Weird Electrical Issue '17 FC450

I was doing some basic stuff on my FC450 today. Pulled the seat of and noticed rust on my battery terminals. I pulled the battery and cleaned all my terminals and reconnected the battery and all I got was a click. Voltage on the battery which is the original was at 12.6 volts. I got an intermittent shot to the starter and fuel pump and then back to clicks, then nothing. My battery voltage is now at 3 volts. The only issue I was having was maybe in my head thinking I needed a new battery as the bike seemed to turning slower than my buddies 18 KTM SXF450 and another buddies 17 SXF350. I ran the bike yesterday for a bit and the day before when I washed it. I checked the fuses and they are OK. I have the battery on a non-desulfating battery tender waiting to see what I get when I get it back to 12.6 volts. I went through my service manual real quick but didn't find anything other than the main fuse. I was checking into batteries today and will probably go with a MPS featherweight replacement. Has anybody experienced anything like this? Do these batteries just flat out die all at once? Any help here so I don't lose sleep overnight? Thanks.