Hi @Barneyboy1970
The battery percentage shown is based on the Rated Capacity in mAh of the battery. Batteries also have a MaximumFCC (Full charge capacity), which can be higher than the Rated Capacity. So if you're lucky, you can have a phone with a battery that has a bit more capacity, so it feels like your battery is lasting really well but in reality it just had a bit more capacity than the minimum required. So the Battery Health stays on 100% until that extra capacity disappears as the battery degrades
I have an old iPhone 8 that was used for two years but it still shows a Battery Health of 100%, simply because I was lucky enough to have a battery with a bit more capacity. My iPhone 12 Pro is down to 97% after 10 months. Apple say that a battery should have 80% capacity after 500 charging cycles, with a charging cycle being a charge from 0 to 100% (this can be 2 charges from 50% to full or 10 charges of 90% to full etc).
It is possible to see what the Rated Capacity, Maximum FCC, Nominal Charge Capacity (actual current capacity) and the number of charge cycles of your battery but it's a bit of a faff to be honest.
