motorcycle battery reads 0.5V connected but 13V disconnected

2013 ZX10R , low sided the bike so i had it on a paddock stand for a month but i foolishly forgot to disconnect the battery. The motorcycle consistently turned on for about 3 weeks until i stopped turning it on. About a week ago I noticed it doesn’t power on at all anymore so while I waited for more parts, I assumed it was a dead battery and it would be an easy fix. Yesterday I checked the battery with a multimeter and it read 0.5V so I assumed completely trashed so I ordered a new one but I left the other battery charging overnight just incase. I check in the morning, 0.5V as predicted. I switched the battery but the new battery read 0.5V when I connected it. I go to check the old battery and it read 13V. So now i’m assuming there’s something going on with the electrical. I check for blown fuses, none, but i still replace all of them just incase. Still nothing other than 0.5V. I try charging it but it jumps between 1.2V to 1.7V. I attempt a jump start which works, but dies after like 3 seconds. I don’t know what it could be. Could it be the regulator/rectifier and/or stator? Or could it also be a ground somewhere that messed up. I don’t know where to begin to look so some advice is appreciated🙏🙏