How to Test Stability of your USB Connections? I have a Macbook Air 2013-mid which has been now 5 times in Apple care: new motherboard 1x and new SSD 3x. 
It cannot read anymore two USB devices: etc one phone and USB stick impossible. Only one device at the time. 
I think the power system may be in bad shape. 
Battery is good according to battery softwares. 

I can also feel the accumulation of electricity in some parts of the corners of the computer. Touching a computer with one finger gives you a little electric current anywhere in the computer after computer being untouched a short while. 
This is a second evidence about the power system. 

I am considering getting this device back to Apple because I have still warranty (5 years in total). However, I should have some sort of evidence that there is some problem. 

The observation is just that you cannot use two USB devices at the same time, and the accumulation of the electric current. Sometimes, strange read/write errors. 



How can you justify dysfunctional USB connections?
Look in:

Apple Menu > About this Mac

Then click "System Report" button

From USB in the sidebar you can see for each device "current available" and "current required" if you plug in a device and it needs more than 500MA then it need an external power source!

If however you are plugging in two and each one requires more than 500MA they will need an external power source!

If this is not the case and each device is 500Ma or less you have youre proof!

I think static on the case alone should warrant Apple fixing the device.