[JustSomeNobody]

> So even if they make a breakthrough making a thinner battery, they would rather use that space to pack something else in, instead of making the battery last longer.

This is exactly what happens in the laptop market. We used to have 6 cell batteries, but because battery density has increased so much, we now only have 3 cell batteries. (Well, actually, we don't as they are more shaped to fit super thin chassis now.) Nevertheless, we the consumer have not benefited in battery life due to the increases in battery density. We have benefited from more efficient chipsets and screens, though.

[dom0]

Actually it's more like 50:50 between improved energy density in batteries and "other stuff" - most importantly "better" CPUs and smarter power management in other hard/software.

The battery in the laptop used to write this comment could power most "ultrabooks" (or it's direct successors) for >24 hours (that is, from the claimed battery life of the newer devices)