As
@russell_314 and
@mrochester stated, the original capacity is typically above the designed, targeted, advertised level, which is good. However, because Apple doesn’t show err report to the user greater than 100%, the perception is skewed as to how long the battery maintains full capacity.
As
@FeliApple described, capacity over use has an initially medium slope, then things almost level off as the battery chemistry settles into a comfort zone (so to speak), and the capability finishes with a nosedive.
The following graph provides a decent depiction.
Here’s an actual Apple device example.
I had it plotted, and it looked similar to the referenced graph. Unfortunately, I can’t locate which battery thread I originally posted the log visualized.
By the way, the recommendation to replace a battery at below 80-percent capacity is just an averaged, estimated guideline. Also:
This is why everyone (including Apple) suggests following the 80/20 rule.
These 20-80, 40-60, whatever rules have emerged primarily due to facts being taken out of context. Let’s begin with the 80-percent cutoff.
Apple said:
Your iPhone uses on-device machine learning to learn your daily charging routine so that Optimized Battery Charging activates only when your iPhone predicts it will be connected to a charger for an extended period of time. The algorithm aims to ensure that your iPhone is still fully charged when unplugged.
To improve the lifespan of your battery, your iPhone learns from your daily charging habits.
support.apple.com
An extended period of time being, for example, six hours a day (e.g., overnight charging, on a charging stand in an office setting). A more prudent example would be using your MacBook mostly docked, when it could be connected to a charger, on average, 16+ hours per day.
The five, ten, and/or 20-percent low battery alerts vary by device type and are just that, notifying you the device may turn off/shut down in a(n unexpected) short period.
Deep discharging shouldn’t be dismissed. Although, when your iDevice or MacBook automatically powers off due to low battery, there’s still a significant amount of charge, presumably ~10%. Not only does this help prevent deep discharge, it also extends — that is, a user doesn’t need to prepare/plan for it — the usefulness of features such as Find My. Furthermore, ‘smart’ charging has been common for decades. In other words, a battery in an extremely low voltage state can often be recovered.
Apple said:
Store it half-charged when you store it long term.
If you want to store your device long term, two key factors will affect the overall health of your battery: the environmental temperature and the percentage of charge on the battery when it’s powered down for storage. Therefore, we recommend the following:
- Do not fully charge or fully discharge your device’s battery — charge it to around 50%. If you store a device when its battery is fully discharged, the battery could fall into a deep discharge state, which renders it incapable of holding a charge. Conversely, if you store it fully charged for an extended period of time, the battery may lose some capacity, leading to shorter battery life.
- Power down the device to avoid additional battery use.
- Place your device in a cool, moisture-free environment that’s less than 90° F (32° C).
- If you plan to store your device for longer than six months, charge it to 50% every six months.
Depending on how long you store your device, it may be in a low-battery state when you remove it from long-term storage. After it’s removed from storage, it may require 20 minutes of charging with the original adapter before you can use it.
Maximize the life and lifespan of the rechargeable lithium-ion batteries in your MacBook, iPod, iPhone, and iPad.
www.apple.com
Basically, as long as the battery state doesn’t remain at the extremes (i.e., 0%, 100%) for long periods, it’s unlikely to affect the typical rate of degradation.
One more thing… Occasionally recalibrate.
All newly-installed smart batteries should be calibrated as soon as possible. This helps your system get an accurate reading on the battery’s state of charge. W
help.ifixit.com
It’s like routine maintenance to ensure a
float gauge is making its complete travel, not becoming stuck, in a tank with buildup that reduces liquid capacity over time.
