Wet cell starter battery releasing pressure?

[arhoton]

I've attached a picture of my starter battery, which one cell appears to be building pressure and blowing small amounts of liquid out (see picture). When I unscrew the top you can hear built up gas escape. The other cells are fine. I've kept them topped up, and this past winter while she was on the hard I had a service which among other things, would come by and charge and top them if needed. I'm no battery expert. Any ideas what's going on?

[mynameismud]

If it's building pressure somehow one of your vents are clogged.

[arhoton]

Researching now on how to un-clog. Suggestions welcome from those that might know or done this before

[GordMay]

Overcharging.
Hydrogen evolution, or outgassing, is primarily the result of lead acid batteries under charge, where typically the charge current is greater than that required to maintain a 100% state of charge due to the normal chemical inefficiencies of the electrolyte and the internal resistance of the cells. This excess charge electrolyzes the water in the electrolyte mix of sulfuric acid and water, causing free hydrogen and oxygen to be vented from the battery.
The higher the voltage under re-charge, the greater the outgassing.

[Hanksy]

Grab a volt meter and check the voltage at the battery when you're charging. Either via alternator or charger/solar panel etc. It will help you find the problem. An over charging alternator is a likely cause

Sent from my HUAWEI SCL-L01 using Cruisers Sailing Forum mobile app

[roland stockham]

If one cell is gassing significantly more than the others you have a dead battery. Expected life for start batteries is 3-5 years. Indicative check is to take the plugs out and look in the cells, if the others are giving the occasional bubble while one cell is bubbling away merrily that cell has gone. It will also generally show a drop in voltage to about 12-13v when charging and under 12v off charge. It may still start the engine for a while, depending on how big the battery is!

[Xthewater]

Overcharging but likely also have a bad cell which has lost ampacity and boiling out. Beware explosion from hydrogen gas!

[arhoton]

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?

[GordMay]

Originally Posted by arhoton

Here is some more info to help narrow it down. ... 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.
... 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.

Your described symptoms Do suggest a plugged cap vent.

[arhoton]

One quick experiment I'm doing now. Just switched cell caps with one of the other cells. If the cell that was working fine, but now has the cap from the bad one, builds pressure from charging, then it is looking like a faulty cap. Will let you know the results.