even without venting, im sure the heating that can happen during charging could be enough to cause concern in a sealed environment.
I hope to be doing some empirical tests on this in the next couple of weeks so I'll keep you posted. I got a +/- pressure gage that reads to 1/4 psi so I'll build an enclosure (with a relief valve) and see what kind of pressures develop. I'm just using a standard charger so I don't think the charge current is very high...I think 1.5 amp max for a single cell or something like that.
Does the piezo switch require a source current or voltage? I've been thinking about using one of these. So far I've been using a MOSFET/reed setup what works nicely.