If you have an old ferro resonance battery charger, common brand seems to be Pro Mariner, DO NOT LEAVE IT ON 24/7 when you are not using the boat. If you use it alot make sure you regularly check the water in the batteries.
Larry Deja had two batteries cook from his promariner charger. These are old, in a metal box housing generally with an analog amp meter. When batteries cook and boil out all their water they create Hydrodgen gas and Sulphur Dioxide. During normal safe charging batteries should have no odor. If you smell them, there is a problem. Hydrogen is explosive. Larry went to check is battery water levels and the poofed pretty good. Fortunately he didn't get covered in acid.
You may have noticed I have been on a little that became large mission rewiring my upper station and now lower station on my boat. I discovered a company called Pacific Yacht Systems out of Vancouver BC. He has about 700 videos about all things boat electrical https://www.pysystems.ca/resources/ This is where I discovered by old Pro Mariner charger I have used for 25 years probably killed all of my batteries prematurely. I generally only turn it on when I have interior lights on when I am around. His page also shows the results of a battery explosion. No fire but endless corrosion from acid. If you were in there when it happened you would be seriously injured.
Larry has a Sea Ray, while helping Larry another member came by who recently replaced his said he had the old style, also a Sea Ray. Was helping another new member on an 86 Sea Ray that has been in the club for years and has the old metal box battery cooker.
Obviously these are on all brands of boats. "Smart Chargers" were not a thing back then but are now. Here is a video about the 3 stages of charging with a smart battery.
Most boat fires are caused by 12 volt wiring issues! Please check your charger and make sure your batteries have water.
