air, from a chemical sense, is impure nitrogen; as a gas it has an average moulecular weight of about 29AMU.
Nitrogen (N2) has a molecular weight of 28AMU
Oxygen (O2) has molecular weight of about 32AMU.
Hydrogen (H2)has a molecular wieght of about 2AMU
Water vapor (H2O) has a molecular wieght of about 18AMU
in light of the above, and in general terms, and assuming absolutely still, isothermal air, two of the gasses out of a battery (H2, H2O) will tend to rise. the O2 will have a slight tendancy to sink; but the tendancy is so slight that any reasonable thermal or convective current will mix the air and draw the oxygen out too. As long as you can generate a reasonable temperature differential in the 'chimney', there should be no problem with convective venting. . . . though personally, I'd use a brushless fan in the chimney, maybe with a delay circuit to run it for a little bit after charging stops, just to be sure.
(I've seen one small fuel-air explosion [propane-air] up close, and i've no interest in a repeat. . . . 'nuf said.)