Firstly, I understand that there's a number of forum threads out there that ask how to run the Raspberry Pi off a battery; but this is my own take on the subject.
Being a geek that I am, I have limited electronics skills, but I learn quickly thanks to Google and also websites explaining the basics of electronics. I've been thinking of running the Raspberry Pi off a battery source, but the problem that remains is the battery source itself - what do we use?
I was searching eBay for some rechargable batteries, and I found NiMh D-Sized batteries that have a whopping 10,000mAh capacity. I have seen batteries in AA size that offer 3000mAh, but D Sized batteries give you more oomph for the price. Now, NiMh batteries are usually around 1.2V. A quick calculation reveals that 4 D batteries in a series would not suffice, that's ~4.8 volts (roughly). Bump that up to 6 D sized batteries, that's 7.2V. That falls inside the 6V - 24V (?) specification that the Raspberry Pi unit requires.
According to google, 1 Watt is 1000mA. That means, at full load, you'd get 7 hours to battery life - on par with some other Android tablets. This is the calculator I used to work out the battery life, which gives you a ballpark figure. Let's say the Raspberry Pi idles at 250mA, punch that into the calculator, and that's 28 hours of battery life!
So, running the Raspberry Pi off D Sized batteries is a possible solution. Forget AA Batteries if you're looking for long-lasting battery life - rechargable D side batteries exist and at 10,000mAh, they are a tank. One would connect the batteries in a series, allowing you to get 7.2V with capacity of 10,000mAh, maybe attach a LED to indicate the battery status (ie. Green/Red) and then connect the output of the batteries to the Raspberry Pi's board. Maybe you could solder them onto the DC jack's inputs?
Could one of the Raspberry Pi devs please give a ballpark figure of a vanilla Raspberry Pi's (either model A or B, I'm planning to get model B) current draw when idle?