Battery technology hit a brick wall several years ago. Nothing much ahs changed since the original DS came out.
The only way to increase battery life is to
A) have a huge battery (most likey this is the case with PSP2, the thing is massive)
B)have more efficient compontents, this comes from die shrinks and more efficient backlighting. Since NGP is using an OLED, I bet the power consumption is much much less for the screen compared to 3DS, meaning all the power is being drained by the CPU GPU.
If Nintendo released an OLED screen for the 3ds in a future revision (can still be same resolution) the battery life would probably be better. However since OLED is already expensive, and 3D is probably worse, they had to pick a regular LCD backlight over OLED to keep the cost down. In 2-3 years when I expect a revision, OLEDs will be much cheaper and they might put one in.