Car Lead-Lead acid batteries are considered fully charged around 13.2
volts. The charging circuit should supply at least 13.2. 14.4 would be
even better. Put your volt meter across the terminals with car off.
If charged is should read at least 12v to 13.2v. Less than 12v means
something is wrong... bad battery or bad charging circuit.
Start the car, turn off all accessories, run at 1000 rpm and the reading
should be around 14.2 plus/minus 0.2v.
Next, disconnect the negative terminal, car off, doors shut, etc.
Measure the amp draw between the neg battery terminal and the neg wire.
It should be less than about 0.7 amps. Anything much over that and it
could mean trunk light on, key not turning fully off, etc.