Had this happen on my W124 last year!
Had windows down and could smell (as well as see) battery vapors being emitted.
Did the voltmeter check and found 17.5 volts with the engine running!
Ended up replacing the alternator on mine...the regulator is internal...
You do have to remove the lower panel to get to the alternator.
The 2 19mm bolts that anchor the alternator can be a bit stubborn, and there is little room to move around for the top bolt. Good time to replace the serpentine belt if it's looking aged in any way.
Sorry...I'm talking as if I know what model car you own?