IF I DON'T DRIVE MY VIBE EVERY DAY THE BATTERY DIES

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I JUST PURCHASED AN '05 VIBE ABOUT 3 WEEKS AGO. IT RAN FINE UNTIL LAST WEEK. IF I DON'T START IT UP EVERY DAY THE BATTERY DIES AFTER SITTING 2 DAYS. THE MECHANIC SAYS THE BATTERY TESTS NEW, THE ALTERNATER IS FINE, THE STARTER IS FINE. WHERE IS THE ELECTRICAL LEAK? HOW DO I FIND IT? THANKS, MS PRIEM