You should get 5 years out of a battery that is kept charged and not being drawn down constantly. Two times a year is nothing. I'd keep going with what you have and just check the battery (with the charge controller turned off or unplugged) periodically.
I don't know what Zoeller uses for a battery, however the manufacturer is unimportant. What you want to do is match the volts, amp hours and type of battery (the type is probably plain old lead acid, although its possible that it uses a deep cycle battery) so the charge controller won't have any issues charging the new battery. The information should be on the battery label.