I had the challange, that the JBL 760NC did not charge anymore and the headphones had no power at all. Hard reset and stuff did not work. I followed the instructions of the video to remove the earpads from the headphones. Then i took hard plastic to open the left head phone with the battery enclosed. As in the video indicated, you have to start from the the upper side where there is the cover shape visible. You have to pull quite with some force and use rather hard plastic (iFixit Tools) instead of the hard knife in the video (i did it wiht my finger nails and the weak one got a small dent). With this method, the plastic cover wont get borken or taking another color.
When opened, I saw that the battery was not connected. So I plugged in the battery and the headphones were running again. I was able to charge and the headphones had power from the battery enclosed.