RTC_BAT (sensor) Severity : critical

This means the CMOS battery on the motherboard needs to be replaced.

RTC_BAT should be >2.90V, I think low-voltage is <2.75V, you can check IPMI sensors to confirm.

Newer systems CMOS Battery RTC_BAT sensor should be ~3.00V.

If CMOS batter is replaced quick enough, AFTER system is powered off and power drained (WHILE OLD CMOS IS STILL INSTALLED), then it should retain BMC settings. Otherwise, while the system is still using old CMOS battery generating the errors, note/screenshot BIOS setting pages so you make make the necessary changes if CMOS battery replacement defaults BMC settings.