About this replacement
Fuji Electric H12HS / H1021H — 3.6V Li-MnO2 Replacement Battery (NXT2)
This is a 3.6V, 2700mAh lithium manganese dioxide cell that replaces the NXT2 backup battery in Fuji Electric H12HS and H1021H programmable logic controllers. It powers the real-time clock and SRAM memory retention circuit inside the control module. Without a functioning cell, these controllers lose their program data and clock sync when main power is removed.
- H12HS and H1021H compatibility: Both models share the NXT2 cell format, the same 3.6V SRAM retention rail, and the same battery connector pinout inside the control module housing. One cell fits both.
- Bench tested on actual hardware: We confirmed cell voltage, polarity, and BMS handshake against the NXT2 specification. The cell held stable open-circuit voltage and passed retention threshold checks before shipping.
- Hot-swap procedure — critical for this controller: Always replace this battery with the H12HS or H1021H powered on and in RUN mode. Removing the cell while the controller is powered off drains SRAM instantly. If that happens, a full program reload from the programming device is required before the controller will run again.
Why the H12HS loses program memory after a battery swap
The H12HS holds its ladder logic and configuration in volatile SRAM. That SRAM needs continuous voltage — supplied by the NXT2 cell — to retain data when main power is off. The moment the cell is disconnected and the controller is not powered, SRAM collapses to zero and all stored data is gone. This is not a fault with the replacement cell. It is how the memory architecture works. Reload the program from your programming device and the controller will resume normal operation.
Battery alarm not clearing after the new cell is installed
The Fuji Electric H12HS does not auto-clear a low-battery alarm once a new cell is fitted. The alarm flag is written to a status register and stays set until cleared manually in the programming software. Connect to the controller via your programming terminal, navigate to the system status or special relay section, and reset the battery alarm flag directly. After clearing, confirm the cell reads at or above 3.5V on the controller's diagnostic screen before closing out the maintenance record.