Hi [NAME],   Yes, it seems to be the mainboard issue. I assumed, the battery is a known good working battery, have you left the server connected to the power for a few hours to check on the issue? Also, could you confirm if the battery socket has no residue and it's seated properly into it.
Hi, I have my dell r620 server error BAT0002, after clear system event log (SEL) and restart server, it is okay, but if you unplug the psu, then plug in back and turn on, the error will appear again, already replaced CMOS battery, update firmware, change psu still same if unplug and plug it back, clear sel and restart okay if the plug is still plugged in. Can anyone help me? Already doing this for about 1 week. Thanks in advance 
Error BAT0002 System board battery has failed