Lead/acid batteries use sulfuric acid which has a very low volitility. The 'gases' given off during charging are hydrogen and oxygen, so the oxygen could contribute to accelerated rusting/aging/corrosion but I don't think it would be much unless done in an enclosed area. The hydrogen that is released is the cause behind all of the fire/explosion warnings around batteries and jump-starting cars, etc. Under the hood, and inside the batteries, between the liquid and top of the cap, you can get explosive mixtures of the hydrogen and oxygen.
My method, which has portability consequences (its heavy) is a deep-cycle marine battery (Sears). I haven't weighed it, and don't remember, but it feels like 25-35 pounds in its 'Recreational Vehicle' carrier. On the other hand, when I tried to discharge it a couple of weeks ago, for the 'deep' part of its cycle, it took 72 hours through a 60 watt light bulb plus an Orion dew heater. I think I have enough juice to observe on a weekend campout <g>. (I did get one of the larger Sears batteries; I may mount it into my pop-up camper for lights in there when we're not observing.)
Rich klappal@xnet.com