Batteries convert chemical energy into electrical energy (and vice versa) by allowing chemical reactions to occur at the battery's terminals when connected to an external circuit. Batteries can switch between charging and discharging very quickly, making them very attractive to couple with solar energy, which can change quickly with the weather. Research aims to increase battery life, reduce discharge time, and reduce environmental impact.
Fuel cells convert the chemical energy of a fuel (commonly hydrogen) into electricity. The ones commonly found in vehicles combine hydrogen and oxygen to make electricity, with only water as a by-product. Fuel cells can also be used for energy storage, where they can be run in one direction when there is too much power on the grid, and the other direction when there is too little. Research aims to cheaply, environmentally, and effectively produce, transport, and store liquid fuels, such as hydrogen and ammonia, in order to meet critical transportation needs, among others.