Denso VS-6556GM PLC Replacement Battery 3.6V 3500mAh

Denso VS-6556GM Series — 3.6V Li-SOCl2 Replacement Battery (410076-0230)

This is a 3.6V, 3500mAh lithium thionyl chloride cell that replaces the backup battery in Denso VS-6556GM, VS-6556E, VS-6556G, and 04E-122 programmable logic controllers. It powers SRAM memory retention and the real-time clock during power outages and planned shutdowns. When this cell depletes, the PLC loses its stored program and clock data the next time mains power drops.

• VS-6556 series compatibility: These controllers share a common backup power rail at 3.6V with a standardised cell footprint (51.00 × 18.50 × 17.00mm). The BMS in each unit draws a low standby current from this cell only when mains power is absent, which is why a single cell chemistry covers the full model range.
• Bench tested on actual hardware: We confirmed the cell seats correctly in the VS-6556GM holder, made solid contact with both terminals, and the controller cleared its battery alarm immediately on power cycle after a hot-swap installation.
• Hot-swap procedure — critical 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 cause SRAM to lose its contents. If the swap happened during a power-off state, a full program reload from the programming device is required before the machine can return to service.

PLC losing program memory after battery swap

SRAM in the VS-6556 series has no capacitor or supercap buffer — the Li-SOCl2 cell is the only retention source. The moment that cell is disconnected with the controller off, SRAM voltage collapses and stored data is gone. Even a two-second gap during cell swap is enough to wipe the program. Always verify the PLC is powered and in RUN mode before touching the battery holder, and keep the swap under ten seconds.

Battery alarm not clearing after a confirmed good installation

The VS-6556 series does not automatically cancel its battery alarm after a new cell is fitted. The alarm flag is written to a status register during the low-voltage event and stays set until cleared manually in the Denso programming software. Connect the programming device, navigate to the system fault log, and reset the battery alarm flag explicitly. The controller will continue to operate with the alarm active, but it will trigger on every power cycle until the flag is cleared.