Flow batteries charge the battery fluid and store the charged fluid in a large tank insolution. It is pumped through the discharge electrodes and the charge drives the load. Depleted fluid goes to a used fluid tank. To charge it is pumped back through the same electrodes = charged = into the charged tank. They suit large stationary uses. The power density is low = limited by the solubility in the transport fluid and the nature of the 1/2 cell in the Vanadium ReDox cycle.1.15 to 1.55 volts). Not sure what DOE patented, but it is far far safer than Lithium non flow storage, as well as cheaper. I am sure this can easily be made in the USA = mainly large tanks etc., but it is not a 'cool' tech. Lots of Vanadium out there. I suspect the low voltage is part of the problem with lithium having 2-3 times the volumetric efficiency.<p><a href="https://en.wikipedia.org/wiki/Vanadium_redox_battery" rel="nofollow">https://en.wikipedia.org/wiki/Vanadium_redox_battery</a>