The Zoeller 508 Battery Backup Sump Pump System battery box will accept all group size 27, 29 and 31 batteries. This includes wet cell and maintenance free batteries. The larger the battery AH (Amp Hour) or CCA (Cold Cranking Amps), the longer runtime your system will realize. That said, the system comes with a ONE year warranty. IF you purchase a Zoeller Aquanot WET CELL BATTERY or MAINTENANCE FREE BATTERY at time of purchase, that system warranty will be increased to THREE years.
If you have any other questions, please let me know.