If you think your gear runs out of juice at inopportune moments, consider the infantrymen of the world, whose lives depend on their gadgets maintaining a charge. Thus the MetalCell, a magnesium-based battery that charges with salt water. Or urine.

The MetalCell, developed by engineers in South Korea, is an update on the Baghdad Battery, a device originating in Mesopotamia nearly 2000 years ago that is thought to be one of the first technologies to create voltage.

The updated version produces a charge when sodium—from soldiers' rations or their bodily fluids—reacts with the magnesium plates inside the battery's rugged box. The researchers claim that it can keep a laptop going for four hours on the initial charge, with additional sodium generating more juice. Barring any stage fright, that is. [PopSci]