Batteries

Batteries  : All types of batteries used topside (out of water) can also be used undersea. There are a few factors unique to using batteries underwater. One is that some batteries can produce gases that are potentially explosive. In the open air this is usually not a problem because the small volume of off-gases is quickly diluted. However, in sealed pressure housing, gas build-up can cause problems ranging from water leaks to explosive failure. Pressure relief valves can safely vent gas to eliminate these problems.
Some batteries and other subsea electronics use pressure-balanced, oil-filled packaging. In this method, all air spaces and voids in a housing are filled with oil or hydraulic fluid. A flexible membrane on the housing transmits ambient water pressure through the housing. Because the pressure inside the housing is the same as the outside water pressure, it is said to be equalized. Since there is no pressure gradient, a heavy pressure-proof housing is not required.
Seawater batteries utilize seawater as an electrolyte. Most use electrodes of magnesium for anodes, and oxygen dissolved in the sea water as oxidant. Advantages of seawater batteries are that they have a very long shelf life while dry, can be used to very deep depths, have high energy densities, and have long duty cycles. Disadvantages are that the cell voltages are low, necessitating DC/DC converters and that the output power can be influenced by factors such as the water circulation, quantity of dissolved oxygen, temperature and salinity.