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RTC battery/CMOS battery - this battery pack provides power for the internal real time clock, maintains the system configuration and the SETUP program values. It will maintain this data for one month when fully charged. "Error in CMOS" may occur at startup if the RTC battery has discharged. To resolve this, plug the notebook in to an external power source, turn the notebook on, and let the RTC battery charge for 24-48 hours.