Smart battery case broken?

Just got a smart battery case for my xs and I wonder if the thing is not working correctly. Yesterday after my first fully charge the case showed 89% battery for around 8 hours and then dropped to 0%. It also charged very slowly, It had got 3% battery on my phone and 1% on the case (using a 60w type-c power-delivery charger) in 3 hours . Today the case seemed to stop working at 43% and when I put it into the charger it showed that the case was not charging at all. After replugging my phone in it the case had 1% battery. Any tips?

Edit: im running latest ios version.