I'd use a digital multimeter across the battery posts, everything off, note the reading. Start the vehicle take second reading across battery posts, the second reading should be higher than the first, if alternator is charging. If alternator isn't charging, the first thing I check is voltage at the L terminal for alternator.
Another possibility is a battery drain, with everything turned off. The first to check is for any lites left on, interior, under the hood, anywhere. Best time to check is in the dark. If that checks ok and the drain is on a fused circuit, take battery cable loose, use a testlite between cable and battery post, if the lite comes on bright, you have a drain. You pull/replace fuses one at a time. When you pull fuse on a faulty circuit, the lite goes out. Then you have to see what all is on that circuit. Go from there.
Sunday, February 6th, 2011 AT 9:53 PM