Light model is BF48L20.
The goal is to use a 5VDC power bank to power the light. That should replace the 4 AA batteries.
- Open the lid, remove the batteries.
- Take the board out by removing the screws.
- Make 2 holes in the case (3mm or so) for the wires and remove any sharp edges.
- Solder the wires on the board.
- Do not put the batteries back. You don't need them anymore!
Use a USB female connector and solder it on an external PCB board or buy a PCB with the connector if you like. Make sure you have the plus and minus on the correct side of the connector.
Take your favorite power bank and test it.
Happy biking... with lights!