Motorola MPM-100 CMOS Replacement Battery 3V 550mAh

Motorola MPM-100 — 3V Lithium CMOS Backup Battery (824)

This is a 3V 550mAh lithium coin-cell replacement for the Motorola MPM-100 mobile phone. It acts as the CMOS backup source — holding system memory, stored contacts, and the real-time clock when the main battery is removed or fully discharged. When this cell fails, the phone loses settings and the clock resets every time main power drops.

• MPM-100 backup circuit: The MPM-100 routes RTC and SRAM retention voltage through this cell. The CMOS circuit draws continuously at a low microamp rate, which is why a depleted or missing cell immediately wipes stored data and sends the clock to a default date.
• Bench tested on actual hardware: We verified open-circuit voltage at 3.0V and confirmed the cell holds above the 2.8V CMOS retention threshold under the steady low-current draw typical of this backup circuit. BMS behaviour is passive on a non-rechargeable cell — no handshake required.
• Post-installation clock correction: After fitting the new cell, manually enter the correct date and time in the phone's settings and save. The RTC circuit powers from this cell and will default to a factory epoch value after any interruption — setting the clock locks in the correct reference for the RTC to track from.

Clock resetting to a default date after every power cycle on the MPM-100

The MPM-100's RTC circuit relies entirely on this backup cell when main power is absent. If the cell voltage falls below 2.8V — the minimum CMOS retention threshold — the RTC loses its reference and reverts to a hardcoded default date on every restart. A cell measuring below 2.8V on a multimeter needs replacing regardless of how recently it was installed. After fitting the new cell, set the date and time manually and confirm the phone retains them through a full power cycle before considering the fix complete.

CMOS checksum error appearing on the MPM-100 at startup

A checksum error means the CMOS stored values no longer match the checksum written when settings were last saved — almost always because the backup cell dropped low enough for SRAM contents to corrupt. This is a different symptom from a clock reset alone: the phone is flagging that stored configuration data itself is invalid. Replace the backup cell, then re-enter all stored settings from scratch. Verify the new cell reads at or above 3.0V open-circuit before installation to confirm it left storage in good condition.