A flow battery is a rechargeable fuel cell in which an electrolyte containing one or more dissolved electroactive elements flows through an electrochemical cell that reversibly converts chemical energy directly to electricity. Electroactive elements are "elements in solution that can take part in an electrode reaction or that can be adsorbed on the electrode." Additional electrolyte is stored externally, generally in tanks, and is usually pumped through the cell (or cells) of the reactor, although gravity feed systems are also known. Flow batteries can be rapidly "recharged" by replacing the electrolyte liquid (in a similar way to refilling fuel tanks for internal combustion engines) while simultaneously recovering the spent material for recharging. Many flow batteries use carbon felt electrodes due to its low cost and adequate electrical conductivity, although these electrodes somewhat limit power density due to their low inherent activity toward many redox couples.
In other words, a flow battery is an electrochemical cell, with the property that the ionic solution (electrolyte) is stored outside of the cell (instead of in the cell around the electrodes) and can be fed into the cell in order to generate electricity. The total amount of electricity that can be generated depends on the volume of electrolyte in the tanks.
Flow batteries are governed by the design principles established by electrochemical engineering.
Redox flow batteries, and to a lesser extent hybrid flow batteries, have the advantages of:
* Independent scaling of energy (tanks) and power (stack), which allows for a cost/weight/etc. optimization for each application
* Long cycle and calendar lives (because there are no solid-to-solid phase transitions, that cause degradation of lithium-ion and related batteries)
* Quick response times
* No need for "equalisation" charging (the overcharging of a battery to ensure all cells have an equal charge) 
* No harmful emissions
* Little to no self-discharge during standing
* Full recycling of electroactive materials
Some types also offer easy state-of-charge determination (through voltage dependence on charge), low maintenance and tolerance to overcharge/overdischarge.
They are safe because they typically do not contain flammable electrolytes, and electrolytes can be stored away from the power stack.
Flow batteries have three main disadvantages compared to batteries with solid electroactive materials
* Low energy density (you need large tanks of electrolyte to store useful amounts of energy)
* Low charge and discharge rates (compared to other industrial electrode processes). This means that the electrodes and membrane separators need to be large, which increases the cost of power.
* Flow batteries have a lower energy efficiency, because they operate at higher current densities to minimize the effects of the cross-over (internal self-discharge) and to reduce the cost of power.
Flow batteries typically have a higher energy efficiency than fuel cells, but lower than lithium-ion batteries.