Is your voltage is too low? Is your current draw is too high? Find out with the USB In-Line Voltage Meter with OLED Display.
Just plug it in between, say, your ethernet cable and your Arudino, and the OLED screen with information about the current, voltage, wattage and current hour. All the data passes through.
The tiny tool is ideal for testing USB devices above 7 VDC, checking load levels and figuring out what’s wrong with your battery chargers. It can handle anything in the range of 3.7-13VDC input voltage and up to 3A of current.
The only drawback: it’s not open source.
You can pick up this handy piece of equipment at Adafruit, for $19.95.