After my first Arduino experiment where i had it measuring and storing temperature data, realised that running the default board configuration on battery was not ideal. A 9V battery ran out in hours even a 6 pack of 2000mAH eneloop AA batteries did not last more than 30 hours. Far from ideal for a sensor data logger where you want it to log data overs many days, if not weeks and months.
So i needed to experiment and get the power consumption down, the pre-requisite for anything like this is to be able to measure the actual power consumption at a point in time. A simple way obviously is to put a multi-meter between the battery and the board and read off that. However, to test things you need to keep uploading new sketches to the board with various settings, and you need the USB connection to do that, and at least for me if the battery is connected the USB doesnt always work.
So i needed to measure the power consumption from the USB port itself. After searching the net to see if some software could accomplish this, could not find a foolprrof way (you can go into device manager and look at the power tab of a USB port, but its not accurate), i decided to build this small hack
Using this have ascertained that a normal load is around 54mA which drops to 39.4mA in the Power Down Sleep mode
While the circuit / schematic for the above is fairly simple, pins 1-3, and the body from one USB female adapter connect to the other, an amp meter / multimeter in the DC Amp mode is inserted between pin 4s.