Discussion about batteries in iPhones

So I have in the past replaced a lot of batteries in various iPhones without any issues. Recently however I have had a couple of iPhones generally in the 6/6s range that have had a poor battery life and a new battery hasn’t fixed the problem. I almost certain that it is not a bad connection and I would like to think that the batteries that I am buying are not the issue either.

I am not really looking for a straight answer, just wanting to start a discussion and maybe people can relate or even have some solutions about the issue.