[pbanders]

Have you tried charging the battery on a charger? Don't assume that just driving around is getting you to full charge. When a battery is fully charged, after you let it sit for a few hours to dissapate the surface charge, you should read a voltage of 12.75 V. Anything lower than that (e.g. even 12.6 V) means the battery isn't fully charged,

If your system voltage is 13.8 V, your alternator is working. If you start with a fully charged battery and it's not staying at 12.75 V after you drive the car, the battery needs replacement.