This comment is for the benefit of all, especially our over-the-top nervous Nellies.
All lead-acid storage batteries release hydrogen when charging, at very low levels when trickle charging and at much higher levels when fast charging.
The lower level of hydrogen that can result in an explosive concentration is 4% by volume.
In a well ventilated room, with only one battery, the risk level approaches zero.
It is reckless to tell people that there is no danger using flooded Pb batteries indoors without ventilation and isolation from possible ignition sources.
It's not filling up the room with hydrogen folks should be concerned about. If you're engaged in a heavy charge cycle, flooded Pb batteries can heat up and generate just enough hydrogen to fill the headspace inside the battery with enough coming the vents to hit the ignition sweet spot.
One spark ignites the outside gas and fuses back to the hydrogen built up inside the battery and BAM! You just got a complimentary sulfuric acid and plastic shard shower, quite possibly in your face. As a chemist for over thirty years who's worked with more hot mineral acids than most folks (outside of plating facilities), I have a healthy respect for acid. I've seen what sulfuric acid does to skin and to your eyes. I guarantee that most people in that situation would not have a happy outcome if that happened to them.
Choosing an AGM Pb battery is the way to avoid this. The electrolyte is absorbed into the porous plates and the design resists H2
generation. It's not that much more expensive than flooded batteries. They're better-designed for the job.
I am not saying that such batteries cannot be used safely; rather I am trying to make it clear that it is quite possible to use them unsafely. Knowing and understanding the potential dangers is a wise course of action before committing to bringing them into the house and making good use of them.
What he said.