i'm not getting any power at all, trigger does nothing, battery works on my drill

Diagnosis

The total lack of power in the Milwaukee M18 FUEL String Trimmer (2725-20) indicates a complete failure of the electrical circuit, meaning the motor controller is not receiving the signal to engage the brushless motor. Since the battery is confirmed functional via a drill, the issue is localized to the trimmer's power delivery system. This is most commonly caused by a failure in the continuity of the electrical path, which in this specific model often occurs at the connection point between the motor head and the shaft assembly. Because this tool uses a modular design, there are internal wiring harnesses that bridge the battery terminal to the motor controller. If the shaft is not seated perfectly or if the internal locking mechanism has shifted, the electrical pins may not be making contact. Additionally, the 2725-20 features a safety interlock system; if the trigger assembly is jammed or the electronic speed control (ESC) has entered a "fault mode" due to a sudden surge or debris, the tool will remain completely unresponsive to prevent motor burnout.

How to Fix It

Follow these precise steps to restore power to the unit:

1. Hard Reset the Electronics: Remove the M18 battery. Hold the trigger down firmly for 15 full seconds. This drains any residual capacitance in the motor controller and clears temporary electronic faults. Reinsert the battery and attempt to engage the trigger.
2. Verify Shaft Alignment: Loosen the shaft locking collar. Pull the upper and lower sections of the shaft apart completely, then push them back together with significant force. Ensure the alignment arrows on the shaft housing are perfectly aligned. If the shaft is off by even a millimeter, the internal power pins will not connect, resulting in zero power.
3. Inspect Battery Terminals: Use a flashlight to look into the battery port of the trimmer. Check for bent or recessed copper pins. If a pin is pushed back, use a small precision pick to gently pull it forward so it can make contact with the battery's terminals.
4. Clear the Trigger Housing: Check the gap between the trigger and the handle casing. Use compressed air to blow out any packed sawdust, grass, or debris that may be preventing the trigger from fully depressing the internal micro-switch.
5. Test with a High-Amp Battery: Attempt to power the unit using an M18 High Output battery (e.g., 6.0Ah or 12.0Ah). Some FUEL controllers require a specific voltage threshold to "wake up" if the battery is slightly depleted, even if it still runs a smaller drill.

If That Didn't Work

• Check for Internal Wire Fatigue: If the tool has been stored in a cramped position, the wiring harness inside the shaft can pinch or fray. Carefully disassemble the shaft connection and inspect the wires for any visible breaks or "crimping" marks. If a wire is severed, it will require a solder repair and heat-shrink tubing to restore the circuit.
• Inspect the Trigger Switch Assembly: The internal switch on the 2725-20 can fail if moisture enters the handle. If you have used the tool in heavy rain or washed it with a hose, the switch may have shorted. You will need to remove the handle housing screws and check for corrosion on the switch terminals.
• Test the Motor Controller (ESC): If there is no "click" sound from the relay when the trigger is pulled, the electronic speed controller may have blown a capacitor. This is typically indicated by a faint burnt-plastic smell emanating from the motor head. In this case, the control board must be replaced.
• Check the Battery Terminal Spring Tension: Use a multimeter to check for continuity between the battery terminals and the motor leads. If the voltage is reaching the controller but not the motor, the internal fuse or a lead wire has failed.

Heads Up

• Avoid "Dry-Firing": Never run the trimmer for extended periods without a line installed, as the lack of centrifugal resistance can occasionally cause the brushless motor to over-speed and trigger a safety shutdown.
• Battery Storage: Store your M18 batteries in a cool, dry place. Extreme cold can cause the battery to report a "low voltage" state to the tool's controller, preventing it from starting even if the battery is technically charged.
• Debris Management: Always clear the head of wrapped-up grass and twine after every use. Debris buildup creates immense drag, which can cause the motor controller to trip its internal circuit breaker to prevent overheating.
• Connection Maintenance: Periodically apply a small amount of dielectric grease to the battery terminals to prevent oxidation and ensure a high-conductivity connection.