Measuring Power on Peripherals: USB Drawbox

disclaimer: For extreme geeks only. This device description is for informational purposes only. Attempts to duplicate this could result in fire, personal injury, or destruction of expensive electronics. Do not attempt to duplicate this.

When I write peripheral reviews for ultra-mobile devices, people constantly ask me how significant the power draw is for the device. Previously, I had no answer. Today, I whipped up a rough way to find a solution.

I call it my USB Drawbox. Attached to a multimeter, it measures the number of milliamperes (mA) any given device pulls over USB. Generally, devices will request up to 500 mA. Above that, standard ports will shut down power to the device. Some devices, notably external hard drives and CD-ROM drives, can pull power from two USB ports simultaneously using a Y connector.
This little DIY hack requires a tiny bit of solder, a tiny bit of Dremel skills, and a whole lot of testing and tweaking. The way I did it - in a metal Penguin Mints tin - required some VERY careful insulation on the screws that went through the banana jacks. Other than that, it was pretty straightforward:
Parts:

• A project box. Plastic will make it easier to mount the banana plugs without worrying about shorts. I had to get very creative with heat-shrink tube, plastic washers, and bolts to avoid shorts in my tin housing.
  
• Banana plugs! One red and one black.
• A female USB type A plug.
  
• A male USB type A cable.
• Solder and heat shrink tube.
• A soldering iron, drill, Dremel tool, heat source, and the standard geek tools.
• A Multimeter that can measure pass-through amperage.

Steps:

1. Mount the USB port in the box.
2. Attach the USB cable to the box so it won't pull through or twist.
3. Wire the black, green, and white cables straight through. Black to black, green to green, and white to white. Test them for connectivity with the multimeter. Use heat shrink to insulate.
4. Mount the red banana plug close to the male USB cable and the black banana plug close to the female USB plug.
   
5. Attach the red wire from each to it's corresponding plug.
6. Test to make sure there are no shorts through the chassis, plugs, or anywhere. You don't want to cook your USB port!

At this point, you will have what amounts to a little USB extension cord with a broken power lead. The multimeter completes the power connection and measures the power going through it. The box is plugged into the computer and the peripheral is plugged into the device. It's probably a good idea to make sure the multimeter leads are SECURE in the banana plugs before attaching it to a computer to prevent damage by flicking the peripheral on and off rapidly.
Here are some examples of power readings:

A mouse, taking 0.018 Amps or 18 mA. Make sure your display is calibrated for mA or A.
A cell phone, while charging, taking exactly 500 mA and not more. This is a well calibrated device: Taking exactly the draw maximum and not more.
This LaCie CD-ROM is intended to be used with two USB plugs attached. I only attached it to one. It detected the device, but when it spun up to read the disc it requested well above the per-port allowance (879 mA, as shown above) and the computer shut the device down.

How does this help measure battery life? Well, that's more complex. My standard eo battery has 2400 mAh at 10.8V. All USB devices are 5 volts so by strict math it would have 5184 mAh at 5V (2400*(10.8/5)). A device taking 500 mA would be powered for 10.38 hours. There is, of course, some loss because voltage dropped from 10.8 to 5 volts causes heat, but it gives the user a rough translation on what to expect. It's easier if all the units are converted to watts and watt-hours before any actual estimation happens here.

I am sure some EE can correct me on this. I make no promises on math: I just know that the above lets me measure how much power a USB device is taking. Engineers: Can you give me a calculator in my comments? Say my UMPC battery (2400 mAh at 10.8V) gives me about 2.4 hours of battery life. What would it do to the battery life if I attached a 500 mA USB device?