Comment by nomel

I understand, but that's my point. Currently the case is 25% of the battery. Only the case can be removed. If you move the fragile guts of the battery into the frame (in no way reducing their mass or the frame's), you're effectively turning that 25% case to 0%. So, the most you can save is 25% battery weight.

That doesn't really nudge the power density needle all that much, especially when you consider that you don't throw batteries out the back as their energy is depleted, as you do with fuels.