Replace batteries with USB
While this page is based on powering Efergy transmitters that use three AA batteries, it is general info that can apply to many other consumer devices, if you adjust for expected voltage.
USB and batteries: Some basics
Typical U.S. batteries used in electronic devices and toys (A, AA, and AAA) supply a nominal 1.5 volts each. They are almost always used in series, so two batteries would provide 3.0 volts, three batteries 4.5 volts, etc.
Although they are labelled "1.5 volts", in fact, standard alkaline batteries start out with ~1.6 volts, then decline to approximately 1.0 volts, before biting the dust.
So if a device needs, say, three batteries, it is designed to handle ~4.8 V at first, and it will keep working until the voltage drops to ~3.0 V.
Said another way, consumer battery-powered electronic devices expect large variations in voltage. Said yet another way, they have no problem with using USB power (5.0 V), if it's within their design range.
If your device wants 3 batteries and can handle 4.8 to 3.0 volts, then a 5.0 volt USB adapter is very close. The adapter's output can be easily modified to supply what is needed.
To be continued