I keep my old Huawei P8 plugged in 24/7 tethering to my router so it's constantly under charge, that's just not good for the battery but I don't care.
The battery swelled up a lot recently almost entirely popping off the oled display and it's dangerous to leave it plugged like that as it might explode or catch fire anytime.
So I just went ahead and replaced the battery with the old one I still had lying around.
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https://www.youtube.com/watch?v=rChHd7znuPY