Gauge was running great then all the sudden it started to go low. Checked the battery and it had a half charge on it so I charged it full. Drove the car about 20 miles and the gauge went low again. Checked the battery and it had gone from a full charge to half a charge again. Any idea if this is the battery or the alternator?

 

Do you have an auto parts store near you that has the machine to test the charging system? Preferably the kind that can be taken outside to your car without you having to remove anything. If so, utilize their free service.

 

Had to pull both and it was the alternator. Totally dead!