My battery was also flat. I ordered a new battery and put it in, very easy. I can finally see 100% battery again. But now I have the problem that the app is going crazy and I can no longer log in.

After 3 years of systematic use, I encountered this described problem!
What I have done is to change the battery. On 'Chinese' apps, original batteries arrive at competitive prices.
After changing the battery, it works perfectly!
Small investment, big result.