Battery health

Lithium batteries degrade over time, this is why your battery might not last as long now as when you originally purchased your device. It should reach around 80% after two years.

You can find out more information about battery, charging and optimising your battery to get the most out of it here

Do you mean how much should a battery health reduce in a year??

If so then 8 to 12% reduction would be about normal for the smaller phones, a bit less in the big ones. It really depends on how much you use and therefore charge it.

Some people have a better experience than that.

Have Apple replace it at 80%