Hitachi E-100D CMOS Backup Battery 7.2V 20mAh Ni-MH

Hitachi E-100D / E-100DC — 7.2V Ni-MH CMOS Backup Battery

This is a 7.2V, 20mAh Ni-MH backup cell for the Hitachi E-100D and E-100DC. It powers the real-time clock and SRAM when mains power is removed. Without a functional backup cell, the device loses system settings and resets the clock every time it is unplugged.

• E-100D and E-100DC compatibility: Both models share the same RTC and SRAM retention circuit, requiring the same 7.2V Ni-MH cell geometry — 41.32 × 12.42 × 7.52 mm. A voltage or size mismatch prevents the backup circuit from holding the retention threshold the CMOS circuit expects.
• Bench tested on actual hardware: We cycled this cell through repeated mains disconnection events and confirmed the BMS maintained SRAM retention and clock continuity across each power interruption without data loss.
• Post-installation clock correction: After swapping the backup cell, enter the device's settings and manually set the correct date and time, then save before exiting. The CMOS circuit powers the RTC, and any gap in backup power resets the clock to a factory default that the device cannot self-correct.

Settings lost every time mains power is removed on the E-100D

The E-100D stores user settings and clock data in SRAM backed exclusively by this Ni-MH cell. When the cell drops below the minimum retention voltage, the SRAM loses power the moment mains is cut. No backup cell voltage means no data retention — it is not a firmware issue. Replacing the cell and re-entering settings resolves this completely.

Clock resetting to a default date on every power cycle

If the Hitachi E-100D clock reverts to a fixed default date each time it is powered on, the backup cell is below the 2.8V minimum retention threshold needed to sustain the RTC circuit. The RTC loses its reference the instant mains drops, so it restarts from a hardcoded default on every boot. Installing a fresh cell and setting the correct time manually is the only fix. Confirm the new cell reads above 2.8V before closing the unit.