But it was maybe a few months old at best. Maybe it had a defective battery from the start but I contacted Xiaomi and I've been told it was "normal" in "winter". Then when I looked online for this issue with Xiaomi phones, the people on the forums said it was "normal", and that I expected too much.

In the end it was probably a defective battery. I couldn't believe that they were selling millions of these and that people always just kept them warm all the time. Like, they have a proper winter too in some parts of China, and I can't imagine millions of people having their phone dying on them as soon as we get into sweater weather.

But obviously this left a bad taste in my mouth. This and having to ask permission to root my phone.