Battery venting excessively in 300E.

[G-Benz]

Got into car at work this afternoon and started it. Idle was suddenly down to 500 rpm but didn't stall. Radio was on but audio signal to speakers would shut down, then reactivate moments later. Pulled car over and shut down. Restarted and idle was back to normal, though radio was still doing its thing.

Engine and radio operated normally on the highway and the rest of the trip, but when I got back to city driving, I could smell what seemed to be battery acid. Pulled over and found acid vapors venting out of a vent hole on the side of the battery case. Continued home.

Got home, waited for car to cool down. Pulled vent caps off and noticed one cell was almost dry. Refilled with distilled water and retested.

Engine and radio were fine, but battery continued venting. Could hear what sounded like electrolyte boiling inside the case. Checked OVP fuse and fuses in box and none were blown.

My sudden fear is that the alternator is overcharging. What's going on??? Any ideas?