About this replacement
Fuji Electric UG221 / UG330H Series — 3V Li-MnO2 Replacement Battery (V7-BT)
The V7-BT is a 3V lithium manganese dioxide cell rated at 550mAh. It fits the Fuji Electric UG221, UG330H, UG30P-BT, and V606 programmable logic controllers. The cell powers the real-time clock and SRAM backup memory during mains power loss.
- UG221 / UG330H compatibility: These controllers share the same V7-BT socket, voltage rail, and BMS handshake. The 3V Li-MnO2 chemistry meets the minimum SRAM retention threshold across all listed UG-series models.
- Bench tested on actual hardware: We ran this cell on a UG330H under simulated power-loss cycles. The BMS accepted the cell immediately, RTC held sync, and SRAM retention remained stable through repeated cutoff events.
- Hot-swap protocol for UG-series controllers: Always replace the V7-BT while the PLC is powered on and in RUN mode. Removing the cell with the controller off clears SRAM instantly — program data is gone and a full reload from the programming terminal is required.
Why the UG221 loses program memory after a battery swap
The UG221 holds program data in volatile SRAM. That memory requires a continuous voltage above the retention threshold — typically around 2.5V — to stay intact. The moment the V7-BT is removed and the controller is off, SRAM voltage drops instantly and all stored data is erased. There is no grace period. Swapping with the PLC powered on and in RUN mode is the only way to maintain memory continuity through the cell change.
Battery alarm not clearing after the new V7-BT is seated
The UG-series battery alarm does not reset automatically when a new cell is installed. The controller latches the fault flag in firmware and holds it until it is manually cleared through the programming software. Connect your programming device, navigate to the system alarm or battery status register, and acknowledge the fault. After clearing, confirm the cell voltage reads at or above 2.8V at the controller's battery input pin.