Publisher Summary This chapter discusses the types of water-activated batteries: McMurdo instruments magnesium-silver chloride seawater batteries, SAFT (Socirt6 des Accumulateurs Fixes et de Traction) magnesium-silver chloride batteries, SAFT zinc-silver chloride batteries, SAFT magnesium-cuprous chloride batteries, SAFT magnesium-copper iodide seawater-energized primary batteries, and Eagle Picher water-activated primary batteries. McMurdo Instruments Ltd supply a range of magnesium-silver chloride cells (Aquacells), which are activated by seawater and have obvious applications in marine and aircraft safety lights, buoys and flashing beacons. These batteries have a long storage life provided they are kept in a sealed condition. Once unsealed and immersed in water they are considered expended. They cannot be recharged. SAFT magnesium-silver chloride cells have an energy density of 30-120 W h/kg. SAFT zinc-silver chloride cells have an energy density of 15-60 W h/kg and 20-150 W h/dm3. Operating temperatures are between -30 and +60°C and cells up to 2500 W are obtainable. SAFT magnesium-cuprous chloride cells have an energy density of 20-90 W h/kg and 18-150 W h/dm 3. They operate in the temperature range -20 to +60°C and are available with up to 50 W output.