Replace Lithium Battery with Permanent Power Source
A fully charged Lithium is 4.2V.
Just power it from 5V USB, with a standard 1n4001 diode in series to drop a volt.
You might need an electrolytic capacitor across the battery place e.g. 1000uF to reduce noise and supply peaks. (batteries have a low impedance, and it has only 3&quot; of wire)