This is something that always bothered me about this movie because there was no explanation for it. It is well established throughout both films that Tony Stark intends the Iron Man suits to be used exclusively by himself. Now, it makes sense that he would have built multiple suits--backups, special purpose, etc. But all of the suits are/would be powered by an arc reactor. Stark has an arc reactor imbedded in his chest, and it is clear in both movies that his chest piece is what powers his suit. So how is it, then, that Rhoades is able to steal one of the suits? Rhoades doesn't have a chest piece, and it would make absolutely no sense for Stark to build the suits with their own power source since it's: a) not needed, and b) allows someone other than him to use it (contrary to his express intent).