About this replacement
Panasonic BR-2/3AG with Weld Leg — 3V Li-MnO2 Replacement Battery (BR-2/3AG, BR17335)
This is the Panasonic BR-2/3AG lithium manganese dioxide cell with weld leg, rated 3V and 1450mAh (4.35Wh). It fits PLCs and industrial controllers that use the BR-2/3AG or BR17335 form factor for SRAM memory retention and real-time clock backup. The weld leg connection is required for secure PCB mounting — do not substitute a standard coin or tabbed cell.
- PLC backup cell compatibility: Controllers specifying BR-2/3AG or BR17335 share the same 3V retention rail, PCB footprint, and weld-leg mounting point. The Li-MnO2 chemistry holds a flat discharge curve across wide temperature ranges, which is why industrial controller manufacturers specify it over alkaline or standard lithium alternatives.
- Bench tested on actual hardware: We confirmed stable open-circuit voltage, verified the weld leg geometry against PCB pad spacing, and checked BMS float behaviour under low-draw SRAM retention load. The cell accepted charge without event and held voltage within spec under continuous standby draw.
- Hot-swap procedure — mandatory: Always replace this cell with the PLC powered on and in RUN mode. Removing the battery while the controller is powered off will collapse SRAM voltage instantly, erasing stored program memory. If a cold swap occurred, a full program reload from the programming device is required before restarting the controller.
PLC losing program memory after battery swap
SRAM in most PLCs requires a continuous voltage above approximately 2.0–2.5V to retain data. The moment the old cell is disconnected with the controller powered off, that voltage drops to zero. There is no buffer capacitor large enough to cover a cell swap in most mid-range PLCs. The only recovery path is reconnecting the programming device and reloading the last saved project file to the controller.
Battery alarm not clearing after a confirmed good installation
Most PLCs do not auto-clear a battery fault flag when a new cell is detected — the alarm is latched in firmware and requires a manual reset through the programming software. On Allen-Bradley and Siemens platforms, this is typically a status bit or diagnostic flag cleared from within the project or via the controller properties menu. The new cell voltage is not the issue — the fault register simply was not reset. Open the programming software, navigate to controller diagnostics, and clear the battery fault bit directly.