The spartan laser is the only known battery powered weapon and as we all know only appeared after Halo 2. In the book Ghosts of Onyx Dr.Halesy and a team of spartans retreived a cache of forerunner technology. If you'll notice the sentinal beams from Halo 2 have a similar laser and also have battery power. I think that the Spartan Laser is Forerunner technology that the UNSC has taken and changed into somthing that spartans and other UNSC personel can use. The book never said what the technology they captured did.