House Battery Failure

A day or two ago I noticed fresh liquid marks on the pavement under the battery compartment ...I didn't think much of it at the time.

This morning I noted the marks still there, and a small one also near the front of the battery compartment. I opened the compartment door and saw traces of leakage under the house battery bank tray. I checked the batteries and found the cap off the positive end. None of the batteries felt more than slightly warm, and all appeared to be normal on the monitoring panels. Fluid level was ok everywhere else, but there I could see the plates in those cells. I added a considerable amount of distilled water and then noted water running on the ground. I could see no damage to the batteries, but then I removed the batteries I found it split from top to bottom in three places on that rear corner, and the top was separated from the sides on that side and about half of the back.

There was no other indication that anything was wrong ...all 12v items and power monitors were normal. I have a triple-stage charger that goes to float mode so it will not overcharge the batteries, and appears to be operating properly. There is a heat monitor that will shut it down if the batteries over-heat. The terminals were a little dirty, but not at all bad. These batteries are almost exactly 4 years old. I check the water levels every other month .

I see nothing to suggest there is any problem with the charging system. Was this likely the result of an internal battery short? I've not experienced this kind of battery failure before.

Ron