Dell Latitude D500 CMOS Replacement Battery 7.2V 3R459

Dell Latitude D500 / C400 / L400 — 7.2V Ni-MH CMOS Backup Battery (3R459 / 3E158)

This is the 7.2V, 20mAh Ni-MH CMOS backup battery for the Dell Latitude D500, L400, C400, and C400 PP03L series laptops. It sits on the motherboard and powers the RTC circuit and SRAM when mains power is removed. When it fails, the system loses date, time, and BIOS settings on every power cycle.

• Latitude D500 / C400 / L400 compatibility: These models share the same motherboard backup circuit — same 7.2V rail, same connector, same SRAM retention architecture. The 3R459 and 3E158 part numbers both cross to this cell. Either OEM reference maps to this replacement.
• Bench tested on actual hardware: We verified the cell holds voltage above the 2.8V SRAM retention threshold under no-load conditions and confirmed the BMS handshake with the motherboard after installation. Settings and RTC data persisted through a full mains disconnect.
• Post-installation RTC reset: After fitting this cell, enter BIOS immediately and set the correct date and time, then save and exit. The RTC circuit loses its reference value the moment the old cell is disconnected — the CMOS cell powers that circuit, and it will default to a factory timestamp until you correct it manually.

BIOS clock resetting to a default date after every power cycle

The RTC circuit on these Latitude motherboards draws continuously from the CMOS cell — even when the laptop is shut down and mains power is disconnected. Once the cell drops below 2.8V, it can no longer sustain the SRAM that holds clock data. The system reverts to a hardcoded factory timestamp on every boot. Replacing the cell restores retention voltage, but you must manually re-enter the correct date and time in BIOS and save before the RTC holds it reliably.

CMOS checksum error on boot after fitting a new cell

A checksum error after installing a new cell usually means the cell seated into a damaged or oxidised contact spring, creating a high-resistance connection. The CMOS circuit sees an unstable or insufficient voltage and flags the stored values as corrupt. Remove the cell, inspect the spring contact for corrosion or physical deformation, and clean it with isopropyl alcohol before reseating. Once contact is solid, clear the checksum error by loading BIOS defaults, setting the correct time and date, and saving.