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Wikipedia definition (similar term):
A flow battery, or redox flow battery (after reduction–oxidation), is a type of rechargeable battery where rechargeability is provided by two chemical components dissolved in liquids contained within the system and separated by a membrane. Ion exchange (providing flow of electric current) occurs through the membrane while both liquids circulate in their own respective space. Cell voltage is chemically determined by the Nernst equation and ranges, in practical applications, from 1.0 to 2.2 volts. The performance of these devices is governed by the considerations of electrochemical engineering. A flow battery is technically akin both to a fuel cell and an electrochemical accumulator cell (electrochemical reversibility). While it has technical advantages such as potentially separable liquid tanks and near unlimited longevity over most conventional rechargeables, current implementations are comparatively less powerful and require more sophisticated electronics. The energy capacity is a function of the electrolyte volume (amount of liquid electrolyte) and the power a function of the surface area of the electrodes.
Source: Wikipedia - Flow battery