I'm designing a home-made laptop and have everything sorted except the battery. The reason this is a problem is that I will be using a full size desktop graphics card. This is the one thing that really steams me about laptops insufficient graphics upgrade-ability.
I'm having trouble calculating the required battery output. What would the formula be to calculate the required battery output for all the components, and how would I then calculate the lifetime on a full charge? I have found plenty of lithium ion "packs" that are already chipped to prevent overcharge/ over-discharge, and am not opposed to using two batteries (one for Video card, one for everything else.)
Battery capacities are in Ampere hours. For example a 2800 mAh battery will ideally/theoretically/spherical chicken allow a 2.8A drain for an hour before full discharge. With P=I2R, P=IV, a good ammeter and component specs, you may be able to back-calc the system power requirements (or delta up from the specs on the video card).