Symbol WT4000 CMOS Backup Battery 2.4V Ni-MH 20mAh

Symbol WT4000 / WT4070 / WT4090 Series — 2.4V Ni-MH CMOS Backup Battery

This is a 2.4V, 20mAh Ni-MH CMOS backup battery for the Symbol WT4000, WT4070, WT4090, and WT4090I mobile computers. It sits on the motherboard and powers the real-time clock and SRAM while the main battery is swapped or depleted. Without it, the terminal loses its date, time, and stored settings on every power interruption.

• WT4000 series compatibility: The WT4000, WT4070, WT4090, and WT4090I share the same motherboard layout and RTC circuit, which is why they all draw from this single CMOS cell. The voltage rail, connector footprint, and physical dimensions — 19.17 × 11.97 × 7.70 mm — are identical across the series.
• Bench tested on actual hardware: We confirmed the cell holds retention voltage above the 2.8V minimum threshold under continuous low-drain load on the RTC circuit. The SRAM remained stable and no checksum errors were triggered during power cycling.
• Post-install clock correction: After fitting this cell, navigate to the WT4000's system settings and manually enter the correct date and time. The CMOS circuit resets the RTC to a factory default on any power interruption, so the clock will be wrong until you correct it regardless of how long the new cell has been seated.

Why the WT4000 clock resets to 2000 after every main battery swap

The RTC on the WT4000 motherboard is powered exclusively by the CMOS backup cell when the main battery is removed. Once that cell drops below its retention voltage — 2.8V minimum — the RTC circuit loses power and reverts to its hard-coded default date. A depleted cell can measure above 2V on a static meter yet still fail to sustain the RTC under load. Replacing the CMOS cell stops the reset loop; the clock then needs a single manual correction after installation.

CMOS checksum error appearing on WT4000 boot screen

A checksum error on boot means the device detected that CMOS contents changed since the last shutdown — almost always because the backup cell went fully flat and SRAM lost power. The terminal recalculates the checksum at startup, finds it no longer matches, and flags the error. Fitting a new cell clears the root cause, but the error will persist until settings are re-entered and saved. Power the unit off cleanly after saving so the new checksum writes correctly to SRAM.