Thanks for the message. For a battery, I would definitely recommend a lithium polymer battery such as this one: http://www.adafruit.com/products/328 but depending on your design, and how you want to arrange your components, a lithium ion battery such as this one: http://www.adafruit.com/products/1781 could also be useful. I went with the first one as this allowed my case to be slightly thinner. Both of these batteries output at 3.7v and so you need to boost the voltage to 5v to power the pi. I have just received the new raspberry pi, and while i have given emulation on it a go, I havn’t tested it to see how power hungry it is whilst emulating, and so it might be a good idea for you to do a couple of tests to get an idea of how much power you would be needing. Send me a private message with your email and I can send you the model file.
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