Or, what alternator did you fit? Some of the cheap ones are very poor and although they deliver the ‘voltage’ they can’t deliver much current.
But if you never need to charge the battery then the alternator must be providing adequate current and a dodgy earth may be the fault.
Final thought, check your lights. In the dark run the engine and keep the lights off, look carefully for a dimly glowing bulb. If you find one you’ve also found the problem. Incorrectly wired and earthing through the bulb.
