I thought I had a problem with my charging system after buying a used 91. After a little research and advice from this forum, I removed the left passenger foot rest to inspect the condition of the ground, leading from the R/R. Sure enough, there were heaps of corrosion where the R/R heat-sink, (which is the footrest itself), connects to the rest of the frame. So, out came the Dremel tool with a wire brush to clean both male and female connection points.. I reassembled, adding dielectric grease, not only to that, but on all connections I could get to. Battery charging system worked fine after that.. Few years later, I opted for a lithium battery.. I like the power and lightness of it. So far, it's been running great (6-7 years later)
Hope this helps..
Good luck with that.