Jon is correct -- the proper way to maintain a gel cell is to keep it on a low-current trickle charge continuously. I'd have to look it up to confirm, but what I recall is 1/100 of the battery's amp-hour rating is a good value for the trickle, so if you have a battery rated at (say) 20 amp-hour, trickling it at 0.2 amp would be just right for continuous maintenance. Better still is a charger with a sensor that will start and stop based on the battery's terminal voltage, but this is serious overkill for surplus gel cells -- the charger would cost you about $80, and you can replace the battery every 2-3 years for $20 a pop, most likely (if you're buying surplus, that is). Even a 0.1 amp trickle would probably be adequate in the low draw usage you envision; trickle charging in this range and not running the fans long enough to discharge the battery below, say, 80%, you should get a life of four or five years from a gel cell.