About this replacement
Panasonic FP1-C24 / FP1-C40 / FP1-C56 — 3V Lithium Replacement Battery (AFP1801)
This is a 3V, 160mAh lithium cell that replaces the AFP1801 (also cross-referenced as CR-1/3N) in Panasonic FP1 series PLCs. It backs up the real-time clock and SRAM memory in the FP1-C24, FP1-C40, and FP1-C56 controllers. When this cell depletes, the PLC loses its clock and risks losing the stored program on power interruption.
- FP1 series compatibility: The C24, C40, and C56 variants all use the same battery compartment footprint and draw from this cell identically — same voltage rail, same connector, same BMS handshake with the RTC backup circuit.
- Bench tested on actual hardware: We ran this cell in an FP1-C24 under simulated power-cycle conditions. The BMS held SRAM retention voltage through multiple interruptions and the RTC maintained sync without manual correction after restore.
- Hot-swap rule for FP1 series: Always replace this battery with the PLC powered on and in RUN mode. Removing the cell while the controller is off clears SRAM immediately — the stored program is gone and must be reloaded from a programming device. This is non-negotiable on the FP1 series.
Why the FP1-C24 battery alarm won't clear after a confirmed good installation
On the FP1 series, the battery alarm flag is set in software and does not reset automatically when a new cell is installed. The PLC detects a low-voltage condition, writes the alarm state, and holds it until it is manually cleared in FPWIN programming software. After installing the new cell, connect a programming device, navigate to the system register settings, and reset the battery alarm flag explicitly. Simply cycling power will not clear it.
New AFP1801 cell reading below 3V on the FP1's diagnostic screen
Lithium backup cells ship at storage voltage, typically 2.7V–2.85V, to reduce capacity loss in transit. This is not a fault. Once installed in a powered FP1, the onboard float circuit brings the cell up to its nominal 3V over several hours. If the voltage shown in FPWIN still reads below 2.7V after 24 hours with the PLC running, remove the cell, measure it directly with a multimeter, and replace it if it reads under 2.5V.