How Much Power is Needed?

This is supposed to be a 'bot that can scurry around the floor so obviously it needs to be battery powered.   The Rover comes with an integral six battery pack but that will not be enough so I am dedicating it to power the motors and sensors (other than the compass which I am powering from the Arduino's 3.3v power supply. 

The Raspberry Pi message boards indicate that the RPi runs at around 2.5 or so watts.   To this I have to add the  overhead of the USB hub, a wireless card, a webcam, and the Arduino.   Due to some power supply issues, namely a wimpy regulator that is supplying power to all the smart stuff, I have not been able to measure actual current draw from the RPi yet but I was able to see that all the rest of the stuff was drawing about 200-230 mA.

Assuming that the 2.5 watts (or 500 mA) is accurate, and adding 240 mA for the USB hub and the stuff it powers, I will burn 750 mA or 3.75 watts at 5v.   My battery pack for the smart stuff is the same as for propulsion so with old technology rechargeable batteries I would have between 600 and 1000 mAh at 7.2v or 4.3 to 7.2 watt hours.   Assuming that I lose 20% of my power to translation from 7.2v to 5v that leaves me with from 3.5 to 5.8 watt hours.   Burning 3.75 watts means from 0.9 to 1.5 hours of run time.

Power for the 'Brain'

New technology NiCd are supposed to deliver from 2000 to 2450 mAh which would yield run times of 3.6 to 4.4 hours which would be a huge improvement.   I have some on order (with a new power regulator) but for now have been using some worn out, old technology, NiCds and they don't really give me squat (though that may also be the wimpy power regulator that I am using since I fried my better one).

Here is a spreadsheet that calculates this stuff.

If you are doing ANYTHING with an RPi you will need to worry about power.  I can testify that it is VERY sensitive to how you feed it.