New Battery

Broke down and bought a new battery for my truck today. Cleaning the terminals helped, but it didn't cure the problem. If I let the truck sit for a day without driving it, then the next time I started it, it wouldn't idle without my giving it some gas. A couple instances of this was enough to convince me it was time. Besides, the old battery was looking pretty grundgy.

When I pulled the old battery out, I noticed this little round disk popping up in the center of the battery tray. What a weird thing. It seems to be loosely mounted on a spring. What the devil is it? I pull on it and it comes right out dragging a couple of wires behind it. It must be a sensor of some type. All I can figure is that it is a temperature sensor to sense when the battery is overheating. What would cause that? Overcharging? A short? Whatever, I stuffed it back in it's hole and installed the new battery and we're good. I hope.