About this replacement
Worx WU137 / WU161 — 12V Li-ion Replacement Battery (WA3540)
This is a 12V 5000mAh Li-ion replacement for the Worx WA3540 battery pack. It fits the WU137 and WU161 cordless tool platforms — drills, impact drivers, and compact saws that share the same 12V slide-in battery interface. Voltage and capacity match the original pack exactly.
- WU137 and WU161 platform compatibility: Both models run the same 12V rail and use an identical battery slot with the same connector pinout and BMS communication handshake. One pack works across both tools without modification.
- Bench tested on actual hardware: We ran this pack through charge cycles on a WU161 platform and monitored BMS response during motor-start inrush. The protection circuit handled repeated trigger-pull spikes without nuisance tripping at rated load.
- Break-in load cycling: On first use, run the tool at half load for two cycles before applying maximum torque. This lets the BMS log the motor's inrush current signature and calibrate overcurrent thresholds before you push the tool hard.
BMS cutoff on motor-start inrush surge with the WU137 drill
When you pull the trigger on a stalled or high-resistance load, current spikes sharply in the first 20–50 milliseconds before the motor reaches speed. On a 12V pack, that inrush can briefly exceed the BMS overcurrent threshold, tripping the protection circuit before the motor has even turned. This is more common with worn tool internals or a dull bit increasing stall torque. If the pack shuts off on trigger pull, release the trigger, wait five seconds, and try again — the BMS resets passively on this platform.
Charger won't recognise the pack after storage
Li-ion packs stored discharged can drop below the charger's acceptance voltage — typically around 2.5V per cell. When cell voltage falls that low, the Worx charger refuses to begin a charge cycle and either blinks an error or shows no response. To recover, apply a brief low-current trickle charge using a compatible lab charger set to 0.1C until the pack reaches 3.0V per cell. Once above that threshold, the standard Worx charger will recognise the pack and complete the charge normally.