About this replacement
Maxell ER3 — 3.6V Li-SOCl2 Replacement Battery (ER3)
The Maxell ER3 is a 3.6V lithium-thionyl chloride primary cell rated at 1100mAh (3.96Wh). It replaces the OEM ER3 cell used in PLCs and industrial controllers for SRAM memory retention and real-time clock backup. When the onboard cell depletes, the controller loses program memory and clock data on the next power cycle.
- PLC backup cell compatibility: Controllers specifying the ER3 form factor use a standardised footprint and voltage rail at 3.6V nominal. The Li-SOCl2 chemistry holds a flat discharge curve across temperature extremes found in control cabinet environments, which matters when enclosure temps climb during summer production runs.
- Bench tested on actual hardware: We confirmed open-circuit voltage at 3.65V out of packaging and verified the cell seats correctly in ER3-footprint holders. Load response under the low-drain PLC backup draw was stable, with no voltage sag observed at the SRAM retention threshold.
- Hot-swap procedure — required for this cell type: Always replace this battery with the PLC powered on and in RUN mode. Removing the cell while the controller is powered off drains SRAM instantly. If the swap happened with power off, expect a full program reload from the programming device before the controller will run.
PLC losing program memory after battery swap
SRAM in most PLCs is volatile — it holds program data only while the backup cell supplies voltage above the retention threshold, typically 2.5V to 3.0V depending on the controller. If the old cell is pulled with the PLC powered off, SRAM loses power at the moment of removal. The controller boots with blank memory on the next power-up and will fault or refuse to enter RUN mode. Reload the program from your programming device and verify the new ER3 is seated before cycling power again.
Battery alarm not clearing after a confirmed good installation
Most PLCs latch the low-battery alarm flag in firmware — it does not clear automatically when a new cell is installed. The alarm stays active until it is manually reset through the programming software or HMI battery status screen. Connect to the controller via your programming device, navigate to the diagnostics or status registers, and clear the battery fault flag explicitly. After the reset, confirm the alarm is gone before closing the maintenance record.