Dave Akerman wants to shoot his Raspberry Pi into the sky on a balloon.  In order to do that the Pi has to run on battery power.  Dave dives deep into the core of the Raspberry Pi’s power configuration and decides that unless you are running a USB device that needs 5v, you can power the Pi on as little as 3.3 volts.  Now I can appreciate any article that starts out with a disclaimer about voiding your warranty!  Dave strips off the on-board linear regulator and goes through the option of wiring up your own LDO (possibly more efficient) or using a switching regulator.