Originally Posted by 5picker
A FLA battery off-gases highly flammable/explosive gases while being charged. With your caps off and the battery not enclosed in a vented container/area, should those gases happen to concentrate near a spark... KA-BOOM. (think brushes in a motor for instance) Also, the tools you mentioned produce a lot of dust/dirt. With your caps off... just sayin'. While all those chances are slim... it is a possibility nonetheless.
I agree it might be a concern if the spark was real close but the gas given off from a charging battery is Hydrogen. It dissipates quickly as it's lighter than air. Also, a trickle charger will not cause the battery to boil and release large quantities of Hydrogen rapidly so the risk is far less.
I would use common sense and keep my "sparking" devices as far away as possible but other than that not be overly concerned.
As for "caps off", I haven't seen people do that with trickle chargers. Especially when the batteries are continuously connected. Too much chance of contamination from airborne dust/dirt and even bugs.
Also, the cap acts somewhat as a Flame Arrestor. Should there be a spark that ignites any Hydrogen an open cell will allow the flame to enter the battery and cause a bigger issue. With the cap(s) in place the flame should stop at the orifice or baffle.
Last thought, under the hood of your motor vehicles there are numerous sources if ignition near your charging battery. Not considered a hazard there as long as any Hydrogen has a means of escaping.
Bigger hazard in your shop are the power tools. I just got in a fight with a power tool and it resulted in three badly bruised fingers (maybe even one broken one). My "Driving Finger" is now twice the normal size from second knuckle to tip but is getting better.
Batteries on a charger? Not even on my radar.