There is a type of cell known as Redox Flow or Vanadium Fuel Cell. A conventional battery has solid electrodes such as lead that participate in the chemical reaction and an electrolyte that serves as an ion transport medium. A redox flow cell by contrast has a solid ionic membrane that permits the exchange of ions but not electrons and two liquid electrolyte compartments. Carbon electrodes are used to connect electrically to the electrolyte but they do not participate in the chemical reaction.

The electrolyte consists of vanadium sulfate in different oxidation states. In the positive electrolyte vanadium has an oxidation state of V5+ when charged and V4+ when discharged, in the negative electrolyte vanadium has an oxidation state of V2+ when charged and V3+ when the cell is discharged.

There is a design by which a lot of these cells can easily be placed in series to obtain the required output voltage (1.6 volts per cell charged, 1.1 volts discharged).

The only limit to the capacity of the battery is the size of the electrolyte tanks. The power output is limited by the active area of the membrane. Because the active chemicals do not undergo a phase transition between liquid and solid as they do in lead acid cells the cell does not deteriorate with charge discharge cycles. The self discharge rate is also extremely low.

My idea is that these could be used in an electric car. And then you’d have an infrastructure where instead of plugging them in and charging them, when you’ve discharged the electrolytes you go to a recharge station and the spent electrolyte is drained from tanks and fresh electrolyte supplied. The spent electrolyte is then recharged in a large stationary cell at the station during power off-peak times.

At the risk of a slightly more complex filling system (because it also has to accommodate draining and keep the positive electrolyte separate from the negative), you can have the convenience of a gasoline car but powered by electricity, and the exchangeable electrolyte system would allow charging to occur during off-peak hours regardless of when you actually refill the vehicle.

I do not know the energy density or power density of this type of battery and as a result I do not know if size and weight limitations would render it impractical for vehicle propulsion. Nearly all the information I can find is intended for stationary applications such as power load leveling where size and weight are not important considerations.

I also do not know how dangerous vanadium sulfate salts would be if released in an automotive accident. I’d love to here from someone knowledgeable on these issues that could supply this data.