Energy can be stored in a variety of ways and for different purposes. Stored energy is referred to as potential energy. Water held behind a dam, a tensioned spring or a wound-up toy car is all examples of stored energy. Once they are released the energy held inside them is also released and can be used to perform certain tasks, in a sense be converted to another form of energy.

Electrical energy in a circuit is associated with the movement of electrons in the circuit. Stored electrical energy creates an electric potential that can be used to drive a flow of electrons. This flow is called an electric current. Electrical energy used in the circuit and electrical devices is stored in several ways, with the most commonly used being capacitors and batteries. Capacitors and batteries are also available in different sizes, and capacities, and made of different materials.

Batteries and capacitors are similar in function but vastly different in how they work. Batteries store the electric energy in chemicals, and the energy is converted from chemical energy to electrical energy through a chemical reaction. On the other hand, capacitors are a less common alternative to strong electrical energy. In capacitors, energy is stored in an electric field, which in essence are two conductive plates separated by an insulating material.

The electrical energy stored in these components is used in electrical circuits to power the components. Nowadays, majority of the devices including smartphones, cars, toys and other devices all use the stored energy in one way or another. Thus, engineers have to choose whether to use a battery or capacitors in the circuits or a combination.

Since batteries and capacitors are not interchangeable, in this article we will look into important factors to consider such as the required energy and power density, charge/discharge cycle requirements, voltage and current requirements, and size and weight constraints. Other factors to consider include cost, lifespan, and environmental impact.

What is a battery?

A battery can be defined as a device that stores electrical energy in the form of chemical energy and converts it into electrical energy through electrochemical reactions. Electrochemical reactions create electrons that move through the circuit.

A battery primarily constitutes one or more electrochemical cells, each containing a positive electrode, a negative electrode, and an electrolyte. When the battery is connected to a circuit, a chemical reaction takes place, generating a flow of electrons from the negative electrode to the positive electrode. The battery generates a voltage difference known as a potential difference that forces electrons to move from the negative to the positive terminal. The battery is thus a consistent supply of power, which is used to power devices.

Different types of batteries are designed to perform different types of functions. Some differ in size, chemical constituents, capacity and other factors. In size, some batteries are made to be used in small devices such as watches while others are designed to hold power for plants such as lead acid batteries. Batteries are commonly used in a variety of applications, from powering small electronic devices like cell phones to powering electric vehicles and providing backup power for buildings.

Figure 1: A variety of battery cells

The major classification of batteries is a primary and secondary batteries. The primary battery refers to batteries that contain chemicals whose chemical reactions are irreversible and thus can only be used once and disposed of. On the other hand, secondary batteries are ones whose chemical reactions can be reversed and thus can be recharged once depleted. These batteries are also referred to as rechargeable batteries.

Overall, the choice of battery will depend on the specific application and the performance characteristics required, as well as factors such as cost and environmental impact.

What is a capacitor?

A capacitor also known as a condenser is a device that stores electrical energy in an electric field between two conductive plates separated by an insulating material, called a dielectric. When a voltage is applied to a capacitor, it charges up and stores electrical energy. When the voltage is removed, the capacitor discharges, releasing the stored energy. Capacitors are used to store electrical energy temporarily and release it quickly.

Although capacitors resemble batteries in certain ways, yet they serve quite different purposes. Capacitors are designed for fast energy discharge, in contrast to batteries, which use chemical processes to store electrical energy and release it gradually through an electronic circuit. Capacitors are used in electronic circuits for a variety of purposes, such as filtering out noise, smoothing voltage fluctuations, and storing energy for short periods.

There are many different types of capacitor s, each with different characteristics, advantages and disadvantages. With each of these types of capacitors, different factors such as dielectric material, type of conductor, and size determine the amount of energy that can be stored.

Figure 2: Different types of capacitors

Different types of capacitors use different materials as their dielectric, such as ceramic, electrolytic, film, or tantalum. The better the insulator used as the dielectric material in the gap between the conductors the more energy can be stored. This can affect their capacitance, working voltage, temperature stability, and other performance characteristics.

The Difference Between Capacitors and Batteries

Batteries and capacitors are both energy storage devices, but they work in different ways and have different characteristics. When choosing between a battery and a capacitor, it's important to consider factors such as the required energy and power density, charge/discharge cycle requirements, voltage and current requirements, and size and weight constraints. Other factors to consider include cost, lifespan, and environmental impact.

Some of the key differences between batteries and capacitors include:

1. Energy density

Energy density refers to the amount of energy that can be stored in a unit mass of a device. Typically, batteries have a higher energy density (i.e., the amount of energy they can store per unit mass or volume) than capacitors, which means they can store more energy for a given size or weight.

It is important to note that the energy density of capacitors has been improving and steadily growing over the years with the introduction of new capacitor technologies such as supercapacitors, and lithium-ion capacitors. These are approaching the energy density of batteries.

2. Power density

Capacitors typically have a higher power density (i.e., the amount of power they can deliver per unit mass or volume) than batteries, which means they can deliver more power quickly. Regardless of the higher power density, it is important to understand that capacitors are limited by their energy density.

The higher power density makes capacitors well-suited for applications that require short bursts of high power, such as starting a car engine or delivering power to a motor. This means that capacitors store less energy and make them less suitable for applications where sustained energy delivery is required for a long period such as powering devices.

3. Energy storage mechanism

Batteries store energy through chemical reactions, while capacitors store energy in an electric field.

4. Charge and discharge cycles

When compared to capacitors, batteries have very slow charging and discharging cycles, this is due to the chemical processes that take place to change the chemical process to electrical energy. On the other hand, capacitors store energy on an electric field, thus making the charge/discharge cycles faster.

Batteries can only be charged and discharged so many times before they begin to lose their effectiveness. For instance, a common lithium-ion battery for a laptop or smartphone could only last 300–500 cycles before losing capacity. The battery may only hold a small portion of its initial capacity after several hundred cycles.

Capacitors, on the other hand, maybe charged and discharged repeatedly without degrading. Depending on the precise kind of capacitor and the working circumstances, a well-designed and maintained capacitor can endure hundreds or even millions of charge and discharge cycles.

5. Size and weight

In general, capacitors are smaller and lighter than batteries for the same amount of energy storage. This is because capacitors do not require heavy or bulky chemical reactants or separators, as batteries do. Capacitors can be made from lightweight materials such as ceramics or polymers and can be designed with thin electrodes to reduce their size and weight.

6. Voltage

Batteries typically provide a steady voltage output until they are depleted, while capacitors can discharge rapidly but their voltage output drops as they discharge.

Overall, the choice between batteries and capacitors will depend on the specific application and the requirements for energy density, power density, cycle life, size, weight, and voltage. Batteries are generally better suited for applications that require more energy and longer cycle life, while capacitors are better suited for high-power applications that require quick energy delivery and have a shorter cycle life.

Table 1: A Comparative Look at the Differences between a Capacitor and a Battery

It is vital to remember that depending on the unique chemistry or design of the device, the operating circumstances, and the necessary performance requirements, the specific properties of batteries and capacitors might vary significantly.

Overall, the choice between a battery and a capacitor will depend on the specific application and the requirements for energy density, power density, cycle life, size, weight, and voltage. Batteries are generally better suited for applications that require more energy and longer cycle life, while capacitors are better suited for high-power applications that require quick energy delivery and have a shorter cycle life.