Since version 4.0, I've been using the same value as Coconut Battery and imazing to calculate battery health.
Maybe it wasn't a good idea but
I haven't received any error reports so far (1007 downloads).
On my 12 mini with a third-party battery, the battery health remains the same every day.
On my 13 mini, I have a battery replaced by Apple, and my battery health changes every day.
Unfortunately, I don't have an explanation for this at the moment.
My analytics files haven't been the same since 18.4.
I'm waiting for iOS 18.5 to see if this changes again.
Is the battery genuine? Are you using different chargers? Fast or slow?