The problems of rechargeable lithium ion batteries overheating and even spontaneously combusting are well known. Conversely, cold weather is anathema to such power supplies as they fail to function when it gets too chilly. Now, researchers at Pennsylvania State University have developed a battery that self-heats if its temperature falls below the freezing point of water. "It is a longstanding problem that batteries do not perform well at subzero temperatures," explains Chao-Yang Wang. "This may not be an issue for phones and laptops, but is a huge barrier for electric vehicles, drones, outdoor robots and space applications." The new "all-climate" battery has a nickel foil attached to its negative terminal with the other extending outside the cell to create a third terminal. A temperature sensor allows current to flow when the temperature falls leading to resistive heating and switches off again once the battery is warm enough.