Make a battery substitute power connector

Step 6 Test and finish up

Test the contacts with a voltmeter to be sure the polarity and voltage are correct and then test the fit of the connectors and the external power supply in an expendable device.

If there are no voltage anomalies and everything is working then cut a notch in the battery compartment cover in a place where the cover can be closed large enough for the leads to come through.

If everything is still working with the cover closed then you are good to go.