Nice to have some options to compressed air and pumped hydro. Flow batteries are different from traditional batteries. With your car battery, the discharge reaction goes on until all the liquid acidic electrolyte is spent. Then you need to charge it back up again. There is a limit to how much energy can be stored.
With flow batteries, they have two tanks of electrolyte, one in a charged state and one in a discharged state. The charged state electrolyte is pumped into the battery as it is discharging and converting that electrolyte into spent material. When the battery is charging, the flow reverses and the spent electrolyte is zapped into charged up electrolyte.
Like a conventional battery, the amount of peak power available from such a battery is determined by the size of the battery. Unlike with a conventional battery, the energy capacity of the battery depends on how big of tanks of electrolyte you hang off it.
Alternative Energy Storage and Blood in the Streets - Seeking Alpha - 13 Oct 08
"ZBB is a transition stage manufacturer of zinc bromine flow batteries and the clear leader in the 'hours of discharge time' product class. Their pre-production prototypes are priced in the $600 per kWh range and will compete primarily with pumped hydro and compressed air storage.
There is no question that pumped hydro and compressed air will be the technologies of choice for utility-scale diurnal [aka daily] storage installations. But when it comes to small-scale storage for homes, businesses and remote villages, flow batteries like ZBB’s are likely to be the best choice."