Diagnosis - either the battery is running flat because power is being used when the vehicle is not in use - possible but unlikely - or the battery is running flat when the car is in use - because the alternator is not charging it - more common.
The "battery" light should come on when the ignition is on and the engine not running. As the engine revs up a bit then the battery light should dim and go out. This suggests that the alternator is good.
The voltage at the battery terminals on fast tick-over should be about 13.5 - 14.4.
If the battery light never comes on then the bulb may be blown which will cause the battery to run flat because it stops the alternator from working.
If the battery light always remains on it strongly suggests a broken alternator.
Look to the battery light...
Good luck...
Peccavi, March 2010
