Naitch: a couple of final questions, if I may:-
Oh, go on then!
1) Do all three cells tend to be affected in each case, or just one in each pump (the one furthest to the right - most positive, for example)?
From my observations, it's just the ONE battery in each pump, but in any one of the three positions.
2) And ... have you found many pumps of similar age where the cells are all OK?
I simply don't know, as (in most cases) I don't know the age of the pump.
I'm wondering if "one bad cell" leads to over-charging of the good ones. These cells should always be changed as a set (rather than individually).
Don't think it's applicable, as the corrosion BREAKS the continuity at the terminal - the cell has usually held it's charge! I don't always replace the cell, as it continues to work after I've cleaned the corrosion off. It's probably bad practice, but my depatment is on an economy drive, so I'm NOT always replacing the battery.