a couple of notes about buying surplus gel cell batteries. Take a volt meter along and measure any cells before buying. A 12 volt battery should measure no less than 10 volts and preferably be running around eleven volt. If the battery reads less than 10 volts then one or more cells may be damaged internally.
Charging gel cell batteries is a controversial subject but I've had good luck charging with a current limited supply at about 13 or 14 volts. I limit the charge rate to 1/20 of the rated capacity. For example a two amp-hour battery is charged at no more than 0.1 amps and when the voltage rises to about 13 volts, it's done. :-)
--- eric
and here are some other hints from my gel-cell archive. probably more than you want to know but it's real good information if you want to run astronomy appliances off batteries. Feel free to archive for your own use.