Forbes: "Most batteries are packaged into a confined space, which makes them handy for powering your iPad or your Tesla. But if you wanted to play Real Racing HD on your iPad for a month straight or drive your Tesla from Seattle to Miami on a single charge, you'd need a battery the size of a freight train.
And even then, it'd be pretty expensive. But there is a way to make a battery last a long time, or store lots of power, or both. It requires blowing up a battery's tight little package, storing the guts of the battery outside its body.
These batteries are called flow batteries. The chemical that allows the battery to charge or discharge is held in vessels outside of the battery and it flows between the electrodes when it is time to do some work. Want it to last longer? Build a bigger tank. Want it to store more power? Beef up the electrodes."