So, what happens? My sister's 15 Pro, bought in May last year, now has a battery life of 91%.

While my 16 Pro Max is still 100% now, after 9 months. She is impressed.
She is probably going to switch to the iPhone 17 when it comes out, but not immediately. She wants to get more rebates from online platforms, so she plans to do so in November or December.
Her daughter is using her previous iPhone 12, and she intends to pass her phone to her daughter once she upgrades. However, after seeing her battery life vs mine, she wants to take over my 16PM for her daughter.

So, she proposes that I trade in her 15P to Apple, and she will pay me the difference.

Anyway, I have a gift card from Apple that I received earlier for my iPad Mini 6 trade-in. Can I trade in an old iPhone and also use the gift card for an iPhone 17 purchase?
I bought 15 pro at the same day as 16 pro (Oct 2024) (my wife uses the 15 pro, full day blasting douyin / yt / whatever video app Tongsan has) and still at 100% battery health. I am at 211 charge cycles, I suppose she is nearer to 250-270 I guess.
Both looking strong but I believe once it dips below 99% will start seeing some linear decay. Its normal, the 100% is not really 100%, instead it is from 105-10x% to 100%.
edit - add date.