Battery health issues, hardware or software?

In the battery settings if iphone says that your peak battery performance is 66% or anything below 99%, can we solve this issue by changing the battery to a new one? Will the peak performance increase or is there some other hardware or software problem that’s draining the battery and how to fix it? Apple technician cannot determine the real root cause but just says that its hardware problem and cannot be repaired. I just wanted to know what hardware is causing the problem. I tried replacing the battery but still drains though there is a little more back up power then it was before. And just to do away with my phone seems hard as iphone aint cheap. What can we do to fix the battery issue of iphone?