In this photo I am simulating a sailboat heeling with flooded batteries. For years I have been wanting to write this article but until my friend Dave let me shoot these polycarbonate cased UPS batteries it was going to be tough to explain. A picture is worth a thousand words. This one may be worth more than a thousand.
Here we can see the three individual cells of this 6V battery. A 12V battery would have 6 cells. Even heeled like this the positive and negative plates are not and have not "uncovered" and this is good. The vents are also not having electrolyte try to escape out the vent holes. In a port starboard orientation, like this, the cells are in their "thin" orientation compared to port or starboard tacks. Cells running port to starboard, not bow to stern, is the preferred method for installing flooded batteries on a sailing vessel. Sorry for just telling you this now, if you have them running bow/stern, but I needed the pics before the article could happen.
This is a problem I discovered back in the late 80's. I was on a friends fathers boat that we had put a "state of the art" charging system on due to chronic short battery life. At the time we incorrectly assumed it was "overcharging" or what we through at the time was "overcharging"... Even after the new alternator and regulator he still had short battery life.
He went through the first bank in less than a season. One day while helping him work on it I noticed electrolyte spilled in the cases. The plastic cases for the batteries were black and so were the battery boxes so it was tough to see.
I removed the batteries and discovered quite a bit of electrolyte. I cleaned it out, neutralized the acid, and we went for a sail with no charging. We put her up on ear beating to weather and sure enough electrolyte began to drain out of the batteries fill/vent ports.. The cases were not cracked or leaking but it was spilling out when heeled.
It then dawned on me that the orientation of batteries on sailboats mattered. Doh'......
I looked at the cell layout, and fill/vent ports, and simply rotated these batteries to port/stbd. After that quick fix he never again had a single drop of spilled electrolyte. This may NOT be an easy fix on all boats as many builders really stiff you one this.
If I recall that bank lasted 5 or 6 years, after those first few days of spilling. This after multiple years of replacing batteries almost annually. I can't say all the issues were related to the exposing of his battery plates, and chronically low electrolyte, but it certainly did not help any.
This problem is only compounded with batteries that have "off set" fill caps meaning they are closer to the edge of the battery than the center. Orient offset cap batteries in a bow/stern orientation and you'll have some real leakage issues on one particular tack.
Exposing the plates, especially under charge or discharge, can lead to shorter battery life. Is a failure immediate and catastrophic? No, but if the opportunity exists to orient them more optimally then it should be something you consider. Monohull sailboats should ideally have flooded batteries installed to minimize electrolyte spillage and to minimize or prevent the positive and negative plates from uncovering during normal sailing activities.