I have a 98 sportster 1200. 30k miles.
Last fall just before riding season ended my battery took a $#1t.
I debated replacing then or parking then and waiting for spring.
(Warranty only 6 months on those)
I decided to take a chance that I had some good riding days left last fall and replaced it then. I think I rode it one time after the battery replacement.
I put a tender on it around Christmas time and left it on. Only rated at 3/4 of an amp.
I did start it up and warm it up a couple of times over the winter no problem.
I have ridden it to work once this year (70 mile round trip) about 2 weeks ago.
I didn't put the tender back on. I've let this bike sit a couple months without a tender connected many times and when ready it fired right up without issue those times.
I went to start it last night and Stone dead.

I jumped it (my bench top charger wasn't enough boost, has to get the big snap on wheeled battery charger out)
And it took a good while even on hi boost to do more than click click click. I finally got it going. I let it run about 10 minutes (battery charger disconnected as soon as it started) and my headlight was pulsing, which it has never done before. It finally stayed on solid for a few minutes then it and the tail light just died. Dash lights brake and turn still work.
I put a multimeter on it after the lights died and got 13.89v while running so it is charging.
Shut it off and battery had 10v.
Took battery out and without being hooked up to the bike it was at 6.some volts. I put it on the bench top charger, and it's an "automatic" charger. Shortly the red 'frown" lit up indicating a bad battery. Reset it and left it on all night. Went out this morning the red frown was on again on the charger, and battery was at 4.some volts.
I took it back to interstate and the guy was gonna "loan" me a battery and charge mine, told me to come back Monday. He put it on his charger while we were there and about 10 minutes later decided to swap it out with a new one. (He initially said he can't warranty it, it's just run down. When I asked him what difference his charger might make when it had been on mine all night already) it seemed to go over his head but he finally gave me a brand new one.
I'm about to go put this latest new battery in and see what it does.
I'm worried I screwed something up running it with that bad battery and fried something related to the lights. I don't see any fuses on this bike. What do you guys think? I've never seen the lights flicker nor have I ever seen them "not" work anytime the key was on.