My bow thruster battery gave me quite a scare today. I'm lucky it didn't catch fire.
The thruster and battery live inside a plastic cover under the v-berth. It's a very "out of sight, out of mind" location, and I've never given it much thought -- until today.
First symptom: I smelled a very faint odor, like maybe a problem with the toilet, but couldn't find the source.
Second symptom: While grabbing some tools from under the v-berth, I happened to glance at the charger, and noticed it was pushing 20 amps. I thought it was odd because the boat had been on shore power for 24 hours, so the battery should have been fully charged.
It was still at 20 amps an hour later, and it finally dawned on me -- oh no, could that odor be from my battery?!?
So I stuck my head down into the compartment -- sure enough, I could hear the hissing sound of water boiling off. Then I noticed that the entire plastic cover was hot, and I got scared fast! I shut down power and removed the cover. The battery was so hot that I couldn't keep my hand it for more than a second.
I pulled the battery a couple hours later after it cooled down to where I could touch it. Luckily it didn't crack.
What if I hadn't noticed it? I'm pretty sure the charger would have continued happily dumping 20 amps of current into the battery. Eventually, I guess it would have melted and/or caught fire.
Lesson learned. And now I'm shopping for a new charger with a remote display.