Mitsubishi Q6-BAT PLC Replacement Battery 3V 1700mAh

Mitsubishi Q02CPU Series — 3V Li-MnO2 Replacement Battery (Q6-BAT)

This is a 3V lithium manganese dioxide cell rated at 1700mAh (5.1Wh), direct replacement for the Q6-BAT used in Mitsubishi MELSEC Q-series PLCs. It maintains SRAM program memory and the real-time clock during any power interruption. Compatible models include the Q02CPU, Q02HCPU, Q06HCPU, Q12HCPU, and nine additional Q-series CPU modules.

• Q-series CPU battery socket: All listed Q-series CPU modules share the same battery compartment format, connector polarity, and 3V SRAM retention rail. The BMS on each unit expects a Li-MnO2 cell — not lithium-ion — because the flat discharge curve of MnO2 holds voltage steady well into depletion, which the Q-series memory retention circuit relies on.
• Bench tested on actual hardware: We ran this cell against the Q6-BAT specification on the bench. The BMS accepted the cell immediately, the battery low alarm cleared, and SRAM retention voltage held within the required window across the full test period.
• Hot-swap procedure on Q-series CPUs: Always replace this battery with the PLC powered on and in RUN mode. The Q-series CPU has no supercapacitor backup — the moment the old cell is removed with the controller powered off, SRAM loses its voltage rail and the program is gone. Keep power on throughout the swap.

Why the Q02CPU loses program memory during a battery swap

The Q-series SRAM holds the ladder program, device comments, and file register data entirely on battery-backed volatile memory. There is no internal capacitor to bridge a gap between cell removal and cell insertion. If the PLC is powered off during the swap — even for seconds — the SRAM supply drops below the retention threshold and the entire memory contents are lost. With the controller powered on in RUN mode, the main power supply holds the SRAM rail, and the battery swap becomes a live replacement with no data risk.

Battery alarm not clearing after new Q6-BAT installation

The Q-series CPU latches the battery alarm flag in a special relay (SM51) once the low-battery condition is detected — installing a fresh cell does not automatically reset it. The alarm must be cleared manually using GX Works2 or GX Developer by resetting SM51 through the diagnostic or device monitor screen. If the flag is left latched, the CPU continues to report a battery fault even with a fully charged cell installed. Connect via USB or Ethernet, open the device monitor, confirm SM51 is ON, then force it OFF to clear the alarm.