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IT Support & Hardware · Foundational

CMOS Battery

A coin-cell battery (usually CR2032) on the motherboard that powers the real-time clock and firmware settings.

Definition

When the CMOS battery weakens, the system loses its date, time, and BIOS settings each time it loses power. Symptoms include incorrect time, 'CMOS checksum error' messages, and certificate errors in browsers due to the wrong date. Replacement is inexpensive and simple.

Example

A PC that boots with a January 1 date every morning needs a new CR2032.

Exam tip

Wrong system time after power loss is the classic CMOS battery symptom.

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