Honeywell DCP551 PLC Backup Compatible Battery 3V 1350mAh

Honeywell DCP551 — 3V Li-MnO2 Replacement Battery (81446140-001)

This 3V lithium manganese dioxide cell delivers 1350mAh of backup capacity for the Honeywell DCP551 programmable logic controller. It maintains SRAM program memory and the real-time clock when main power is lost. If the existing cell has depleted, the DCP551 will lose time-of-day accuracy and may drop stored process data on the next power cycle.

• DCP551 backup circuit compatibility: The DCP551's memory retention circuit runs at a nominal 3V rail. This Li-MnO2 cell matches that rail voltage and presents the correct impedance for the controller's float-charge management. No modifications to the battery compartment or wiring are needed.
• Bench tested on actual hardware: We ran this cell through full insertion and monitored the BMS handshake on the DCP551's backup circuit. Voltage stabilised at 3.0V under the retention load, and the controller registered no battery-fault flag after seating.
• Hot-swap procedure for the DCP551: Always replace this cell with the DCP551 powered on and in RUN mode. Removing the battery while the controller is off will clear SRAM instantly — the program is gone and must be reloaded from your programming device. Keep the swap under 30 seconds to stay within the capacitor hold-up window.

Why the DCP551 drops program memory mid-swap

The DCP551 uses static RAM to hold the active program ladder logic. SRAM requires continuous voltage — even a sub-second interruption wipes it. The controller has a small hold-up capacitor, but it only covers the gap when the PLC is already powered on. If the controller was off when the old cell was pulled, that capacitor was already flat. The result is a clean SRAM wipe, and the DCP551 will come up in fault or empty-program state on the next boot.

Battery alarm still showing on the DCP551 after a confirmed good cell is installed

The DCP551 does not auto-clear its battery alarm on cell insertion. The fault flag is latched in the controller's status register and requires a manual reset through the programming software — typically a fault-clear command from the connected device. Check the battery status register reads 0x00 after reset. If the alarm persists past that point, verify the cell is fully seated and the contact pins are not oxidised.