Publisher Summary This chapter discusses magnesium perchlorate primary batteries. Magnesium dry batteries are now finding widespread use in military communications equipment. With increasing use and production, the cost of these batteries has dropped significantly. There are two main types of primary battery: the non-reserve type, which is ready for use off the shelf and requires no activation or pre-use conditioning, and the reserve type, which usually contains the active electrode materials but requires the addition of water or an electrolyte just before use. The advantage of reserve manganese dioxide- magnesium perchlorate batteries is that they can be stored for years in an inactivated condition and require activation with the electrolyte before use. However, unlike the non-reserve type, it is necessary, once they are activated, to use reserve batteries fairly soon, as they undergo self-discharge. Reserve batteries are recommended for use in portable transceivers, locator beacons, electronic counter measures, and expendable electronic equipment. Further, applications for non-reserve batteries include fire and burglar alarm systems, barricade lights light packs for emergency slide crates in commercial aircraft, and emergency location transmitters on aircraft.