Differences between capacitor, ultracapacitor and battery

Capacitor and Ultracapacitor

Conventional capacitors are devices having two (or two sets) of electrodes separated by a dielectric medium. They are of two broad types- electrostatic and electrolytic. Electrostatic capacitors have been made since the discovery of Leyden’s jar, while electrolytic capacitors are about a century old. Ultracapacitors were developed towards the end of 20^th century, and most of its development has taken place since past two decades.

Electrostatic capacitors have distinct separate dielectric material. Electrode material is a conductor either as a separate plate, foil, some other shape or a coating, or deposit of conductor on the dielectric surface (as in metallized capacitors). Dielectric thickness varies from a few microns to several millimeters. Voltage rating is usually over 100 V. Values of electrostatic capacitors are from ppF to few hundred microfarad range. These are rated for both DC and AC applications. They have very low ESR, and very low power factor.

Electrolytic capacitors have dielectric in the form of a thin oxide layer on electrode surface, and anode electrode is in the form of foil, plate or some other shape.  An electrolyte effectively works as cathode (negative electrode), which also serves to maintain and replenish the oxide layer. A cathode collector (foil, plate or some other shape) is used to carry the current from electrolyte to external negative terminal. Values of electrolytic capacitors are from few microfarads to few thousand farads (few milli farads).

Ultracapacitor (also called Supercapacitor) is an electrochemical capacitor (EC), based on nanotechnology materials. Electrode is made from a highly porous material like activated carbon having extremely high effective surface area (e.g. 2500 sq. m./gm), or a Pseudocapacitor material. Electrolyte is from a large number of aqueous or organic compositions which can impregnate the porous electrode and form contact with the large surface area available. Energy storage is one through double layer capacitance or pseudocapacitance. Electrochemical basis of charge storage of these capacitors They fill the gap between a capacitor and battery regarding energy storage and power delivery.

The construction makes extremely large values of capacitance possible, which go in Farad range, or even thousands of Farads. Because of the extremely high values possible, these capacitors got the name ‘ultracapacitor’ or ‘supercapacitor’. They are also known as Electrical Double Layer Capacitor (EDLC), Double Layer Capacitor (DLC), Digital Energy Storage Device (DESD) etc. going by their construction and usage.

Voltage of these capacitors may vary from 1.8 V to 4.2 V depending on construction. Voltage rating is decided by decomposition voltage of electrolyte, which limits the voltage. The extremely high values enable them to store energy far above that of electrostatic or electrolytic capacitors, though about an order lower than batteries.

Ultracapacitor and Battery

Though level of stored energy in ECs is an order below that of battery, their main advantage lies in its fast response to voltage / current changes. They accept electric charges during charging and store energy them as electrical charges (or electrical energy). There is no conversion of energy is involved while discharging or charging, and charges are directly discharged into circuit. Hence their response time for voltage change is several times faster than batteries.

Further, since no chemical reactions are involved, and it is only acceptance / delivery of charges, charge / discharge life of ultracapacitors is several times longer than batteries. Chemical reactions in batteries during charging and discharging make the processes slow, and reaction heat is lost in both processes. This limits their ability to deliver peak power.

Typically, time constant RC of EC capacitors is of the order of 1 sec. or lower. So a large capacitor can be charged fully between 1-5 seconds. If 1000 F capacitor is charged to 2.5 Volt within 1 second at constant current (2500 coulombs), the current is a whopping 1000 Amps! Corresponding power works out to 2700 W (2.5KW). This is what gives ultracapacitors a power capability several time higher than batteries.  This makes ECs popular as battery backup device. 

Materials in ultracapacitors do not undergo any chemical reactions (which depend on temperature), and are materials naturally occurring in nature. They can withstand much wider temperature range from -30 to +70℃. Batteries typically have working range from -10 to +40℃. Their performance deteriorates at temperatures go below 10℃, and above 35℃. Ultracapacitors score hands down in this respect.

Energy density of EDLCs varies typically 5 to 20 Wh/Kg, with low ESR up to 100 Ω, while pseudocapacitors and hybrid capacitors have much higher densities. Lead acid battery has energy density of about 45 Wh/Kg, while Lithium ion batteries have densities ranging from 80 Wh/Kg to over 200 Wh/Kg.

Ultracapacitor thus scores over battery in all respects, except energy density. New developments in past few years have taken storage levels of ultracapacitors much higher – from 45 Wh/ Kg to 80 Wh/Kg, and researchers are working on ways to improve this still further.

Life of ultracapacitors in terms of charge / discharge cycle stability is way ahead of batteries, often over a million cycles, compared to just 500 – 1000 cycles for batteries. This makes them last for decades without degradation. This makes them ideal replacement for batteries in some high power applications, or in remote / inaccessible locations.

To summarize

Ultracapacitors fill the gap between capacitors and battery. They can be used independently as energy storage devices for few applications, and can extend battery life significantly when used with battery. Ultracapacitors have made several new technology applications feasible and easier because of their unique properties.

Property	Capacitor	Ultracapacitor	Battery
Energy storage	Watt Second of energy	Watt Second of energy	Watt Hours of energy
Energy Density	< 0.1 Wh/ Kg	1-10 Wh/Kg	35- 200 Wh/Kg
Power Density	High, > 10,000 W/Kg	High, ~ 4000 W/Kg	Low, 50-200 W/Kg
Charging Method	Voltage across terminals from source	Voltage across terminals from source	Current and voltage method from source
Fastest Charging Time	< 1 millisecond	0.3-30 sec.	1-16 Hr.
Fastest Discharge Time	< 1 millisecond	Seconds to minutes	0.3 to 3 hours
Operating Voltage	1.2-4.2 V/Cell	5 V – 600 V	1.2 – 4.2 V/cell
Voltage Stability	Decays with discharge	Decays with discharge	Constant Voltage
Charge/Discharge cycle life	300-1000	>500,000	>500,000
Self-discharge rate	Negligible	Up to 30% per month	<5% per month
Charge-Discharge efficiency	~ 100%	90-98%	70-85%
Operating Temperature	-20 to +100℃	-20 to +70℃	-20 to +55℃