The battery must be 12.4 volts or higher to be able to crank engine over. My suggestion is have the battery load tested. When it passes the test charge it up to within specifications and install back in the car. With a voltmeter on the battery posts, start it up and put headlight and heater On and measure the voltage.
If its not in the range of 13.5 - 15.5 the alternator is not putting out and you're just running of the battery available voltage till its completely gone.
See charging circuit below
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Thursday, December 6th, 2007 AT 12:00 AM