This just happened to me on a thumb stick repair. During 4 tear down and reassembles to get the thumb stick fixed, I went to test and the joy con wasn’t powering on. My aging eyes thought the battery was properly connected but it turned out the north side wasn’t fully clicked into place. Gave it a push, it clicked, and now all is well. Also if you somehow managed to insert the connector upside down it won’t work, the contacts are at the end. Holding the joy con vertically, hot south ground north. Good luck.
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