Fast Charging Vs. Slow Charging On Smartphones And Tablets

[JAS]

I see references made to fast charging vs. slow charging on smartphones and tablets.

Is there a recognized standard for what is fast and what is slow?

I seems to recall that USB is 5 volts.

 

[vicv]

Standard usb spec is 5v0.5a. Usbc is 5v3a. Usbc pd is up to 20v5a. There are also proprietary systems like Qualcomm quick charge and other company specific ones.
I don't think any standard in what is officially fast. Both my iPhone and iPad can charge at 18w. But the iPad has a much bigger cell so it's not very fast. The phone is fast. So it depends. I'd consider any charging over 5v to be "fast charging"

 

[JAS]

spec is 5v0.5a. USB-C is 5v3a. USB-C PD is up to 20v5a. There are also proprietary systems like Qualcomm quick charge and other company specific ones. I don't think any standard in what is officially fast. Both my iPhone and iPad can charge at 18w. But the iPad has a much bigger cell so it's not very fast. The phone is fast. So it depends. I'd consider any charging over 5v to be "fast charging"

That is good to know. Here I thought the voltage remained 5 volts, but it is great to know that it is more than that.

Thank you for the reply.

 

[Graylock]

There are standards, but the topic is a bit...complicated. Details vary by chemistry, but for example with standard Li-ion batteries, charging is normally done at a "C-rate" of 0.5C - 1.0C to an end-of-charge voltage of 4.2V, where C is the battery capacity. Thus, a 3,000 mAh battery would be charged at 1.5 - 3.0 Amps. Note that charging at 1C means that the battery will be charged theoretically in one hour*, whereas 0.5C will take twice as long. The C-rate can be pushed to 2C or even 3C, but this leads to more rapid degradation of the battery.

So, for Li-ion, I suppose anything above 1C would be considered "fast charging". To do this without prematurely wearing out the cells, more sophisticated approaches to voltage and temperature regulation are needed, which requires different charger and battery designs. This is what Qualcomm is doing with their "Quick Charge" technology.

A basic overview of these approaches is provided here.

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*This is not true in practice because normally when the cell reaches the final voltage (4.2V), the charger begins to decrease current in order to hold voltage at the target value. Charging then proceeds until the current drops to a specified level--I usually terminate at 0.1 or 0.2 A when charging my batteries. This slows down the final phase of charging significantly.

 

[Kestrel]

The simple answer for USB-A is that it starts at the 0.5A default and then handshakes up from there, depending on the device, cabling, etc.
I have some nice Anker cables that permit a little more of that compared to my other data-rated cables.
But I haven't kept up with the specifics, nor what USB-C does.