I have an S21 plus that I've been considering upgrading because the diagnostics function in settings says my Battery is weak also the adhesive on the back failed and the back is completely off. Also the fact that software support ends on the s21 plus this year. Ive cleared my phones cache and it still says the same thing. I found a way to find out the exact battery health (*#9900#) and it's at 87 percent. That seems pretty good to me. Is it saying it's weak because as far as I know the battery is original and therefore old or is there something else I dont know about? As far as I can tell the battery isn't swollen.
@Kath01 Since the back is coming off and the battery diagnostics indicate that the battery has an issue, I would be inclined to think that your battery is failing and swelling. The swelling is typically what causes the back cover to come off.
You need to get the phone into repair for diagnostic and repair.
