Its not that they need to be changed that often. But when they do, it ends up taking a couple of days before I get to the hardware store, etc. Very minor issue, but minor issues add up.
I'm redoing my whole RO/DI system and trying to make is as efficient as possible, and eliminating as much maintenance as possible. This was just one more little nuisance to tackle.
Thanks for your suggestion. I'll be rooting around the electrical scrap heap at home looking for a suitable charger. I'm thinking there were two little button cells in there, so maybe a 3v brick? I'll check first.