First things first, check batt voltage with a multimeter.(I say multimeter, mean voltmeter, but if you don't have a multimeter, get a decent one, you will find yourself using it thousands of times in your life) If it does indeed check out (12V+ engine off) as good, test your fuse panel for voltage with the multimeter. It may be bad connections...if you've got 12V at the batt and say 8V at the fuse panel, your gauge isn't wrong, and your battery is good, you've got a bad connection.