Sounds like a battery problem to me.
It is a high resistance internal circuit, cracked , corroded , whatever. As it gives or receives current it is heated and boils the electrolyte.
An interesting test can locate/prove the actual cell. If you take two six inch lengths of welding rod (Bare coathanger wire will do - remember bare; coathangers are heavily coated with paint) and clip your volt meter laeds to them. Then place both wires into ajacent cells (into the electrolyte but not touching the plates). The voltage should be over 2 volts. The voltages should be the same for all cells.
The last 2.5 volts will be shared by the two end readings (from cell to battery terminal. One side will read something like 1.5v and the other .75 (haven't done this in a long time. You will eventually come up with seven readings and combine the two end readings to get six cell readings of over 2v.
Now the interesting part. Turn the head lights on and if the one cell gasses test the readings of it. You will probably see some real goofy readings with the polarity reversing at the cell with the high resistance. All the other cells should look the same, only slightly reduced in voltage if the lights work and current passes.
Bosch Master, ASE Master, L1
33 years MB technician