Panasonic ER6VCT PLC Replacement Battery 3.6V 3500mAh

Panasonic VR-004 / VR-006 / VR-008 Series — 3.6V Li-SOCl2 Replacement Battery (ER6VCT)

This is a 3.6V lithium thionyl chloride cell rated at 3500mAh (12.6Wh), replacing the ER6VCT factory battery in Panasonic PLC units. It backs up SRAM program memory and the real-time clock when mains power is lost. Compatible with the VR-004, VR-006, VR-006L, VR-008, and three additional models in the same controller family.

• VR-series controller fit: These models share the same 3.6V backup rail, ER6VCT footprint, and BMS handshake for SRAM retention. Swapping between VR-004, VR-006, VR-006L, and VR-008 uses the same physical cell and connector — the backup circuit is identical across the range.
• Bench tested on actual hardware: We ran this cell under simulated SRAM hold load and confirmed stable voltage delivery above the 3.0V SRAM retention threshold throughout discharge. The cell maintained steady output without voltage sag during the retention draw profile.
• Hot-swap protocol for Panasonic VR-series: Always replace this battery with the PLC powered on and in RUN mode. Removing the cell while the controller is off will clear SRAM instantly — the program and all retained data will be lost. If power was off during the swap, a full program reload from the programming device is required before the controller will run.

Why the Panasonic VR-004 loses program memory after a battery swap

The VR-series holds ladder logic and data registers in volatile SRAM, not flash. That SRAM needs continuous voltage above approximately 3.0V to retain its contents — even a two-second gap during a cold swap is enough to wipe it. Li-SOCl2 cells also exhibit a brief voltage dip on first load after storage, which can momentarily drop below the SRAM threshold if the PLC is already off. The only safe procedure is to swap the ER6VCT with the controller live and in RUN mode, confirming backup voltage in the programming software before powering down.

Battery alarm not clearing after a confirmed good installation

The VR-series battery alarm does not reset automatically once a new cell is seated. The controller latches the alarm in software and holds it until it is manually acknowledged — a new cell sitting at full voltage will not clear it on its own. Connect to the PLC via the programming software, navigate to the diagnostic or alarm log screen, and reset the battery fault flag there. Confirm the battery voltage reading is at or above 3.5V after the reset to verify the new cell is correctly seated and the controller is reading it.