Worx WA3503 12V Replacement Battery 2000mAh Li-ion

Worx WU131 / WX125 Series — 12V Li-ion Replacement Battery (WA3503)

This is a 12V, 2000mAh Li-ion battery pack that fits the Worx WU131, WU288, WX125, WX125.1, and over 29 additional Worx 12V compact tool models. It replaces OEM part numbers WA3503 and WA3509. Voltage and form factor match the original pack exactly.

• WU131 / WX125 platform fit: These models share the same 12V rail, slide-in connector geometry, and BMS handshake protocol. Any pack coded for WA3503 will seat, lock, and communicate with the tool's onboard electronics without modification.
• Bench tested on actual hardware: We ran this pack on a WX125 drill and an impact driver from the same 12V platform. The BMS handled motor-start inrush current without tripping, held voltage above the tool's low-cutoff threshold through sustained load cycles, and accepted a full charge immediately after deep discharge recovery.
• First-use load conditioning: 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 set accurate overcurrent protection thresholds before you push the pack hard.

BMS cutoff on motor-start inrush with the WX125 drill

Compact 12V drills pull a current spike the moment the trigger closes — often two to three times the steady-state draw. A BMS that hasn't profiled the motor yet may read that spike as a fault and cut power instantly. This is not a cell failure. The protection circuit trips before the cells even warm up. Running two half-load cycles first teaches the BMS where the inrush ceiling sits, so it stops reacting to a normal start event as an overcurrent fault.

Charger shows no response or blinks red on a new pack from storage

Li-ion cells below approximately 2.5V per cell will fall outside the acceptance window of most Worx 12V chargers. A pack that sat on a shelf — either new or used — can drop into this range through self-discharge alone. The charger isn't faulty; it's refusing to bulk-charge a pack it reads as potentially over-discharged. Connect the pack to the charger and leave it for 10 to 15 minutes — many Worx chargers run a trickle pre-charge mode that nudges cells back above the 3.0V threshold before switching to normal charge. If the light stays red beyond 20 minutes, check cell voltage at the pack terminals with a multimeter; you're looking for at least 10.5V across the full 12V pack.