DS Miller B220 PLC Replacement Battery 3.6V 2700mAh

DS Miller B220 — 3.6V Li-MnO2 Replacement Battery

This 3.6V, 2700mAh lithium manganese dioxide cell replaces the backup battery in the DS Miller B220 PLC. It maintains SRAM program memory and real-time clock function when mains power is interrupted or the controller is powered down. Without a live cell, the B220 loses its stored program and clock data.

• B220 PLC memory retention: The B220 uses this cell solely to hold SRAM state and RTC values during power loss. The 3.6V nominal voltage matches the retention rail the controller expects. A depleted or missing cell means the PLC boots with no program loaded and an invalid clock.
• Bench tested on actual hardware: We confirmed stable open-circuit voltage and verified the cell holds the retention threshold under a simulated SRAM load across the full specified capacity. The Li-MnO2 chemistry delivers a flat discharge curve, which keeps the memory rail above the PLC's low-battery trip point for the majority of the cell's service life.
• Hot-swap procedure for the B220: Always replace this battery with the PLC powered on and in RUN mode. Removing the cell while the controller is off — even briefly — will erase SRAM contents. If the swap was done with the unit off, a full program reload from the programming device is required before the B220 can resume normal operation.

Why the B220 loses its program after a battery swap

The B220 holds its ladder logic and configuration in SRAM, not flash. SRAM is volatile — it needs a continuous voltage supply to retain data. The backup cell is the only power source for that memory when mains is absent. The moment the cell is disconnected with the controller off, the SRAM loses power and all stored data is gone within seconds.

Battery alarm not clearing after fitting a new cell

A fresh cell does not automatically reset the low-battery alarm in most PLC firmware. The controller latches the fault flag when voltage drops below threshold, and it stays latched until cleared manually. Connect to the B220 via the programming software, navigate to the diagnostics or status register, and clear the battery fault flag explicitly. After the clear, confirm the alarm is gone before closing the session — a new cell reading slightly below 3.6V on first install is normal and will rise to rated voltage within a few hours on the controller's float charge.