It's probably a failed SCR (silicon controlled rectifier) or diode which is used to excite the field. No excitation no alternator output. There may be bad fuses in the control circuit too. These are just generic guess based on the simple principals that these devices use to work and no doubt will not be better than having a trained tech meter in hand troubleshooting this in person. Have them fix your fuel problem too or you will wear out starter motors prematurely. You also won't need a higher amp rated battery you were asking about at an earlier date.
