While we've seen more than a few flexible batteries in our day, they're not usually that great at withstanding tugs and pulls. A team-up between Northwestern University and the University of Illinois could give lithium-ion batteries that extreme elasticity with few of the drawbacks you'd expect. To make a stretchable battery that still maintains a typical density, researchers built electrode interconnects from serpentine metal wires that have even more wavy wires inside; the wires don't require much space in normal use, but will unfurl in an ordered sequence as they're pulled to their limits. The result is a prototype battery that can expand to three times its normal size, but can still last for eight to nine hours. It could also charge wirelessly, and thus would be wearable under the skin as well as over -- imagine fully powered implants where an external battery is impractical or unsightly. There's no word yet on whether there will be refined versions coming to real-world products, but we hope any developments arrive quickly enough to give stretchable electronics a viable power source.