After 3 weeks of being not used I went to start the car and I found the battery to be dead.We charged it and when putting it back in it was backwards and blew a fuse.Replaced the fuse and the car started again but a couple of days later it was barely turning over.Charged the battery again and when tested the battery when it was going it was only generating 12.8 volts not 14.25 - 14.55 volts at 2000 rpm as it should. Undid one terminal and left it for a week ( the battery was fully charged ).Reconnected it today and the battery was flat.. I don't know anything about cars and need to keep this one going as its all I can afford ! ANY HELP WOULD BE GREATLY APPRECIATED ! THANKS GUYS

new battery?

faulty voltage regulator inside alternator...get a complete alternator from wreckers.

assumming battery is FULLY charged ?
put multimeter across battery - check while cranking should hold approx.9.0volts if less replace - check wreckers for s/hand price compared to new .

charge voltage can be affected by a faulty battery so recheck should build up to approx.14.0volts-14.5volts.

if still low check with multimeter for voltage at heavy power cable at back of alternator if no power check for blown fuse link.