Motorola flip phones from the 90's had it right when the case back was the battery also and you could choose a nicely integrated slim or extended battery option

Motorola flip phones from the 90's had it right when the case back was the battery also and you could choose a nicely integrated slim or extended battery option based on your preference. For peeps younger than me: http://en.wikipedia.org/wiki/Motorola_MicroTAC http://en.wikipedia.org/wiki/Motorola_MicroTAC