I've had a 500w 36v 12Ah external controller on my bike for a year now. Done 1000km or so, maybe 20 cycles on the battery. Didn't use it for two months but did top it up.
Yesterday I went for a short ride and the battery indicator was green all the way. Got halfway up a steepish hill and it went green > orange > red > dead in 15 seconds or so. The controller was still cool.
I thought the battery indicator was wrong so trickle charged it for 6 hours.
Now I test the battery on the charger plug side at 41.8v, but at the terminal side to the controller its at 0.4v. Any thoughts?