Hi,"Should I just replace the controller?" -> yes please, it's got nothing to do with the battery.Hope this helps and have a great one!
I have an R420 that is displaying an error message, "Single-bit overflow ECC errors were detected on the RAID controller."  The controller is the PERC H710 Integrated RAID Controller, 512MB NV Cache (5CT6D).  The message appeared after rebooting last night and it is stuck on this message.  I'm not able to get the OS to load.  Based on this message I presume the controller has failed.  By any chance could the battery cause this issue?  I don't have any spare batteries for testing.  Should I just replace the controller?  Are there any other troubleshooting steps that I should try?
PERC H710 Integrated RAID Controller Error