About this replacement
Symbol MC30 / MC3000R / MC3090G Series — 7.2V Ni-MH CMOS Backup Battery
This is the internal CMOS backup battery for Symbol MC30-series mobile computers. It holds system memory and the real-time clock when the main battery is removed or discharged. Rated at 7.2V and 20mAh, it fits the MC30, MC3000R, MC3090G, MC3000S, and 74 additional variants in the MC30 family.
- MC30-series memory retention circuit: All MC30 variants in this family share the same internal CMOS backup circuit — the same voltage rail, connector footprint, and SRAM retention path. One cell covers the full range.
- Bench tested on actual hardware: We verified the cell holds the RTC circuit and SRAM above the 2.8V retention threshold under no-load conditions. The BMS accepts the cell without error flags on reconnect.
- Post-installation clock correction: After fitting this cell, enter the MC30's system settings and manually set the correct date and time. The CMOS circuit powers the RTC, and any gap in supply — including the swap itself — resets the clock to a factory default. Save the settings before exiting.
CMOS checksum error on MC30 boot after power loss
A checksum error at boot means the CMOS cell dropped below the minimum retention voltage and the stored configuration was lost. The MC30 compares a checksum of its stored settings against what it reads at startup — when the CMOS supply collapses, both the settings and the checksum value corrupt together. Replacing the backup cell alone is not enough; the device will continue throwing the error until settings are re-entered and saved. After installing the new cell, re-enter all system parameters and allow the device to save to SRAM before cycling power.
Transaction data disappearing when the MC30's main battery is swapped
The MC30 uses SRAM for short-term data retention — and SRAM is volatile, meaning it needs continuous power to hold its contents. The CMOS backup cell bridges the gap when the main battery is pulled. If that cell is depleted, even a 30-second main battery swap wipes whatever is in SRAM. The fix is a functioning backup cell sitting above 2.8V before any hot-swap is attempted. Install this cell, confirm voltage is present, then proceed with main battery replacements.