Dell Latitude D505 CMOS Battery G4221 3V 200mAh Lithium

Dell Latitude D505 / D600 Series — 3V Lithium CMOS Backup Battery (G4221)

This is a 3V, 200mAh lithium coin cell that replaces the CMOS/RTC battery on Dell Latitude D505, D510, D520, D600, and compatible models. It maintains BIOS settings, real-time clock data, and SRAM-backed system parameters when the laptop is unplugged or the main battery is removed. OEM part numbers covered include G4221, 3R459, 3E158, and 0G4221.

• Latitude D-series CMOS circuit: These models share the same RTC circuit voltage rail and coin cell socket footprint across the D505 through D600 range. The cell slots into a spring-contact holder on the motherboard — no soldering required on these units.
• Bench tested on actual hardware: We measured open-circuit voltage on each cell before dispatch. Cells ship at storage voltage, which reads slightly below 3.0V at rest. Voltage rises to the rated 3.0V once the retention circuit draws a load.
• Post-installation BIOS step: After swapping the cell, enter BIOS immediately and set the correct date and time, then save and exit. The RTC circuit resets to a default value after any power interruption — the cell powers that circuit, so the clock must be corrected manually after every replacement.

BIOS clock resetting to 2000 after every power cycle

The Latitude D-series RTC circuit requires the CMOS cell to hold at least 2.8V to retain clock data between power cycles. Once the cell drops below that retention threshold, the RTC register loses its value and the BIOS defaults to a fixed date — typically 1 January 2000. The main battery and AC adapter do not back up this circuit. Replacing the coin cell and setting the correct time in BIOS resolves it; if the clock resets again within days, check that the spring contact on the holder is making firm contact with the cell.

CMOS checksum error on boot after fitting a new coin cell

A CMOS checksum error after a fresh cell install means the BIOS detected that stored settings no longer match the checksum it calculated at last save — this happens because the depleted cell allowed SRAM contents to corrupt before replacement. The new cell does not restore those settings automatically; it only prevents further loss. Enter BIOS setup, review all settings, and use "Load Defaults" if values look incorrect, then save and exit. If the error persists after saving, the cell contact spring may be oxidised — clean it with a dry cotton swab and re-seat the cell.