One of the bigger issues with iPhone repair in recent years has been partial serialization of components. Simply put, Apple chooses to make certain parts of your smartphone identifiable as "genuine" by the phone itself. For example, replacing the LCD on iPhone 11 and newer models works fine with any brand of display. But if it is not the original one, the phone can tell that the display has been replaced and will tell you that it is "Unable to Verify that Display is Genuine".
Apple does this by including a specially programmed tiny chip in the display that has a unique code programmed to it. The phone itself communicates with this chip, recognizes the code programmed at the factory, and thus it recognizes it as the "native display". So when you replace the display, the chip on the new one is not programmed with the unique identifier and your phone can tell that the screen was changed. Everything works fine, but there will be a notation in settings that says "unknown part". It's just an Apple thing; they're very protective of their trademark and unfortunately they have the right to do that as a private company.
This issue is not limited to displays. Batteries have the same serialization, and that began with the iPhone XS/XR/SE2020 era. Beginning with the 11 series, the serialization became much tighter and dealing with it is more complicated. The way it works with your battery is that if you were to replace it with a new one, again, you'll get the unknown part warning, and the onboard diagnostics are switched off. This is Apple's choice to do this, but in my experience the phones work just fine otherwise. It's simply a nuisance more than anything.
At CDC, I'm very sensitive, as you most likely are, to these policies and do my best to make repairs as accurate as possible. With batteries, there is a way to work around the unknown part warning, but it requires some TLC. It is possible to harvest the tiny circuit board from the original, or "native" battery, and reinstall it on a new battery. The new battery is called a "core" and comes without a circuit board. You use a tiny welder to install the board to the battery and reprogram the entire assembly. When done you have a new battery and the phone still sees it as the original "native" one. Not all batteries can be done this way, as occasionally you'll get a phone in with battery problems and it's due to that board being defective, but generally if the board is good, you can make it work on another battery.
The iPhone XR/XS/SE2020 models are a little easier to deal with than later models. The little board itself is not locked down so reprogramming it is straightforward. 11 and newer not so much. Apple has that board locked so that you cannot reprogram it. It requires a special add-on chip called a "tag" that plugs into the battery and allows you to reprogram everything. Without the tag, the capacity and cycle counts cannot be reset.
With all of this now explained, I have a report for you on how well these battery rebuilds are doing long term. Quite honestly, middling at best. I am finding that the XR/XS/SE2020 models seem to be alright. 11 and newer with the add on tag? Not so much. I've had success making it all work, but long term I'm starting to see failures such as no charge, unknown part warnings, and so forth. In a nutshell, it appears that this process, while successful in the short term, might not be so great a year or so down the line. The reasons for this are likely either the little circuit board wears out or perhaps an iOS update comes along that affects it.
Going forward, this is my recommendation. Yes, we can certainly install a non-native battery and put up with the warning message if you wish. I'd propose continuing the battery rebuilds as I'm doing but add that long term reliability may be a problem. I'm not saying all devices repaired this way will have problems, and here at CDC I can say that is certainly the case. But I am seeing a few here and there that are, which means it's something to be aware of. We can always switch to a non-native battery later, but just beware that the BMS transfer might not be that reliable over the long haul.