Sennheiser HD 4.40 BT Not charging.

I all started last month when I forgot to use the headphone for a week. So, when I tried to power it on, it wasn’t switching on. When I tried to put it in charge, there was no indicator, but I felt slight heat on right side of the headphone. I left it for few hours, then it started working, but after 2-3 hours it abruptly shut down without any warning.

Now I again tried to charge, but there is no indicator. but I left it for few hours and then it worked. This method worked 4-5 times but there was never a charging indicator.

But now it stopped working completely. I still can feel it, but it doesn’t power on. I can use it via aux cable but unable to use it wirelessly.

My hunch is something wrong with battery. What do you think? How should I proceed?