Plug in your iPhone or iPad and charge it up, and you’ll notice that while the first 80% or so will go by pretty fast, they actually kind of suck at charging up that last 20%, taking a lot more time to do so than it feels like they should.
There’s a reason for this. Charging batteries up to “full” is a complicated process. There’s no real way to tell if a battery is completely “full” so all you can do is measure the voltage, which (and this is a vast simplification) tells you how much resistance is being met when you try to put more electricity into the battery.
That’s why it takes so long for an iPhone to charge that last 20%. It charges full blast until it measures a certain voltage, then goes into what’s called “trickle mode” to slowly allow small sips of electricity into the battery until it thinks, based upon some software calculations, that the battery is more or less full. But a new algotihm could make the time it takes to charge your iPhone or iPad go by a lot faster..