Now that I've had some time to think about it, I want to say that when I encountered the CHECKSUM message it gave you a Y/N option. If this is the case with your controller, you should be able to get your screen back by pressing "Y". The system should thereafter be fully operational, and should remain so, until you decide to shut off the power at the main panel.
For spare parts (like the Varta battery), contact Tom Vassik @ Num Corporation in Naperville, ILL. Telephone: 630-505-7722 (ext 235). He can transfer you to the tech support department if the CHECKSUM issue continues to be problematic.