T-45d model of power armor was the first version of power armor to be successfully deployed in battle. These suits were lacking the mobility of later versions (such as the T-51b power armor), limiting the wearer's Agility. The first version to hit the frontlines ran on small energy cells, burning through them at an alarming rate. In response, later versions were manufactured with built-in MicroFusion Packs. The T-4x power armor was originally built by the the United States Military and has become used by many factions including the Brotherhood of Steel and the Outcasts and they each have their own design to their power armors.