You may need to find a short circuit or a bad relay. Possibly there is moisture somewhere that is jumping a connection. Automotive electrical problems are difficult to track down yourself and expensive to have someone else do it.
Inside the alternator
It could be an alternator. Check with a mechanic if that is the issue.
Voltage Regulator?
Bad battery, Parasitic drain.
Your belt is correctly tensioned?
Try changing the battery.
you have a short where negative is touching the metal of the car or it is grounded inproperly
Try the cables from your battery and alt. Sometimes these can corrode.
You probably have a bad battery. You probably have a bad battery.
?ignition control module/pickup
Have the battery checked out. It may have a dead cell and not holding a charge,
sounds like your gauge is broken or there is a short in wiring