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If it is a Good battery and you fully charge it and then hook it up to your car while off. then 3 hours later its dead the I agree with "Ieatcamaros" it is the rectifing diodes in the alternator.
I had this exact prob in my 88 stang
the car will charge half ass while reving the motor but idle or off it will drane your batt like a drunk at happy hour.
disconnect the alternator and let the car sit then reconnect it and see if it starts
The retifiers on these cars are shity they don't have a good heat sink and if you run anything that takes much juice they will give out after a while.
If you know how to take apart an alternater(which isn't hard) you will see a plastic part with 3 leads coming off it screwed to the back. This is the rectifier, if you go to your local alternator shop they should be able to test it for you and sell you another for like 25 bucks.
I think this is prob your problem
Good luck