I bought a Braille Absorbed glass mat (AGM) light weight battery for my car. It seems to lose voltage as I drive. When I got it it was low so I charged with my regular battery charger. It would only charge to around 80%. I bought a Shumacher battery charger that would properly charge an AGM battery. It charged to 100% and still the same problem. The battery will be around 12.9 volts when I leave for the track. It will be 12.7 when I get there and 12.6 after 2 passes. I just charge it at the track but this can't be right. I also sent the original battery back and got another one from Jegs. Nothing changed. The alternator is putting out the right voltage, but I didn't load it to check the amp output. I had no problem with the old battery at all.