Don't know about Bosch alt but old 10si Delco alt. the idiot light supplied current to excite the alt to get it to charge. With the Delco if you used too small a bulb it would not excite and not charge. I suspect that may be your problem with LED light . However you said it did charge some sometimes so that has me wondering. I would put the old bulb in and see what happens.
I have a feeling you will be back in bussiness. No guarantee tho.