There is no difference in battery life between medium and max 3D, the only thing the 3DS has to do is to change the positions of the two in game cameras. In other words alter the two images for each eye, the 3DS still has to produce two images no matter what position the slider has, except of course for the off position where only one image is projected thus lowering the power consumption.
The 3DS' top screen consumes the same power whether it's 3D on or not. it's pixels are not square but vertical rectangles. put two together and you get a "square" pixel. when you turn the 3D on, the two pixels start to display different stuff, where each pixel is seen by one eye. when you turn the 3D off, the two pixels display the same, melding into "one" pixel.
that's why they say the top screen's resolution is 800x240, because it is 800x240, halving only when you turn the 3D on (where half are diverted by the parallax barrier for one eye each).
if anything, what consumes more power when you turn the 3D on is the GPU, as moving the slider up from OFF triggers it to render double the images at two different vantage points.
