battery charging - How do I design a 2A or more power supply for my consumer USB devices?

I understand that many of my consumer devices charged by USB ports can charge at higher rates than 0.5A. However, in testing, I've found they don't consume more than this.

Test Setup

• I have a power supply that outputs between 0 and 30 volts, and 0 to 20 amps.


• I set the voltage to 5 volts, then connected up a female USB to the wires correctly.


• When I plug in a Samsung Galaxy S4 Mini, it pulls around 0.44 amps. It is the same with a Samsung Galaxy Tab Pro.


• However, when I connect an iPad Air with an official lightning cable, it only pulls around 0.11 amps.


• Also, I tried connecting a ZGPAX S28 smartwatch and it was still about 0.44-0.45 amps.

The power supply is more than capable of outputting the maximum 2.1 amps, so why doesn't it to at least the tablets?

What do I need to do in my test setup to convince the devices to consume their maximum charging current?

Answer

The reason why your Apple/Samsung devices do not draw more current is simple. It is because there is additional data communication going on between the Apple/Samsung device and their dedicated power supplies. This makes both devices recognise eachother and agree on a higher current to be used. The charger sets certain voltages on the USB data lines and this is recognised by the phone or tablet.

Your 20 Amps supply does not supply these voltages on the date lines so it does not "talk" to your Apple/Samsung device therefore these assume it is a normal "dumb" charger and do not draw more current than what is allowed by USB standard which usually is only 100 or 500 mA

To charge an apple device put thoses voltages on the data lines:

desired current : 2,000mA D- : 2.0V D+ : 2.75V

Also, depending on how full the battery is the charge current is also limited. It will only be maximum when the battery is 30 - 70 % charged (these numbers are just my guess). Charging with a high current is bad for the battery when it is very low or almost full.

Sources:

Adafruit : The mysteries of Apple device charging

Voltaic : Choosing USB Pin Voltages