Reasons why battery not charging.
2 Answers
Bad ground or high voltage drop on black battery cable.
Have you put a voltmeter on battery as car is running? Turn on blower, radio, plug something in cigarette plug. 13.4 or higher? If not you may not have good performance on alternator. I had one in the shop once that was putting out 13.2 volts but that was low end on spec and van could not adequately charge battery.