A Portable Battery That Runs on Saltwater – or Urine. MetalCell was designed with militaries in mind; on the modern battlefield, soldiers rely on a growing array of electronics to execute their missions, but when operating in remote areas or cut off from support, those devices can run out of juice at inopportune moments. But MetalCell can sit in the back of a Humvee, in a remote bunker, or in a locker at a forward operating base for years, waiting to power up electrical devices in a pinch.