Battery was empty again in less than 1 month. ☹️
But i think to have found the reason in all my 5 BT-Button.
The problem (which I suspect) lies in the contact pressure of the contact spring of the negative terminal of the battery.
In my opinion, the factory pretension is not sufficient, which leads to poor contact. I noticed the whole thing when I accidentally restarted a BT button, which can be explained by a loss of contact. At the same time, the BT button was very warm!
The - contact in its original state
and here after my correction :
With this small adjustment, 1 BT button now runs for over 1 month and still has a battery indicator of 100%.