Engine Running

15V+

 

Overcharging. Your battery will overheat, boil the electrolyte out and die.

14.4V

 

Battery charging quickly, this is a healthy electrical system.

> 13.8V

Overnight charging voltage.  If charge remains on the battery for more than a few days, electrolyte will eventually boil out.

 

13.8 V

Battery is connected to a good float charger.  At 13.8V, the battery will be happy indefinitely.

Battery charging slowly, this voltage is easy on the battery, and will give you a full charge, but not very quickly.  Good systems go to this voltage after the battery has been charged.

<13.8V

 

Marginal charge. Battery will most likely not be fully charged by the end of your trip.

12.65V

Fully charged, ready to go

 

At 12.65V or less, car is running off battery, not alternator. Your battery will not get fully charged. Battery life is short because it never gets fully charged.

12.45V

Battery at 75%

 

12.24V

Battery at 50%

 

12.06V

Battery at 25%

 

11.89V or less

Flat.   

 

 Acknowledgement: Keven Coates