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?

Question

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

Answer 1

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.

Source: https://www.quora.com/Why-does-my-phones-charging-rate-decrease-after-reaching-about-95#:~:text=Originally%20Answered%3A%20Why%20do%20phones%20charge%20slower%20as,get%20closer%20to%20100%25%3F&text=It’s%20normal%20because%20if%20battery,to%20maintain%20better%20battery%20Life..