Suppose the current drain (on average) of a LED is 2mA at 4.5v. Now suppose that 250 mAH 1.5v cells are being used to power this thing. I want to keep this LED powered for 504 hours (3 weeks), how many cells are needed and how should they be arranged? Now, here is my stab at the problem. There is a voltage requirement of 4.5v, right? So, place 3 1.5v cells in a series to add the voltage up to 4.5v. So far I know that there will be a series of 3 cells. There is a mAH requirement of 250 mAH, right? So there will be cells in parallel to each other to meet this requirement (since putting cells in parallel adds current). At this point, I get stuck, because I am not sure how to compute how many I will need in parallel. I do know that battery life = capacity / current though... Any help?