Why are my LED string lights not working with an adapter?
If these &quot;3 volt&quot; batteries are little coin cells, they have high internal resistance (at least 10 ohms, perhaps much more):
https://data.energizer.com/pdfs/lithiumcoin_appman.pdf
That high internal resistance acts as a current limiting resistor.  If you have no other resistor on your LEDs, then you absolutely must add one to use with a 5v supply or you will burn up the diodes.