the idea is something i had come up with for my pipboy project it requieres the use of the gpio's and modifying a usb power booster that you use too charge a lipo battery as well as charge your phone from. the idea uses several mosfet transistors or simular too alow power too flow too and from the individual battery you also wire the battery boosters battery indicators too 3 of the gpio pins and set them too sence wen the power drops too one bar the raspberry pi activates the next battery and disables the first using a diferent gpio too activate the next battery . as for charging you use the gpio to sence the power meeter or the light on the booster board. during the charging prosess you could have the power be disabled too the raspberry pi after it swiches too the alternate power sorce.