EDIT / CLARIFICATION: I would like to point out the original phrasing of my question as being the main point on this question. That is:
If you have an old phone with an old battery, can you just perform a factory reset to make the settings -> battery -> battery health value appear as 100%?
Background / original question body under the line. Also, here is a link to a much more detailed question on a treasure trove of advanced data that I have found, and a more general electronics question about how to measure battery charge and health.
Here is another very important context for the case of an iPhone 11. It might indicate that the factory reset trick triggers iOS 14 and higher (mine is 16) to recalibrate the battery health and this takes "a few weeks" and meanwhile shows the health at 100% with an asterisk footnote saying that the real value will appear in a few weeks.
I am in a South American shopping Mecca and there are gazillions of iPhones being sold in many stores. They have a certain grading, new in sealed box, then semi-new, used. Tons of pristine no scratch phones.
But apparently there are shenanigans being played. On the surface Apple iOS 16 should tell us when replacements with non-genuine parts have occurred, but there are workarounds, aren't there?
Specifically the battery life issue. So they are selling 100% batteries, and claim those are replacement batteries, but, they say, they are original American, not Chinese. This whole subject of the "Chinese" vs. "American" stuff is a big deal here.
I am not an Apple iPhone guy, but want to make the girlfriend happy. So I am trying to navigate the cliffs. Checked that no non-genuine part warning is there, checked the serial number (already returned one with the shop, where the serial number came up as "returned". So now I have one with clear serial number -- of course out of warranty.
But I am running a battery test. Just YouTube on web site, to see how quickly a 100% battery is spent. And it is going a bit too quick for my taste. In about 2 hours I am at 40% already.
When I looked more closely to the 100% battery health message, I read the fine print, telling me it is being recalibrated, and it would take, not days, but weeks to actually come up with a result. So that tells me that they played a trick on me. And just how easy that trick is, I am figuring out now. Looks like any time you do a factory reset, the battery history is lost, and the battery health message doesn't say "unknown" but it says "100%".
Was I scammed? I feel I can go back to the store and I don't know if they would do money back, nor what I would do with the money, buy a 93% battery life phone instead? Then they have yet another scam trick?
Anyway, I need to have some hard facts to prove them wrong. I am trying to run a few deep discharge and charge cycles to get the battery health info to come up with some assessment. Or is there some app I should use to come to a better assessment quicker? They told me I had 40 day warranty, would like to resolve this while I am here, in the next few days to avoid unnecessary travel.
From an electronic point of view, I find it amazing that we don't just have an amp-meter which will just integrate the amps over time for charge and discharge and tells us the mAh instantaneously from one discharge - charge cycle.
