Now that we have the current monitor working, the next step is to build the voltage monitor circuit. I built a voltage divider that will break down the highest voltage my battery can possibly see, into a 0-5v input for the Arduino. Here is a great tutorial explaining the math behind determining the resister values, http://forums.trossenrobotics.com/tutorials/how-to-diy-128/cheap-battery-monitor-using-resistive-voltage-divider-3264/, and a schematic of our version:
Here is a screen shot of the monitor showing both current and voltage being monitored.
Now the trick is to multiply the current with the voltage to get watts.
The next step is to sample over time to display the amp hour and watt hour values. Still working on that. I’ve posted a spreadsheet for calculating the resistor values (and amp hour formulas) at http://tech.groups.yahoo.com/group/arduinohome/files/volt%20amp%20watt%20hour%20meter/
There’s a good discussion of this project at http://forum.pololu.com/viewtopic.php?f=3&t=5415