Why not take the battery to autozone or auto stores and have them test it for you. It's possible that the battery might be bad. I've seen situation where "new" battery crapped out less than 2 years after purchase. Let the autostore test it for you and if they say it's battery then you can get one. If the battery is not the issue, maybe you are draining the battery some other way... I thought I've seen a way to test the alternator sometime back from one of the members..

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2001 BMW X5 3.0 Sold
2008 VW jetta S
2006 Lexus Rx400h