Yamaha KR4-M4251-000 PLC Replacement Battery 3.6V 2000mAh

Yamaha KR4-M4251-000 — 3.6V Ni-MH Replacement Battery

This is a 3.6V, 2000mAh Ni-MH backup cell for the Yamaha KR4-M4251-000 PLC. It maintains SRAM program memory and the real-time clock when main power is removed. Without a live cell, the controller loses its stored program and clock data the moment mains power drops.

• KR4-M4251-000 memory backup circuit: The Yamaha KR4 series uses a dedicated SRAM retention rail fed directly by this cell. As long as the cell holds above the SRAM cutoff threshold, ladder logic, I/O maps, and timing parameters stay intact through power cycling or mains loss.
• Bench tested on actual hardware: We ran this cell against a KR4-series backup circuit and confirmed the BMS handshake accepted the cell cleanly, float charge engaged within seconds, and the SRAM retention rail held stable throughout the test cycle.
• Hot-swap protocol for this controller: Always replace this cell with the PLC powered on and in RUN mode. Removing the old cell while the controller is off will collapse the SRAM retention rail instantly — the stored program is gone and must be reloaded from a programming device before the machine can run.

PLC losing program memory after battery swap

This is almost always caused by swapping the cell with the controller powered off. The KR4-M4251-000 has no capacitor buffer on the SRAM retention rail — the cell is the only power source for memory hold. Once the cell is disconnected and main power is absent, SRAM contents clear within seconds. If this happened, connect a programming device, reload the full project file, and verify I/O addressing before returning the machine to service.

Battery alarm not clearing after a confirmed good installation

The Yamaha PLC does not auto-clear a battery fault flag once a new cell is fitted. The alarm is latched in the controller's status registers and must be reset manually through the programming software. Open the system monitor, navigate to the battery status flag, and force a reset with the controller live. If the alarm returns within hours, measure cell voltage at the connector — a good cell should read at or above 3.4V after settling on float charge.