About this replacement
FLEX FX1431A / FX2141R Series — 24V Li-ion Replacement Battery (FX0121)
This is a 24V Li-ion battery rated at 3000mAh (72Wh), replacing OEM part numbers FX0121, FX0311-1, FX0411, and FX0431. It fits FLEX cordless drills, impact drivers, and multi-tool platforms including the FX1431A, FX2141R, FX2141-Z, and FX2241-Z. Voltage and connector type match the original pack exactly.
- FX1431A and FX2141 platform compatibility: These models share the same 24V battery rail, connector pinout, and BMS communication handshake. One pack works across the full range without adapter or modification.
- Bench tested on actual hardware: We ran this pack through repeated high-torque trigger pulls on a FLEX drill platform. The BMS held steady through motor-start inrush spikes and did not trip into overcurrent protection under normal load cycling.
- Break-in procedure for new packs: On first use, run the tool at half load for two cycles before applying maximum torque. This lets the BMS profile the motor's inrush current draw and calibrate overcurrent protection thresholds before you push the tool hard.
BMS cutoff on motor-start inrush surge in FLEX 24V tools
When you pull the trigger on a drill or impact driver, current demand spikes sharply before the motor reaches running speed. If the BMS registers this inrush spike above its overcurrent threshold, it disconnects the pack instantly. A new or storage-depleted battery is more likely to trip here because cell impedance is higher before the pack has been cycled. Running two moderate-load cycles first brings impedance down and reduces the chance of nuisance cutoffs at full trigger.
Charger shows blinking red on a new 24V pack after storage
Most FLEX 24V chargers reject packs where cell voltage has dropped below the charger's acceptance threshold — typically around 2.5V per cell. A pack that has sat unused for several months can fall below this level even if it was stored partially charged. To recover it, look for a "wake" or "trickle" mode on your charger and hold the pack in that mode for five to ten minutes until the charger transitions to normal charge. If your charger has no trickle mode, check that resting voltage across the pack terminals reads at least 18V before attempting a standard charge cycle.