Battery doesn't charging. Red light blink for a second and that's it.

@andriyilchenko could be a bad battery. It may no loner hold a charge. Start by replacing that. Since iFixit doesn't yet have a guide for this, you do want to use something like this video Sightly different model but procedure is identical to yours.

After that, check the charging port. Make sure there is no corrosion, debris etc. that prevents it from working.. Clean it of any debris etc, use a few puffs of air to clean it out.

While you work on your headphones, take lots of pictures. Once you are done with it, take those pictures and create a guide for iFixit. That will help the next person that has the same problem as you do. It's easy and fun to create these guides. https://ifixit.com/Guide/new The iFixit community will appreciate this.

Adding images to an existing question