Here is some more info to help narrow it down. I have a catamaran
with two starter batteries (one for each engine) as well as my house batteries as well as a Cristec charger
. Also only this one cell on just this one starter battery is having this issue. A faulty cell vent in the cell cap could explain why just that cell is having issues.
To answer some of the questions raised: the battery is less than 3 years old. In the cell there is no bubbling. It just builds pressure that releases when I twist off the cap, like opening a Coke. But inside it is not boiling and doesn't look any different inside from the other cells. The battery still works fine and holds a change of 12.6-13.2v.
For those that think it's overcharging, wouldn't that affect all my batteries? Or at least the other cells? Best way to test this theory?