1991 560sel; 126K
At the end of ~100 mile trip Friday, I pulled into my driveway and began to smell something like rotten eggs. My first thought was "catylic converter" but the smell was not at the rear of the car. It came from the front of the car. With the engine still running, I raised the hood and found the battery spewing out a cloud of hot acid steam from its vent. The battery was very hot to the touch. The acid cloud continued to spew for 5 to 7 minutes after I stopped he engine and disconnected the negative battery lead.

Saturday, I removed the battery and found that the heat had actually caused the battery case to bow or bulge out in several places. My first trouble shooting thoughts were: