I believe this is what you are referring to
The MetalCell is a portable battery ready for use anywhere. The battery allows you to generate energy for electronics like flashlights or even laptops from saltwater. Or, if there is no salt water about, it can use urine.
The MetalCell is said to have been conceived in South Korea for use by the military. The battery works by exploiting the chemical properties of magnesium. The magnesium plates react with the sodium in the salt water (or urine) which creates low voltage current. This electrochemical reaction can then be used to charge up any equipment, particularly equipment where there's no immediate power source. Electronics are known for running out of power when it's most inconvenient and when there is no chance to recharge them. For the military, where the problem can be life threatening, this battery is meant to be an emergency back up that can run from materials a soldier has to hand (be that saltwater or urine!).
The MetalCell could be extremely useful for home use in areas where a sudden cut of power is likely. The battery can be stored away safely for years, as the internal workings will not degrade with age. Due to it's compact size, a number of batteries can be stored away in even small storage cupboards. As soon as you add saltwater or urine to the battery, it's ready for use. In theory, the battery can run a laptop for around 4 hours. The battery will require refills of saltwater or urine to keep charge going, though eventually the magnesium plates would also require replacement.
Fitted with magnesium plates inside, the MetalCell can be charged up with nothing more than the addition of saltwater. The sodium in the salt reacts with the magnesium to create a dose of low-voltage power that can power up laptop, a flashlight, night vision specs, etc. when no other source is available. The output can keep a laptop humming for more than four hours and can be recharged with fresh saltwater until the magnesium begins to deteriorate.
Here is what I found on how to make one
"All jokes [about] urine aside, what is needed are low-cost batteries. …" he said. "The other neat thing about this is the fact that it's basically a biodegradable battery."
Urine Power
To make the battery, Lee and his colleagues soaked a piece of paper in a solution of copper chloride and sandwiched it between strips of magnesium and copper. This sandwich was then laminated between two sheets of transparent plastic.
When a drop of urine is added to the paper through a slit in the plastic, a chemical reaction takes place that produces electricity, Lee said.
The prototype battery produced about 1.5 volts, the same as a standard AA battery, and runs for about 90 minutes. Researchers said the power, voltage, and lifetime of the battery can be improved by adjusting the geometry and materials used.
I have an Idea for this; and will model something later