Hi
I was in the exact same situation with my pair and since they were already past the warranty period I decided that I had nothing to lose and simply disassembled them. As I was expecting it turned out that the reason for this behavior was the battery (actually there are two 3.7V VARTA CP1254 batteries in parallel) but not because it was dead but simply because it was depleted bellow the threshold that the charging circuit will allow you to charge it. If you are familiar with li-ion batteries this is a common situation when you leave them not charged for a very long time and the phantom drain or simply the time passed depletes them bellow the threshold. The way to "revive" such a battery is to jump start it from another battery - no need to be the same one - simply anything with the same voltage and equal or higher capacity will do. In my case I used a random IMR 18350 that I had laying around from my Zhiyun gimbal. Giving it a couple of touches for a 5-6 seconds each raised the voltage from the 0.67V dead state to the reasonable 2.8V state. After this I simply plugged in my USB charger and instead of going to green in a second as they did before they kept on charging for a couple of hours and only then turned on the green light. Then I turned them on to check if they were working and surprise - all is fine. I've been listening to them for more than an hour now and the battery is still at 90% in my Android phone Bluetooth settings and the headphones themselves say "Battery full".