Trying to figure out the lowest voltage a device (Phone/Tablet) would charge

[SpookyGhost]

I bought a USB multimeter, and a USB load tester along with loads of adapters, and I am doing a bunch of tests on all my cables and chargers to see which ones are bad, and which ones are good.



There are great cables like Anker 3ft ones which will transmit like 98% of the power with almost no voltage loss, and then there are terrible cables which will do like 15% (Yes really. This is the absolute max current I could get before my multimeter didn't get the 3v required to be powered on)



Thats not even USB spec!

BUT! My tests are not actually relevant unless we know what voltage devices will charge at.

Sure, a 6ft Belkin USB extention will pull 2A at 3.97v which equals almost 8w, not too bad right? Well what devices charge at 4v? None probably.

Lets assume a device will charge at 4.8v, well now you can draw a MAX of 400ma... Thats less than 2w and your device will take a million years to charge



4.7v Maybe? Still at only 2.7w. 4.6v? Still a pathetic 3.5w

I plugged an iPad into my multimeter in place of the load tester, and through the USB extension it drew almost 1A and the voltage dipped to 4.4v and it was still charging. This would lead me to believe the limit is somehwere around 4.2v (Lithium cell charge voltage) It may have actually been 4.2 at the iPad since there was an extra cable going from my meter, into the iPad

But then to make this even more confusing, I plugged in a crappy Micro USB cable, which is now drawing 1.5a at 4.64v. It could get the full 2a before even getting close to 4.2 or even 4.4v...

How on earth is it choosing how fast to charge? If I had some kind of variable voltage power supply I would test it out, but sadly I do not... Its either 5v or no voltage for me!

[jeroen79]

How on earth is it choosing how fast to charge? If I had some kind of variable voltage power supply I would test it out, but sadly I do not... Its either 5v or no voltage for me!

Then I suggest you get or build one.
You can then experiment, first by connecting it directly to the device to charge then by puttin some resistance between it to simulate a cable.
Then log the charging process.

A simple cc/cv charger would simply let current pass until it either hits the maximum current or voltage.
If the resistance of the cable is such that it will drop a lot of voltage even below the maximum current then it will become the limiting factor.

[SpookyGhost]

I have a SFF ATX Computer PSU free, it can do 25a on the 5v rail so it might be something good to use. Could I just throw a variable resistor in line and use that? Or is there something I am missing?

[Ian.M]

Sounds like a good way of frying stuff.  If you make any mistakes the high current 5V supply is capable of melting your USB device's input connector and blowing tracks off its circuit board.

However if you use a LM317 regulator from the 12V rail with a beefy heatsink (e.g. drill and tap a mounting hole in the center of the heatsink off an old CPU, and hook up the fan as well), it will give you an adjustable output voltage while limiting the maximum short-circuit current to a couple of amps.  You can easily limit the adjustable regulator circuit in the LM317 datasheet to under 6V out to minimise the risk to your USB device, by connecting a 4.7V Zener between Adj and Gnd.

[SpookyGhost]

What would cause it to melt? Since it can't go OVER 5v, I assumed it would be okay

[jeroen79]

It's not the voltage but the current that would melt stuff.
The ATX supply would happily pour 25A into an accidental short and at 5V that is 125W.

[SpookyGhost]

Didn't think if that. Do you think it would be a better idea to just buy some kind of variable voltage/current power supply vs making one?