Help! charging sytem acting weird!

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i have an 84 chevy pu i replaced alternator, battery, negative cable, and continuity of all wiring for the alternator the system has only 12.5 volts when it is checked with a voltmeter, you disconnect the positive cable and the cable read 14 volts hook it back up to the battery and it goes back to 12.5 and the system only runs off the battery. have replaced the batery twice, not it, replaced the alternator with three of them all test good. with the key off and the positive cable off a test light going to the cable and the positive side of the battery doesnt light up so no drain on the battery any ideas?