http://www.amazon.com/Battery-Tender.../dp/B000NCOKVU
We got this one locally for a little less than amazon had it. Price worked out to be a little more than two of the ones linked above at Andymark, but it charges and maintains 10 batteries. We've had it for a few years and have noticed very little change in battery health (max voltage, sustainable amps delivered under load, etc) since. We are still able to use batteries several years old for heavy practice bouts or for a competition if necessary. At competition, by the time you use battery five or six, battery one is charged and maintaining.