About this replacement
FLEX FX1431A / FX2141R Series — 24V Li-ion 5000mAh Replacement Battery (FX0121)
This is a 24V Li-ion replacement battery rated at 5000mAh (120Wh), cross-referenced to OEM part numbers FX0121, FX0311-1, FX0411, and FX0431. It fits the FLEX FX1431A, FX2141R, FX2141-Z, FX2241-Z, and additional models in the same platform. The pack slots directly into the tool's battery cradle and communicates with the FLEX BMS via the standard multi-pin data bus.
- FX1431A and FX2141 platform compatibility: These models share a common 24V rail, identical connector pitch, and the same BMS handshake protocol. One battery pack works across drills, drivers, and fastening tools in this family without modification.
- Bench tested on actual hardware: We ran this pack through motor-start inrush cycles on the FX1431A. The BMS held the overcurrent threshold steady across repeated trigger pulls and did not trip under simulated drilling loads at full torque.
- Break-in load cycling on first use: Run the tool at half load for two cycles before applying maximum torque. This allows the BMS to profile the motor's inrush current draw and calibrate its overcurrent protection thresholds before you push the pack hard.
BMS cutoff on motor-start inrush surge in the FX1431A
Every trigger pull on a cordless drill sends a short inrush spike — often two to three times the running current — before the motor reaches speed. A BMS that hasn't profiled the motor yet can read that spike as a fault and cut output instantly. On the FX1431A, this shows up as the tool dying on the first pull, then recovering after a second attempt. Keeping the rail contacts clean and running break-in cycles at half load prevents the BMS from flagging normal motor start as an overcurrent event.
Charger blinking red and refusing to accept the pack after storage
Li-ion chargers have an acceptance voltage floor — typically around 2.5V per cell. If a pack sits unused for months, cell voltage can drop below that threshold and the charger refuses to begin a charge cycle, showing a red blink or no response. The fix is a brief "wake-up" charge: some FLEX chargers enter a recovery mode if you hold the pack on the charger for 10–15 minutes before the LED responds. If the pack resting voltage reads above 18V on a multimeter, it is still recoverable and the charger should eventually accept it.