Technical its not supposed to be a functional backpack per se, even though everyone calls it that way(even in the script i think).
It dont think it was meant by Nolan to be actually able to cary things inside it. Originaly it was devised, that Batman would use the "memory cloth" abbilities of the cape to make it shorter while driving the bat-pod, so that it wouldnt get tangled in the weels and kill the caped crusader
The idea was scrapped later, when it turned out(after building a functional pad-pod, and giving it a go several times) that it is practicaly impossible for the cape the get tangled in the weels, so Nolan left it, especially since he really loved the sheer epicness of the image of a black cape flowing in the wind.
