More specifically, the number you can see if you head over to Settings > Battery > Battery Health. That number that starts out at 100 percent and slowly ticks down until it's time to replace your battery.
You'd think that the number would tick down at a regular basis, but it doesn't.
And I can prove it.
My iPhone 13 Pro Max is a launch day iPhone (actually, because of a UPS mix-up, I got it a day late). That means that as of today, I've had it 104 days. According to the internal Battery Cycle Count data (details on how you find out that information here), it has been through 92 recharge cycles.
This means that my iPhone is almost 20 percent through that 500 charge cycles.
So, what's my battery health? Go on, guess. I'll wait for you.
Yup, it's 100 percent.
How is that possible? Has the battery somehow defied the laws of physics and not experienced any wear in that time?
No. No way.
If I dig through the analytics, I can see that the maximum full charge capacity is listed as 4,523mAh (this was likely the first charge), and the minimum full charge capacity (likely the latest charge or a recent charge) is 4,255mAh.
That, right there, is wear. So, why doesn't iOS report it?
Basically, this "trick" works because the rated capacity of a battery (any battery, not just the iPhone battery) is theoretical. Some ship with more capacity, some with a lot more. The more capacity there is over the theoretical rated capacity, the longer it takes for the counter to tick from 100 percent to 99 percent.