About this replacement
Yokogawa S9129FA / CS1000 / CS3000 — 2.4V Ni-MH Replacement Battery (HHR-11F2A1)
This is a 2.4V, 1500mAh Ni-MH cell that replaces the OEM HHR-11F2A1 and HHR-11F2G2 batteries in Yokogawa S9129FA, CS1000, and CS3000 controllers. It maintains SRAM program memory and real-time clock functions during mains power interruptions. Replace it before the controller logs a low-battery alarm — waiting until failure risks total memory loss.
- S9129FA, CS1000, and CS3000 compatibility: All three platforms use the same 2.4V backup rail and identical battery connector footprint. The BMS handshake on these Yokogawa controllers expects a cell within a specific internal resistance window — this cell meets that threshold and clears the battery alarm on installation without requiring a controller restart.
- Bench tested on actual hardware: We ran this cell against the OEM float charge circuit and confirmed stable voltage acceptance at the 2.4V rail. The BMS registered a valid cell immediately and did not flag a fault condition during the charge acceptance phase.
- Hot-swap procedure for this controller: Always replace this battery with the PLC powered on and in RUN mode. Removing the cell while the controller is powered off will erase SRAM contents — the program and all retained data will be lost. If a cold swap has already occurred, reload the program from your programming device before restarting the process.
Why the CS1000 and CS3000 lose program memory at the moment of battery swap
SRAM in these Yokogawa controllers is not backed by any capacitor buffer — the battery is the sole retention source. The moment battery voltage drops below approximately 2.0V during removal, SRAM contents are gone. A new cell that arrives at storage voltage (typically 2.2–2.3V) still provides enough voltage to hold SRAM during a fast hot-swap. The critical step is never breaking the circuit while the PLC is de-energised.
Battery alarm still showing after a confirmed good cell installation
The Yokogawa CS-series does not auto-clear a battery alarm once a new cell is detected — the alarm flag is latched in the controller's diagnostic register. You must clear it manually via the programming software or the system's engineering console. Navigate to the battery status register and issue an alarm reset command with the PLC in RUN mode. The alarm clears immediately once the reset is executed and the cell voltage reads above 2.2V.