The battery part number for a 7250, 7252, 7050, 7052 or 7750 controller is 2135249 with description "(P28051187), BR2330 3V LI25MAH BATTERY". It is on the CPU card.
If you are losing the system date/time the battery should be replaced. Follow the <Menu>, <Setup>, <System> path to check the date and time.
The generic part number is BR2330 and you can find many sources with a Google search, or sometimes at your local big-box retailer or battery store.
Parts can be ordered, or quotes obtained, by calling +1.877.355.3225 or emailing FCAL Orders