Not many people are aware of these, and they are a very important technology.
Just like a normal battery you can charge it with electricity. But since all the power is stored in the liquid electrolyte (2 tanks of the same chemical, one positively charged, the other negatively), you can also "fuel up" the battery by pumping the spent electrolyte out and pumping charged electrolyte in. The electrolyte will eventually have to be refined as impurities build up, but it has a long lifetime.
Take a few minutes to browse the sites of some of the companies/universities working on them.
http://www.vrbpower.com/energystoragesystem.htmlhttp://www.ceic.unsw.edu.au/centers/vrb/http://www.kepco.co.jp/english/rd/solution/solution_2.htmlhttp://pepei.pennnet.com/Articles/Article_Display.cfm?Section=Articles&Subsection=Display&ARTICLE_ID=143512&KEYWORD=vanteckhttp://www.ceic.unsw.edu.au/centers/vrb/Magnam-VFuel%20website%20info.htm