The battery (currently a Lion 3 cell battery) is temporarily attached to the tail, it will go inside the body in the end. Now it comes handy to access it easily to charge or unplug.

To power Raspberri Pi, I'm using a buck (DC to DC) converter, it's just a few dollars worth but produces pretty stable output voltage. I've tried it with my adhustable power supply between 11-15v and it stays 5 volt without fluctuating. I think it uses LM 2596. The datasheet says it can output 2A steady current which is enough for me since I don't use RPi to power anything yet.

One important thing to express again is that there are three different voltages going around the robot; 12 volt, 5 volt and 3.3 volt. If you are not careful you can easily burn things out or damage badly.