What Is the Difference Between Battery and Capacitor?

Battery and capacitors both store energy, and it is natural to have a doubt about their functions and differences. It is necessary to understand their basic differences.

What is a battery and how does it work?

Battery is an electrochemical device used for energy storage. It has a system of electrodes and electrolytes, the latter being chemicals which react with electrode materials and accept / deliver electrical energy in the process. These reactions can be irreversible (primary battery) or reversible (rechargeable, or secondary battery). Energy is delivered to a circuit by converting chemical energy through chemical reactions to electrical energy when needed.   

When discharging, chemical reactions take place, electrical energy is generated and current is drawn from battery. Both discharge and charge processes are chemical reactions, which involve change of form of energy (chemical to electrical and vice versa). In some batteries, these reactions are irreversible (primary batteries), and battery is dead once the chemicals reactions exhaust the electrolyte. In secondary, or rechargeable batteries, the reactions are reversible- they go in both directions, and chemicals are restored by passing current through battery in reverse direction. Battery is ready to deliver electrical energy again, and charging / discharging of battery can thus be done repeatedly for a large number of times over its lifetime.

There is loss of energy in chemical reactions in both directions by way of heat. It uses electrical energy while charging, to carry out reactions in chemicals stored in it to form new chemicals. Thus, energy output usable for work is much lower than energy spent in charging the battery. Lead acid battery efficiency has typically round trip efficiency of 50-65 % (charging and discharging full cycle), while Li-ion battery has conversion efficiency of around 80 %. Since chemical reactions are involved, there is some delay (response time) in responding to sudden changes in load, till reaction rate is stabilized to the new situation.

Battery is functional so long as its terminal voltage is above a certain minimum level, below which it becomes unusable. It delivers power by generating electricity through chemical reactions. This change of form of energy is what makes it an active component. Battery voltage is more or less constant during its use from fully charged conditions till it is nearly drained of its usable energy. 

How does a capacitor store and deliver energy?

Capacitor stores energy in electric field as electric charge for a given voltage. It is a passive element, and can store electrical energy when the same is supplied to it by applying electrical voltage across its terminals. It does this by virtue of polarization of dielectric material between its electrode plates in the presence of electric field created by external voltage. Electric charges of opposite polarities get stored on capacitor plates in the process, and the system remains in equilibrium till the dielectric remains polarized. Extent or level of polarization is decided by the electric field (or external voltage). Energy is absorbed by the dielectric and held to maintain the polarization state. Thus, electrical energy is stored effectively by way of charges stored on capacitor plates, decided by the capacitance value and applied voltage. When a load is connected across capacitor plates, charges get discharged through it, voltage is reduced, and electrical energy stored gets depleted.

Energy is thus absorbed as electrical energy, and when needed, is drawn as electrical energy. No conversion in other form of energy is involved. So efficiency of capacitor can go over 98 %. The only losses are in wires / connections or very small resistances.  Supercapacitors, having very high values, thus store large amount of energy this way, with a round trip efficiency of over 98 percent. Capacitor can adjust to sudden load change without any perceptible delay (quick response time), since no reaction or change of form of energy is involved.

In a capacitor, energy stored depends on voltage across it, which voltage goes down as its energy is being used up. So while using ultracapacitors for energy, it is necessary to use a booster circuit (or DC/DC converter) which keep output voltage constant for load despite falling capacitor voltage. The converter is effective till its input voltage (capacitor terminal voltage) drops to half of its rated voltage. Hence, about 75% of full rated ultracapacitor energy is usable.

This, then, is basic difference between battery and capacitor. To summarize, here is a table.

Property	Battery	Capacitor
Device	Electrochemical	Electrical
Type	Active Component	Passive Component
Energy Storage Method	Electrochemical Energy	Electrical Energy
Voltage Stability	Stable throughout Discharge	Drops as Energy is Drawn
Round Trip Energy Efficiency	50 to 80%	98% and above
Response Time	Slow	Rapid, almost instantaneous

Battery is an active component, and a source of electrical energy for a circuit. It works so long as its chemicals are active. Its voltage is practically constant till it is able to deliver energy, or its chemicals are not depleted. The chemicals may or may not be restored, depending on type of battery. Most rechargeable batteries get depleted over their life – a few hundred or thousand cycles of charging and discharging.

Capacitor stores energy when it is charged by external source of energy. It can then discharge this stored energy to a component or circuit attached to its terminals. A capacitor cannot work in the absence of a source of electrical energy which can impart energy to it, which can then be delivered / discharged as and when needed. There is no change in the form of energy between storage and delivery, and it works only when a source is available to charge it. This is what makes capacitor a passive element. Unlike a battery, as current is drawn from capacitor, voltage across capacitor goes down. Charging and discharging of capacitor may be done millions of times without any degradation of its properties.