when the battery is said fully charged?

A fully charged battery will be somewhere between 12 and 14.4 volts. Voltage alone is not an indicator of whether the battery needs replacement or not.

If you live in a very hot or cold climate and the battery is 6 years old, I'd think about getting it replaced soon.

Thanks Ray. The thing that I cannot determine is how could you tell whether a battery is full charged or not even if its voltage is below

12V? How long I need to drive on highway in order to have it full charged? thanks.

Thanks Ray. The thing that I cannot determine is how could you tell whether a battery is full charged or not even if its voltage is below

12V? How long I need to drive on highway in order to have it full charged? thanks.

If a battery is not at 12 volts then it is not fully charged.

If a battery is at 12 to 14.4 volts, then it is fully charged by it may not have enough reserve capacity to get the car started on a cold day.

How long you need to drive on the highway to get it fully charged depends on several factors. One, the charge state of the battery, the amperage output of the alternator, and how fast you're driving. If everything is in good working order, you should be able to fully charge a battery on the highway in 10 or 15 minutes.

Actually the voltage is a really good indicator of battery condition. Fully charged at about 13.8 to 14.2 volts and a voltage check under a load test will confirm its abilty to deliver the power properly. Basically with a 150 amp load for 15 to 30 seconds, the voltage should stay well above 11 volts the whole time. It it drops steadily during the test without leveling out after a few seconds or so, the battery is getting soft or not properly charged.

I said that voltage alone is not an indicator of whether the battery needs replacement or not because I'm pretty sure that the OP did not have a battery load tester.

You said "Actually the voltage is a really good indicator of battery condition" implying that I was incorrect but then you go on and add that the voltage needs to be measured under a load.

I don't mind being corrected if I'm wrong although it does bug me to be correct and have someone who agrees with me claim I'm wrong ;-)

The

should have the information, but here is state of charge verses voltage information:

12.6V >75% charged 12.2V 50% 12.0V 25% 11.7V 0%

Measurements taken only a few hours after the engine has been turned off will read higher because of surface charge, so when in doubt turn on the headlights for 30 seconds and then turn them off before measuring.

The battery should also be tested under load by turning on the high beams, ventilation fan on high, and rear window defroster but not running the engine. The voltage should remain at 9.5V or higher for at least 30 seconds.

The final charge rate can be a better indicator of battery condition but requires use of an automatic charger with an ammeter. A final rate of less than 1A after several hours indicates good condition, but a battery over 5 years old can still fail suddenly. Ammeters found on chargers aren't highly accurate, but a final meter reading of over 2A after overnight charging likely predicts failure within 6 months.