When you remove the CMOS battery (or, preferable to removing the battery, reset the NVRAM using the jumpers on the motherboard), it resets all BIOS settings to default.  The default for the RAID controller is OFF.   Go to BIOS Setup (F2), Integrated Devices, and Enable RAID. If your OS still doesn't boot, your RAID memory (which was more likely to have needed replacing) will need to be replaced before the OS will boot successfully.
I have a Dell PE 2850, Wndows server 2003. The onboard battery needed changing as indicated by error message that the memory or battery had a problem. All devices OK but items in cache had been lost. The server would POST and then try to start but auto restart after several minutes of trying to load Windows.  I removed and put it back the CMOS battery.After this While booting up, the server doesn't detect the PERC 4e/Di module at all. Before, it would do this right after listing the CPU stats, and would proceed to initialize and spin-up the drives. However, this is skipped now and it goes directly the DRAC, and so on. It then comes up with the boot error, and to strike F1 to try reboot, or F2 to setup. F1 obviously gets me nowhere.  Help will be much appriciated  Thanks & Regards Aneesh Joseph
Power Edge 2850 PERC 4 controller is no showing after CMOS battery replacement