Vast amounts of excess heat are generated by chemical, steelmaking, and power plants, leaving engineers scrambling for ways to harness the wasted energy and put it to work. Efforts have focused on thermoelectric devices that produce electricity by subjecting a solid-state material to a temperature difference of several hundred degrees. But such devices don’t work well when the excess heat is less than 100 °C. Researchers led by Gang Chen of Massachusetts Institute of Technology and Yi Cui of Stanford University have now found an alternative approach based on the thermogalvanic effect (Nat. Commun. 2014, DOI: 10.1038/ncomms4942). To begin, the researchers heated up an uncharged battery made from a copper hexacyanoferrate cathode and copper/copper(II) anode. While at the higher temperature, the battery was charged. Once fully charged, the battery was allowed to cool. Because the charging voltage is lower at high temperature than it is at low temperature, the battery delivered more electricity than was used to charge it. This concept was proposed in the 1950s, Chen says, but suitable electrode materials with low polarization and high charge capacity were not around at the time. Chen cautions that work is needed to improve the speed of battery charging and to ensure battery reliability over an extended period.