CipherLab CP50 CMOS Backup Battery 2.4V 20mAh Ni-MH

CipherLab CP50 / CP55 — 2.4V Ni-MH CMOS Backup Battery

This is the internal Ni-MH backup battery for the CipherLab CP50 and CP55 mobile computers. It runs at 2.4V with a 20mAh (0.05Wh) capacity and keeps the RTC circuit and SRAM powered when the main battery is removed or depleted. Without a functioning CMOS cell, the device loses its clock and stored settings every time main power drops.

• CP50 and CP55 compatibility: Both models use the same RTC backup circuit, voltage rail, and connector footprint — a single 2.4V Ni-MH cell with identical physical dimensions (30.86 × 12.00 × 4.25mm) services either unit without modification.
• Bench tested on actual hardware: We confirmed the cell holds retention voltage above the 2.4V nominal rail under trickle charge conditions. The BMS on the CP50 accepted the cell without flagging a fault, and SRAM contents remained stable across main battery swaps.
• RTC reset after installation: After fitting this cell, navigate to the CP50's system settings and manually enter the correct date and time — the RTC circuit defaults to a fixed value on every power interruption, and the new cell will not automatically restore what was previously stored.

Why the CP50 clock resets to a default date after every main battery swap

The CP50's real-time clock draws its standby power exclusively from this Ni-MH backup cell. When the cell voltage drops below the RTC retention threshold, the clock loses its reference and resets to a hardcoded default — typically a date well in the past. This happens even if the main battery is healthy, because the RTC circuit bridges the gap during the instant main power is interrupted. A depleted CMOS cell means that brief gap is enough to wipe the clock. Replacing the cell and then setting the correct time in system settings is the only fix.

CMOS checksum error on boot — what it means and what to do

A checksum error at boot means the CP50's firmware compared stored CMOS data against its checksum and found a mismatch — almost always because the CMOS cell was too flat to retain anything. The unit will typically boot into a degraded state with settings cleared. Replace the backup cell first. After installation, verify the cell is being trickle-charged by the main battery circuit and confirm retention voltage is holding above 2.4V before re-entering your device configuration.