# Why is it that mobile phones these days promises to charge your phone up to 80 % in like half and hour.. but it takes another 20 minutes to charge to 100%? Why can’t it just go all the way?

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Same with electric cars… I always see “fast charge you car up to 80 percent or so in X amount of time but the rest is not fast?

In: Engineering

The most wear on a battery happens between 0-20 and 80-100%. It’s a measure put in place to help the battery last longer. It’s the same reason why you shouldn’t leave your phone plugged in overnight.

Charging times of lithium based batteries are not linear. Up to about 80% are faster than from there on.

Think of it this way, when it’s at 0% there’s full of empty seat for the electricity to be in but when it reaches 80% they would have to go out their way to find those empty seats.

Or a slightly disgusting analogy, after you take a dump, your first wipe would clean up most of the mess but the subsequent wipes would clean up lesser and lesser until your final wipe where its barely anything but it gets you to a 100% clean bum

The charge flows to the positive charged plate (cathode) of the battery to be stored. The charge is positive and naturally wants to flow away from the positive charged cathode.

Getting positive charge is easier to do when it is empty. Once the charger reaches ~80% the positive charge increases enough to push back on the flow of additional charge entering. It still will fill, but at a slower and slower rate.

It’s like packing a room full of people, when the room is empty it’s easy for people to walk in. After a certain point people keep bumping into each other and it’s harder to squeeze more in.

Imagine squeezing a strong spring. It’s kinda hard to get started, but once it’s going it’s fairly smooth until you reach the end of the compression, when it becomes the hardest to compress further. That’s charging your battery.

imagine you’re trying to fill one of those ice cube trays for your freezer.

you start out by just randomly pouring as much water as you can in the general direction of each cube.

once they’re all *mostly* full, you reduce the amount of water you’re pouring in, and direct it to a single cube. when that one’s full, you move to the next cube, and so on, till each cube is full.

instead of Cubes in an Ice Cube tray, you’re now charging cells in a multi cell battery.

you know that because of the way the cells discharge, they’re all going to discharge *more or less* evenly, so you know that you can fast charge all of them at more or less the same rate.. but, once you get to 80-85%, you have to stop fast charging, and then check the charge of each cell, and just top it up, to prevent overcharging.

All of these answers are missing the crucial reasoning behind why charging slows down. Yes current tappers off, but nobody explained why.

Your phone aims to charge its battery to a certain voltage, let’s say V= 4.2 Volts. The aimed voltage is given by

V = V_B + I * R

The current battery voltage V_B plus the charge current I times the battery resistance R. Let’s say R = 0.1 Ohms, and we keep current at I = 1A.

Your phone wants to charge the battery to full capacity which is V_A = 4.2 Volts. Solve for the current battery voltage at termination V_B:

V_B = 4.1 Volts

Not exactly 100%! In order to achieve true 100%, current is reduced near the end so that the product I * R becomes very small. If we could design low resistances, this wouldn’t be an issue. The only way around this is to reduce current near the end, let’s say I = 0.5A, already that gives us

V_B = 4.15 Volts.

So we reduce more and more until we reach 100%. Of course the lower the current the slower it charges, that’s why it’s done near the end and not beginning.