I find myself thinking that i recall reading about certain thinkpads (or some other brand) had the option of an attached battery that would fit flush with the l

I find myself thinking that i recall reading about certain thinkpads (or some other brand) had the option of an attached battery that would fit flush with the laptop bottom. Thus i find myself thinking about some way to do a combo of frames/cases and batteries so that one can add a frame to a laptop and insert batteries into the frame to extend overall runtime. The idea of the frames being that if one replace the laptop one only need to replace the frame but continue to use the batteries.