After many months of not using it, and having drained the battery stupidly (I left the key in a position that leaves the lights on… mildly), one of my motorcycles had what appeared to be a dead battery. This happened to me before and the guy from the scooter shop convinced me to buy a new one.
I have a battery tender charger whatever, and it just didn’t detect the battery. I wouldn’t charge it not even after many hours!
So I figured out another solution: I used a phone charger (around the same voltage) and charged it WHILE monitoring the charge with a cheap multimeter. When it reached certain voltage (don’t remember what), I just used the battery tender to finish the job, and although the green light (maintenance finished) never came up, it did work, no problem!
Note for BMW GS owners: I experienced a weird issue though: sometimes while riding the power was cut, after which the RPM’s clock doubled the read. Doing some research I found out that that’s what happens when there’s a battery faulty connection. In my case that was exactly the problem, and I kinda suspected it… loose bolt.
So what the story tells us is that surprisingly enough mechanics / technicians ask us to change parts that are actually recoverable / fixable.