2000 Jeep Cherokee Instrument Panel going haywire!

Car seemed to be running fine, stopped at my parents and when I put the key in to leave I heard a sort of clicking that sounded like it was coming from right there where the key was, then all of my gauges wen't from one extreme to the other and the car wouldn't turn over. I jumped it and it ran for a few minutes, I turned it off and tried to turn it back on only for it to freak again and kinda turn but not come on. I looked and the battery had little to no water, so I filled it with water and then hooked the cables back up to another car to jump it. This time I let it charge for at least 15 minutes before trying to start the jeep again and this time there was no clicking or gauge freak outs but it was just a weak attempt to turn over again and then nothing. Dead. I took the cables off and then removed the battery cables from the battery in the jeep, my girlfriend noticed everytime I took the positive cable off then touched it to the battery again the gauges would freak. Do I need a new battery? Also, looking at the enging the battery is on the left side, sitting on a "tray" there are no brackets on top of it but on the right side of this tray there is a kind of plastic bracket that sits over a lip of the bottom of the battery. There's a bolt right in the middle that goes straight down, there was a ton of corrosion (looked like a golf ball over the top of the bolt) that I wiped away and I can't seem to remove it. Is there a nut on the bottom side of this tray where the bolt comes out? If so how am I suppose to access it? Tried with pliers and wrench but no luck. I'm able to turn and turn it (may have stripped) but it won't come out! Please let me know if anyone knows what this may be or how to get the battery out. Right now it's still parked! Thanks in advance! By the way, I do have an aftermarket stereo system that Best Buy installed over a year ago the instrument lights have been kinda dim when the lights are on since but I've never had any shorts or blackouts.I think on 2 or 3 different occasions over the last year the yellow check engine light would come on for about 2 days then go off by itself.