Razer Opus not responding, not turning on.

Hello, I have some Razer Opus headset which are like ~8 months old now. A couple of days ago I think the battery has been drained entirely. However, now when I try to charge them (yes, more than 3.5h), the green light blinks as it is charging but it never gets turned on all the time, as it does when the headset is completely charged. I'm also unable to turned them on or use the bluetooth functionalities. The only way to turned them on is when they are connected to a USB charger.

I'm beginning to wonder if it might be a malfunction of the headset batteries. If it is that case, does anyone have ever replaced Razer Opus batteries? Is it possible to do that?

I would really appreciate your help on this .