[homebrewer]

It's not arbitrary. Lithium batteries charging uses voltage termination, and the usual cutoff for 100% on lithium-cobalt batteries (so not LiFePO which is not yet widely used on phones) is 4.2 volts. It's the same for all batteries I've ever used or heard about. You can charge it to higher voltages, but basically nobody sane does that as it quickly becomes dangerous, and 4.2 is pretty much the standard voltage.

[LiamPowell]

Pouch batteries straight from distributors might all recommend 4.2V, but what's actually used in phones varies a lot. The Samsung Galaxy S21 for example charges all the way to 4.4V. I don't have a huge sample size to work with, but I have seen a number of recent Samsung batteries degrade alarmingly fast, likely because of this.

According to [1], the S24 has gone all the way up to 4.45V.

[1]: https://old.reddit.com/r/techsupport/comments/1ayqveg/samsun...

[hnuser123456]

I was surprised to read the label on a GoPro battery and see it rated to 4.4v or thereabouts.

I used to build quadcopters around 2015, and some of the battery mfgs released "LiHV" batteries, which were supposed to be safe up to 4.35v, however if you actually charged them that high, they would puff up like balloons after only a couple flights. Maybe with the cumulative advancements in the last 10 years, they've been able to push up the max voltage more safely.

[creaturemachine]

LiHV has nearly taken over, especially in racing and 1-2S powered micros.

[rcxdude]

There's been a slow creep in voltage as the manufacturers chase ever higher energy density.