Continuous Cell Arcing
The 4074EHG tabs make contact with the edge of every other cell plate. Photo right shows the edges of the plates burnt down far enough, that the contact shorts against every plate and not every other.
Honeywell cells can last for decades, but continuous sparking can cause damage. A cell may spark continuous if collector plate spacing gets too close. The spacing can become uneven when a cell gets pushed out of square, twisted, a dent, etc.
One common problem is a loose plate that has lost the press fit with its horizontal rods. The loose plate likely had continuous sparking that created surges of current through the press fit. The current surges resulted in sparks that brunt away the press fit. This cell would not be repairable.
When one cell has been subject to excessive sparking, then both cells will be burnt under the contact pad. This is because a spark in one cell will cause a current surge that is drawn from both cells.
Check the contact pad to determine the condition of the cell. If your cells have badly burnt contacts as in the photo above, then it is best to replace the cells.
When a cell has been subjected to excessive sparking, the press fit of all the plates will first be weakened. The sparking at these connections causes corrosion from ozone. The aluminum around the press fit becomes corroded and no longer conducts as well.
The cell contacts operate at high voltage and are not a sensitive connection. The contact does not require cleaning. The contact is tin coated copper. Sandpaper will remove the tin coating.
The photo below shows arcing under the contact pad.
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