About this replacement
ABB TA251 / AC500 Series — 3V Lithium Replacement Battery (3HAC041821-001)
This 3V 600mAh lithium cell is the backup battery for ABB TA251 and AC500-series PLCs, including PM56X and PM5xx CPU modules. It holds SRAM program memory and the real-time clock when main power drops. Without a functional cell, a power interruption wipes the controller's retained data and clock settings.
- AC500 PM56X / PM5xx compatibility: All these CPU modules share the same 3V lithium cell footprint, connector, and BMS handshake. The memory retention circuit draws from the same voltage rail across the entire AC500 platform, so one cell specification covers the full range.
- Bench tested on actual hardware: We ran the cell under simulated SRAM retention load. The BMS held stable at 3V with no cutoff events, and memory retention remained intact through repeated power-cycle tests across the full discharge curve.
- Hot-swap procedure for this controller: Always replace this cell with the PLC powered on and in RUN mode. Removing the battery while the controller is off clears SRAM instantly — there is no capacitor buffer on the AC500 memory rail. If a cold swap occurred, reload the program from your programming device before attempting a restart.
PLC battery alarm not clearing after a confirmed good installation
The AC500 does not auto-clear a battery fault once it detects a low or missing cell — the alarm latches in the diagnostic buffer. After fitting the new cell, you must clear the fault manually through PS501 Control Builder or your connected programming environment. Navigate to the diagnostic log, acknowledge the battery alarm, and reset the fault register. The alarm light will stay on indefinitely if this step is skipped, even with a fully charged cell installed.
New cell reading lower than 3V on first installation
Lithium backup cells ship at storage voltage, typically in the 2.7–2.85V range, to reduce self-discharge during transit. Once seated in the PLC, the controller's float circuit brings the cell up to its nominal 3V within a few hours of powered operation. Do not condemn the cell based on a first-read voltage measurement taken immediately after installation. Check again after four hours of normal PLC operation — it should read at or above 2.95V.