The AirPods have batteries 80 times smaller than an iPhone's. Why don't they charge 80 times faster?
One of the best things about the AirPods is their battery life. While most other fully wireless earbuds last 2 to 3 hours per charge, Apple's earphones promise a solid 5-hour music-listening time, and our own testing showed that as much as six hours of continuous playback can be achieved. Not bad for a gadget that uses a ridiculously tiny battery. And when the AirPods are drained, 21 minutes of charging is sufficient to top them up.
But something about this last figure doesn't seem right. The battery inside each AirPod is about 80 times smaller than the one inside the iPhone 7, so why doesn't it charge 80 times more quickly? The iPhone needs 141 minutes to go from zero to full, which makes the AirPods only about 7 times faster at charging. Well, that's because lithium-ion batteries can be quite the divas.
Lithium-ion batteries: a charging story
Lithium is well-known for being highly reactive and flammable, which makes charging a lithium-ion battery a tricky business. The process must be actively and constantly regulated in order to be both efficient and – more importantly – safe. If mishandled, a battery may suffer irreversible damage. It may go up in smoke if abused further. This is why every Li-ion-powered gadget has specialized circuitry built inside it – circuitry that limits the flow of electricity, keeping it within limits that the battery cell can handle. And what the battery cell can handle is proportional to its size.
The smaller the lithium-ion battery, the slower the rate at which it can be charged. That's because elements inside the battery (called electrodes) responsible for making electric flow possible are physically smaller, which limits the amount of electrical energy that can safely pass through them. Once again, the accent is on "safely". You surely can force a Li-ion cell to charge faster, but this can easily cause irreversible damage or even a smokey, smelly explosion.
Of course, the AirPods are not the only product affected by this phenomenon. When I reviewed the Samsung Gear IconX wireless earphones last year, I noticed that they needed over an hour to fully charge, even though their batteries were also tiny. I'm pretty sure that smartwatches charge rather slowly primarily due to the very same reason and less due to their use of wireless charging.
Just gimme the TL;DR version
What? You didn't read the whole piece? But it wasn't even that long! Anyway, what you should walk away with is that smaller batteries need to be charged at slower rates, and there's not much that can be done about it. Perhaps improvements in battery technology could make quicker charging a reality. Perhaps one day a new technology will emerge and replace Lithium-ion cells altogether. When that's going to happen, however, is anyone's guess.
iPhone 7 and AirPods teardown photos courtesy of iFixit