Best way is to mount a solar pannel on the feeder so you never have to touch the battery. Spend the money for a good metal housing and all wires need to be in metal.... nothing exposed.

 

Buy the little chargers that are set up for both 6 and 12 volts to charge them originally then always have a good solar panel to keep them charged while in use.
When testing batteries with a voltmeter be sure they are always under a load. A bad battery with no load most of the time will measure full voltage either 6 or 12.