So i thought, 'hmm alkaline cells are like 2000mah, maybe i could use them here....'
well the first problem is the voltage. 2xAA cells give you 3v, the pi can run as low as 2.7v, so we could use that!!
So i wired it up, and it worked... for all of 15 minutes... it seems that alkaline batteries drop voltage very fast. we need to boost that voltage!!
So i raided the parts bin, and i found an adjustable buck/boost board, gave that a go
didnt even make the start!! seems the boost board wont work below 3v
so plan C, over to pololu and one of there boost boards boosty which can take an input as low a 0.5v and boost it up so we looked like we were onto a winner
So testing began, and i was expecting big things, 2000mah should technically yield 8 hours, but after just 3 hours, the pi started rebooting. seems that boosing the voltage isnt as simple as it seems!! to get 3.3v @ 200ma on the output, the input needs more current. so as the voltage drops, the current requirements raise.
so as we can see here, things will quickly snowball. as the voltage drops, the current raises, which will force the voltage lower, and the current higher, and so on, until the batteries just give up.
So using lithium batteries is a much better prospect, as the voltage in general is higher, and there is no need for boosting!