A water-activated battery is a disposable reserve battery that does not contain an electrolyte and hence produces no voltage until it is soaked in water for several minutes.
Typically, a large variety of aqueous solutions can be used in place of plain water. This battery type is specifically designed to be more environmentally friendly due to an absence of heavy metals. Water-activated batteries are used most commonly found in radiosondes which cannot contain heavy metals because they regularly fall to the ground or ocean surface and remain there indefinitely.
A carbon-magnesium battery, named NoPoPo, coming in AA and AAA sizes, was released in Japan in 2007. The batteries come with a syringe to inject water or electrolyte, such as juice. Diagnostic kits using copper-magnesium cells activated by water or the liquid sample itself are also in development. However these batteries have failed to make an impact due to their extreme low power output and very short active life-span. Moreover , due to the chemical reaction involving magnesium , these batteries are known to deform and cause damage to products used with these batteries.
Another water-activated battery had been invented by Susume Suzuki of Total System Conductor.[1]
Aluminum anodes are used on many water-activated batteries designed for use with seawater and its high salinity.
A new kind of water-activated battery is the HydroPak, a portable power generator that uses water-activated disposable fuel cartridges. It is a high energy alternative to lead acid battery packs and portable generators that works by adding water to sodium borohydride which releases hydrogen fuel for a proton exchange membrane fuel cell. It can be re-charged simply by replacing the fuel cartridge rather than the lengthy recharging that other batteries require, however the cartridges cost $20 each. [2]