A lmost indistinguishable among the thousands of steel cargo containers awaiting transport from the French port of Marseilles is one that could mark the start of a sea change in renewable-energy production. The six-metre-long container houses a large-scale modular lithium-ion (Li-ion) battery system developed by France's Saft. Later this year, it will be hooked up to a PV farm in Tudela, northern Spain, to provide energy back-up and smoothe power output to the national grid.