Look for a parasitic drain on the battery. It's pretty common for the alternator to be the culprit, but you'll need to measure the amount of amps between the negative battery terminal and the negative battery post. If it's more than around 35 milliamps, then that is a good indication that something is draining your battery. If this is the case, then pull fuses while checking amperage, also unhook alternator and test.
Pete
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