The first thing to understand is that iOS records the performance of the device’s battery. Always. Proof of this is that we can always check your status. But until now the user could do it very superficially, while Apple has access to much more information through an internal diagnostic tool.
Only authorized Apple technicians have access to that tool, and they can do it remotely. In this tool you can see both the general state of the battery and its daily consumption rhythm with graphics like this image:
But the information that most interests us is in this chart here:
This scheme is quite simple to understand: the white point is the state of our battery. When the device is new, it should be in the upper left corner indicating an optimal state. Healthy at 100%. And as we use the terminal, loading and unloading it, the white point goes down diagonally to the right and down.
It is important to understand that there is no situation in which the white point moves horizontally to the right indicating optimal health as time passes. It is impossible to happen, the battery always ends up degrading over time and moving down and to the right through the three zones of the graph.
- The green zone indicates a state of the battery that Apple considers good, concluding that it is not necessary to replace it.
- The yellow zone indicates that the battery has behaved as expected, and simply the time and use have affected its performance, suggesting that a battery change would bring benefits. This is not considered a hardware failure, in the same way that it is not considered a breakdown that the tire of a car will wear out over time.
- The red zone indicates a problem. The battery has deteriorated too fast (it has moved faster downwards than to the right) or it has a defect. It is imperative to change it.
Do not look for this diagnostic tool anywhere because you work for Apple or one of its authorized centers, or you have no way to access it. Although it can be found on the web, this tool is theoretically private and reserved for Apple technicians.