That was my theory as well. They've actually changed it for Halo 3. The plasma fires from the top segment, instead of from in between the two glowing prongs. This leads me to believe that the bottom portion is the battery, connected to the main part of the weapon via electrical field between the two prongs. Someone needs to make an animation for an estimated plasma rifle battery reload. This principle works with either iteration of the plasma rifle, if you think about it.