AGM batteries question

So in my on board charger manual it says that I need proper ventilation for the battery while charging. It doesn’t say why so I thought I would ask here. It is a sealed battery used for starting and electronics etc. My trolling motor batteries sit in a very poorly ventilated compartment under the floor and those are not sealed and I just plug them in. I never paid that much attention to it but maybe I need to? Any thoughts on why it says that in the manual?

All manuals say that, it’s a safety disclaimer. It’s a good idea but not mandatory. AGM batteries are sealed and there are no fumes.

Thanks! I thought the same thing. Kind of like the “hot coffee” warning at McDonald’s

Any non-sealed battery needs to be in a vented area because of the off gases. Bad things could happen. With a buildup of gases and turning on the charger could cause a spark and boom. Side note: never store your batteries over the winter next to your water heater.