What causes battery health decline in iPhone 13 Pro Max?

HarlanTolken wrote:

I got it from a family friend. They purchased it August 2022.

Oh, I had only reached it to 100% once or twice. Should I make it reach 100% once in a while?

Then your battery is doing quite well.

The only way the Battery Health calibrates is when it does charge to 100%, so for example, given that Battery Health drops 1% a month on average, if you do not let it calibrate by charging to 100% for 3 months, once it does charge to 100% then you should expect the Battery Health to drop by 3% all at once. You are not required to charge to 100%, but just understand how battery calibration works so you are not surprised when you see a large drop.

I got this not brand new last November 7, 2024

Any way to know how "not brand new" the phone was when you got it?

The iPhone 13 Pro Max was discontinued by Apple in September 2022, so the phone was at least 2 years old when you got it and as much as 3 depending on when the phone was originally placed into service.

The battery was at 96% in November 2024 and 5 months later it was at 92%. Battery decline is normally about 1% on average per month over longer periods. So, a decline of 4%-5% over 5 months would be normal.

But if the battery just declined 4% in a month, that may indicate that the battery is starting to head downhill. Sometimes...but not always.....that can be a steep fast decline. Other times, things smooth out and slow down again. No way to know in advance what will actually happen.

There is not really anything that you can do other than replace the battery when it is down to 80% Maximum Capacity, assuming that you want to hold on to the phone. A new battery will be about $100 (U.S.).

Personally, I have always ignored the 80% Charge Limit, but that is another discussion.

You bought the phone brand new in November and the Battery Health was 96%? That does not sound right. For an iPhone 13, you can expect the battery to drop about 1% each month on average. The Battery Health cannot calibrate until the phone is allowed to charge to 100%, so if you always stop charging before it has a chance to go to 100%, then you will see a larger than expected drop as it was not possible to calibrate previously.

If you bought a refurbished phone from a source other than Apple, the battery may not be a genuine battery and your results may not be what is expected. The iPhone 13 Pro Max was discontinued on September 7, 2022, over 2 years ago, so not exactly sure the history of the device.