Current Consumption

When I built my accelerometer project (chompr movr), I placed a jumper in the return path to ground. This allows me to measure the current consumption of the device by replacing the jumper with a 1 ohm resistor. Using Ohm's Law (V = I * R), the voltage across the resistor is equal to the current draw of the circuit. The current consumption varies over time, as the peripherals require more or less current. This makes it difficult to measure current accurately with a multimeter. An oscilloscope gives you a much better picture. Here's a capture of the current consumption on chompr mover. The circuit, which includes an Arduino, accelerometer, and microSD card, draws about 36mA, in 20ms bursts. These bursts happen every 2 seconds, which is the period that I'm querying the accelerometer and writing data to the card. 

The rechargeable 9V battery I'm using has a capacity of about 200mAh, so this is a rough estimate of the potential run time:

1 hour = 1800 samples * 20ms a sample = 36 seconds total (0.01 hours)

36mV * 0.01 hours = 0.36 mAh. 200mAh / 0.36 mAh = 555 hours