Oh okay, but still I'd imagine an energy clip looking a lot different than an ammo clip. But it's alright, you were working with PMG after all. But I guess it could pass for an energy cell, what with its straight (rather than curved and much more "magazine-like")shape and all.
Energy clips can be any shape. I usually have them look like regular clips, my e-cannisters on some weapons, or whatever feels right.
Look at stuff like Star Wars or Star Trek, especially their games. We don't usually see the weapon sources, but they are there. Don't play the games, fine. In ST V, there is a scene where one of Kirk's security team is popping in a clip into the hand phaser. How many times did we used to hear, "Phaser banks are tapped out/drained/etc?"
Look at sci-fi role playing games. They use energy beam hand weapons and some sort of limited shot power sources.