Instructables

Step 4: Power

My power system depends on AA batteries.  Since I do not want to spend countless dollars on batteries, I got six rechargeable batteries and a recharger.  This is pretty simple. 

If you get a MintyBoost as I did, you need to make a simple modification to use it on your RaspberryPi.  The Pi requires more power than just 2xAA batteries alone can provide.  All I did was wire two 2xAA battery packs in parallel before connecting them to the MintyBoost.  I don't have an altoids gum case (yet), so I simply dropped the exposed battery pack and minty boost into an anti-static bag.  Connect a micro-USB cable to the MintyBoost.  Your portable Pi power system is now complete. 

I have not done any tests as to how long the battery pack will actually last, but I speculate that it is within the laptop/netbook battery life range.  Of course, you can use as much or as little power as you want.  It all depends on how much you're willing to carry around with you.  This setup is meant to be super-portable. 

Having 4 batteries is CRITICAL.  I can't stress this enough.  Two batteries will run your RaspberryPi, but it is not enough power to power the keyboard/trackpad and WiFi dongle. 
I think going li-poly here would be a reasonable option. Lighter, smaller, and quite a bit more power. The tricky part is dealing with charging and such but even then, there are several pre-made packs that would fit the bill if you don't want to get into rolling your own.