Here's what I expect to see if everything is working correctly.
- All electricals and engine off, 12.8 volts at the battery
- Engine on at idle, 12.8-13 volts at the battery
- Engine on at 1500 RPM or higher with no accessories on, shortly after engine start 13.5-14.5 volts at the battery
- Engine on for more than ten minutes with no high draw accessories, 13-14 volts at the battery
If you don't see numbers very similar to these it is likely a problem with the battery, alternator or voltage regulator (if you have one separate from the alternator). The first thing I'd do is fully charge the battery on a charger and check it (with nothing attached to it) after a few hours. If its less than 12.8 volts, that's likely the problem. . . . If the battery tests good but the other voltages are low, its likely the charging system.