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USB PortsA while back I wrote about playing music via USB in the Model S and some of the challenges with that. In that same post I mentioned that there were different power ratings for the 2 USB ports. 2.1A for the one closest to the driver and 1A for the passenger. I got that data from a couple different reports on the forums. And then the other day I watched a “News from the Frunk” episode from Nick Howe that spoke about increasing USB charging speed in the Model S with a device and I got interested in the real story on Model S USB power and set out to do some testing.

Tools

For the test I collected a few tools:

  • My Model S (created in April 2014), running current 6.0 (first release) software.
  • An iPad 4, iPad Mini (original), and an iPhone 6.
  • A USB Volt/Amp meter.
  • The portapow adapter (universal flavor) that Nick spoke about.
  • 2 USB cables for the i-devices, one from Apple (standard cable) and one Amazon basics certified clone.

One big caveat on the test setup is that i’m using Apple devices and the iPads can draw power differently. If you have another device type (Android based or other) your mileage may vary.

Theory

The theory behind the adapter is that it removes the data lines from the USB connection making the device think it is connected to a USB charging port versus a normal USB data port.

The details are over on Wikipedia, but essentially data ports can only charge so fast and depend on the negotiated USB protocol (USB 1.0 vs 2.0 vs 3.0) while the power port just puts out power and has a higher maximum.

Results

I ran through the combinations and came up with the following data:

USB MeasurementsThe numbers in green show where Portapow is doing what it was designed to do — turn a computer USB data port into a USB power port to provide a faster charging rate. However, the device provided no benefit at all in the Model S. Also interesting is that the device actually reduced charge rates when added to an already-high power powered USB charging only port.

Summary

On my Model S, both ports charge at up to 1A, not the 2.1A and 1A configuration that some people (including me!) have previously reported. Actual measurements show that the Portapow device is useless in the Model S.

My recommendation is to get a good USB car charger that will charge at the maximum 2.1A rate. Those go for under $20 and, while a bit more than the $7 portapow, they will actually do the job.