Supercapacitors

Supercapacitors (or supercaps) store energy at significantly higher volumes than typical capacitors. A typical capacitor will have an energy density of somewhere between 0.01 Watt-hours/kg. A supercap has an energy density of 1 to 10 Watt-hours/kg, thus placing them closer to the energy density of a battery that can be on the order of 200 Watt-hours/kg. Like a capacitor, energy is stored electrostatically on a plate, and doesn't involve the chemical transfer of energy like a battery. Usually, supercaps are made of fairly exotic materials like graphene, which can impact on overall cost. Supercaps also have the advantage of charging to their full potential in seconds, whereas Li-ion batteries will charge within minutes to approximately 80%, but require a trickle current to safely get higher. Additionally, supercaps can't be overcharged, whereas Li-ion can be overcharged and can result in serious safety concerns. Supercaps come in two forms: 

• Electric double-layer capacitors (EDLC): Use an activated carbon electrode and store energy electrostatically  
• Psuedocapacitors: Use a transition metal oxide and electrochemical charge transfer

Supercaps have an advantage over batteries in predicting the remaining time power will be available. The remaining energy can be predicted from the terminal voltage, which changes over time. Lithium-ion batteries have a flat energy profile from fully charged to discharged, thus rendering time estimation difficult. Since the voltage profile of a supercap changes over time, a DC-DC converter is needed to compensate for wide variations in voltage.

In general, the main issues with supercaps or capacitors are the leakage current and the cost. As can be seen in the following table, supercaps have their place. One will often see them in a hybrid solution with regular batteries to provide instantaneous power (for example, electric vehicle acceleration), while the battery supply sustains running power.