Thanks for the post Mark. I spent some time yesterday checking electrical wiring and fittings on the bike since most information I've found so far sent me in that direction and the previous owner gave me some red herring hints that I followed down a wormhole. Your post, however, may hold some water. The bike looks brand new despite approaching 5 years in age, it is pristine, just ridden about town but the previous owner stopped riding it after having a bad fall off his road bike. Now that it's acting up he just wants to be rid of it.
The rear brake cylinder is bad/leaks - this is the 2nd time it has had issues - they are Magura and still under warranty - I will either have them fixed or replace with Shimano once I get the power situation figured out. Still, with no rear brake and despite getting the "No Batt" message on the display, I have ridden the bike up and down the street a few times under power. But the fun soon ends as the display goes dark. But it will power up again and go again for a short spurt.
So until you posted your question I was trusting the charger to tell me the battery was full.
I pulled out my multimeter and something ain't right. Either I have a bad battery or a bad charger or maybe both? I have the gold battery getting a green light on the charger but reading only reading 4.2 volts! When I put the battery in the bike to charge the yellow light flashes on/off with a clicking noise and I think I read somewhere last week where that wasn't a good thing but at the time I was still suspecting wiring issues.
With only 4 volts I don't know how the thing even pushed me down the street, I'm no lightweight.