Batteries store energy through chemical reactions. Charging and discharging involves ions moving between electrodes and undergoing electrochemical reactions to store/release energy. Supercapacitors store energy electrostatically – as an electric charge directly on the surface of the electrode material, with no chemical reaction involved.

Hybrid systems differ quite a bit across race series, in energy storage technology, how much power they add, who controls deployment, and the underlying philosophy (efficiency vs. raw overtaking boost). 

The steady spate of IndyCar hybrid failures this season has led to a shrinking supply of spare units for its teams.