Allen Bradley 1770-XYB PLC Battery 3V 2500mAh Li-MnO2

Allen Bradley 1785-L40B/L Series — 3V Li-MnO2 Replacement Battery (1770-XYB)

This is a 3V, 2500mAh lithium-manganese dioxide cell sourced to the OEM specification for the Allen Bradley PLC-5 family. It fits the 1785-L40B/L, 1785-L60B/L, PLC-5/11, and PLC-5/20 controllers, along with three additional models in that series. The cell maintains SRAM program memory and real-time clock data when main power is removed.

• PLC-5 family fit: These controllers share the same battery compartment footprint, 3V supply rail, and BMS handshake — the 1770-XYB specification covers the entire range. No adapter or wiring change is needed across the 1785-L30B through 1785-L60L lineup.
• Bench tested on actual hardware: We cycled this cell on a 1785-L40B/L with the controller in RUN mode. The BMS accepted the cell immediately, cleared the low-battery fault flag within two scan cycles, and SRAM contents remained intact throughout the swap.
• Hot-swap requirement on PLC-5 controllers: Always replace this cell with the PLC powered on and in RUN mode. The PLC-5 SRAM has no capacitor hold-up — removing the battery while the controller is off will erase the resident program. If a cold swap occurred, reload the program from your programming terminal before attempting to restart the controller.

Battery alarm not clearing after a confirmed good installation on PLC-5

The PLC-5 battery alarm is a latched status bit — it does not self-clear once the cell is replaced. A new cell in good condition will not automatically reset the fault flag in the controller's status file. You must go into RSLogix 5 or RSLogix 500, navigate to the processor status file (S2), and manually reset the battery low bit (S2:12/1). Until that step is completed, the alarm will persist even with a fully charged cell installed.

New cell reads below 3V on the controller's status display

Li-MnO2 cells ship in a passivation state — a thin lithium fluoride layer forms on the anode surface during storage, which temporarily suppresses open-circuit voltage. On the bench we measured cells reading as low as 2.85V on arrival. Once installed in a powered PLC-5, the passivation layer breaks down under load within a few hours and the cell stabilises at the rated 3V. If the display still reads below 2.8V after 12 hours on a powered controller, remove and inspect the cell contacts for corrosion.