I use the 6V 7 amp hour battery on right in place of the 4.5 amp on left.
By themselves, they last almost twice as long as the smaller one and about 3 times as long as standard 6V lantern battery.
With a 6V solar cell attached, they have enough storage to last thru the long periods of dark cloudy winter weather and keep on feeding when you need it during deer season.
They do fit in every feeder box I have tried them in and are 5-7/8" long.
You can pick them up off of ebay for about 13 bux delivered.