FIRE,BURN. Negative terminal on battery burnt insulation

[CamarOklahoma]

I was diagnosing my voltage issues and disconnected the negative terminal. Apon inspection I noticed the insulation nearest to the negative terminal is burnt.

A week or so prior to burnt insulation discovery-
I recall one day sitting in the car at a store parking lot, music playing and all of a sudden the whole cabin area filled with smoke and smelt like burning rubber. I quickly popped the trunk and didn't see any smoke or smell, I then assumed it was a vehicle behind me leaving, possible their exhaust....

[bsn]

Poor or loose connection, the current is the same negative or positive.