Optimized battery charging doesn’t mean it never charges beyond 80% until some crystal ball says you will need more than 80% power in your next session.
Optimized battery charging is based on the fact that rapidly charging a Lithium Ion cell when it is approaching full tends to result in shortened battery life. Slow (“trickle”) charging can extend a battery’s life, but of course, at the expense of taking longer to charge.
Battery chargers these days tend to implement a hybrid approach. They do a rapid charge up to some point and then slow down the rate until the battery is full.
Technically, these two phases are “constant current” and “constant voltage”.
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Constant current charging means that the charger tries to supply a fixed amount of current (some number of milliamps) to the battery. As the battery fills up (and its internal voltage increases), the charger increases the supplied voltage in order to keep the current flow constant.
This is how fast charging works. But as the battery approaches full, the high voltages necessary to maintain the constant current will cause the battery to get hot and can cause the chemistry to break down faster, shortening the battery life.
The charger needs good voltage monitoring circuitry, because a constant-current circuit can push a battery’s voltage beyond its rated limits, which can result in catastrophic failure. So it needs to measure the battery’s voltage and switch off when it is full.
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Constant voltage charging means that the charger supplies a fixed voltage to the battery. As the battery fills up (and its internal voltage increases), the voltage difference (between the battery and charger) shrinks, causing the amount of current to go down, slowing the rate of charging.
If the constant voltage equals the battery’s fully-charged voltage, the charger can remain in this state indefinitely. Once the internal voltage equals the charger’s voltage, the current will go to zero and charging will stop. If the constant voltage is higher than the fully-charged voltage, then there still needs to be monitoring circuitry so it can shut off when the battery is full.
It is common for modern chargers to use constant-current charging until the charger’s voltage reaches some threshold amount and then switch over to constant voltage until the battery is fully charged. Which is why you will see a battery rapidly charge up to a point and then take much longer to complete the charge.
Apple’s optimized battery charging is a variation on this. It uses constant current charging until the battery’s charge reaches about 80%. Then instead of doing constant-voltage at a fixed voltage, it will dynamically select voltages so it will reach 100% shortly before it thinks you will remove the device from the charger. For example, if your usage history says you remove it from the charger at 8:00am every morning, it might try to reach 100% at 5:30am, even though a traditional constant-voltage charger might be able to bring it to 100% at 2:00am.
The idea is that by charging more slowly after the 80% point, it will (hopefully) extend the battery’s useful life. And since you don’t need it to be fully charged when you’re asleep, there’s no practical downside to this slower charging.