Supercapacitors

Supercapacitors are electrochemical storage devices that work like large versions of common electrical capacitors. They are also known as ultracapacitors or electrochemical double-layer capacitors. Unlike batteries, supercapacitors store their energy in an electrostatic field rather than in chemical form.

Ragone chart showing energy density vs. power density for various energy-storage devices

Supercapacitors are capable of very fast charges and discharges, and can typically be recharged hundreds of thousands of times, unlike conventional batteries which last for only a few hundred or thousand recharge cycles. But their power is available only for a very short duration, and their self-discharge rate is much higher than with batteries. Common applications include starting diesel trucks and railroad locomotives, and in electric/hybrid-electric vehicles for transient load leveling and capturing the energy used in braking. In power systems, they are most likely to be used as bridging power sources in uninterruptible power supplies, much like flywheels.

MC and BC series supercapacitors (up to 3000 farad capacitance) produced by Maxwell Technologies