Battery Fail

[Bduck]

As I was rummaging around in the garage looking to work a project, noticed a very noticeable odor of sulfur. Walking by my boat I usually check the onboard charger for the green lights. Number 1 was solid red & number 2 was flashing yellow. So I unplugged the charger which maintained an AGM on #1 and a wet cell on #2. I would get around to troubleshooting a little later after I get a couple of things done. 2hrs later I noticed the sulfur odor was getting stronger as I walked thru the garage, thankfully a breeze was blowing to air out the garage. My immediate attention now is drawn to the boat batteries, when opening battery compartment panel, I could hear the wet cell boiling. The AGM was supplying power to it continuing the boil. I then shutdown batt switch to off and disconnected the wet cell batt finding this batt was too hot to physically to touch. Once I got it removed, I took an infrared digital temperature reader finding batt temp was 171°F. This had disaster written all over it if it had gone any further sitting in the boat. This was an Interstate Costco battery I bought 6yrs ago and replaced with another AGM. This is my last AGM upgrade I was awaiting to do but not at the expense of a battery meltdown. My diesel truck takes 2 AGM's, Caddy Escalade & Buick took 1ea. I was sold long ago on AGM with final upgrade made, $$$$'s but well worth it.