Off gassing is a byproduct of charging, it can be reduced by a lower amperage charge (a solar panel is a low amperage charge) and special battery caps to reduce the venting even more. But it can’t be stopped 100% and the last thing you want to do is seal up a deep cycle battery (ok to do with AGM & gel-cells) and let the gas build up.

I’m almost 50 now, but when I was 17-years old (give or take a year) I was working on a ford car in the yard, the hood was up and I was putting a new start solenoid on it. The solenoid was about 6-inches from the battery and I did not disconnect the battery first. It sparked and ignited the hydrogen gas and blew the top off the battery and acid flew everyplace. My face included. I had glasses on so it saved my eyes and I ran to the house and washed it all off very fast. Other then a little stinging and a bad taste in my mouth for a while there was no harm done (thank god I had glasses on) If the gas can travel in an open yard with the hood of the car up, it can do it even more so in a closed up and wind protected area. Be careful.

Since this time I have had a more then health respect for battery off gassing and acid.

It’s not something to do ½ way. Do it right and follow all safety procedures.