Battery could be bad and therefore not taking a charge. Alternator or regulator could be failing. Grounds might be failing, or wires could be loose.
If you have a one-wire alternator and are using a diode to fool it into working, its overcharging your battery, and the battery may have failed as a result.
I'd encourage you to move this question over to the electrical forum, and provide more details about your electrical system there.
