interesting idea if you are caught short in the dark...

[photoman290]

found this while perusing my morning  email
http://hackaday.com/2014/09/22/a-water-powered-flashlight/#more-133126

something useful for a change.

[Tony]

Reading the comments, its effectively a dry-storage cell, and adding water allows the cell to discharge.  So no energy from the water as such, and a finite operating life.  I'm not sure how useful it really is!

[photoman290]

on a practical level not very useful. on a cool gadget level quite a lot. i suspect  the magnesium dissolves and would need replacing at some point. there are other dry cell type batteries such as zinc air used in guided missiles where recharging them is not a problem they have a shelf life of 10's of years.

[Tony]

Interesting, I'd not heard of those until now and had to google.

http://en.wikipedia.org/wiki/Zinc%E2%80%93air_battery

I guess most other types of batteries die during storage over that timescale.

My favourite battery technology has to be Vanadium Redox.  Shame energy density seems to be too low for vehicle use.  Instant recharge by swapping out the electrolyte, which itself conveniently indicates charge state by colour?  Marvelous.

[photoman290]

yes i agree. i got quite a long way with those. the biggest problem apart from the nergy density is the cost of the nafion membrane. as long as du pont has the patent on it will be too expensive to be practical compared to lead acid. i did try to buy the stock of nafion from a canadian company when they went bust,but the chinese got in first.

[Julian]

Interesting, I'd not heard of those until now and had to google.

http://en.wikipedia.org/wiki/Zinc%E2%80%93air_battery

I used to work for a company making gas analysers.  The oxygen cell we used sounds very much like this description.  Calibration was really high tech ... if it read too high you tapped the end to close up a little hole and if too low you enlarged the hole with a spike!