Bike-powered phone charger from a dead e-scooter motor

Why

I had a dead e-scooter with a perfectly good motor in it. Brushless motors are generators if you spin them. Phones need 5V. Everything in between is the project.

How it works

Spinning a brushless motor produces three-phase AC. A bridge rectifier turns that into lumpy DC, a capacitor smooths it, and a buck converter steps it down to a clean 5V for the USB port. The motor's small roller presses against the rear tire sidewall, like an old-school bottle dynamo.

Steps

1. Test the motor first. Spin the shaft by hand with a multimeter on AC volts across any two of the three leads. You should see a few volts.
2. Solder the three motor leads to the three AC inputs of the rectifier. Order does not matter.
3. Solder the capacitor across the rectifier's DC output, minding polarity.
4. Connect the rectifier output to the buck converter input. Set the converter to 5.1V with no load before you plug anything in.
5. Print or build a bracket that holds the roller against the tire with light pressure.
6. Mount, zip-tie the wiring, go ride.

What went wrong

The first buck converter had a minimum input of 7V and at walking speed the motor only put out 4V, so the phone charged in bursts. One with a 3V minimum input fixed it. A rubber band tensioner on the bracket worked better than a rigid mount.