How Do Batteries Work?

Aurora, IL

Batteries are used in almost all of our electronic devices nowadays.

Batteries are an essential part of almost every type of technology we use today. From smartphones to tablets to smartwatches, batteries keep our devices powered on. Batteries are storage units that contain electric current used to power the subcomponents of a device. What are batteries actually made of though? How do they send electricity to a device?

The first documented instance of a battery is the Baghdad battery, a device composed of copper, iron electrodes, and citric acid. However, it was actually intended to be used for religious ceremonies. The first real battery is attributed to Alessandro Volta, an Italian scientist. His creation was actually in response to a “discovery” made by a fellow Italian scientist known as Luigi Galvani. Galvani touched frog legs with two bars of different metals and discovered that the legs twitched. He credited this to “electricity” inside the animal. Volta did not believe this and decided to disprove him. Volta made a stack of layers of silver and zinc along with saltwater-soaked paper or cloth between the metals. On either end of the stack, he placed two rods of differing metals and found that electricity was conducted. This crude stack of various materials was the first battery ever made and earned Volta recognition from the Royal Society of London. The volt, a unit of electricity, was named after him.

Volta's crude pile of silver, zinc, and paper was the forefather of the modern battery.

Volta’s layers of zinc and silver were able to conduct the electricity because they represented different metals that the electricity could flow through. Electricity itself is made up of electrons, tiny units of energy that carry a negative charge. For the battery to work, the electrons need to flow from one place to another. These areas are called electrodes. Because electrons have a negative charge, they start out in the negative electrode (anode) and flow through a semipermeable barrier (more on this later) to the positive electrode (cathode). This is why many batteries today (e.g. AA or AAA batteries) have plus and minus symbols on them. In Volta’s battery, the zinc was the anode and the silver was the cathode. In batteries today, there is also an electrolyte solution filling each side of a battery. The anode chemically reacts with the electrolyte to make electrons. When these electrons fill up the anode side of the battery, they pass through a semipermeable barrier to reach the cathode side. Semipermeable barriers only allow some molecules through, explaining why only the electrons flow to the other side. On the cathode side, there is another chemical reaction that lets the electrons into the cathode. These chemical reactions are commonly known as reduction-oxidation reactions, otherwise known as redox reactions. Once there are a sufficient number of electrons on the cathode side, they can flow into another device through some kind of wire to power laptops, phones, calculators, and so much more.

A standard potential is the tendency for a material’s reaction to create or take in electrons. Standard potentials define the efficiency of a battery. Ideally, anodes are made of materials that create reactions that have lower standard potentials. Conversely, cathode materials should have reactions with much higher standard potentials. This way, there will be lots of electrons made at the anode and the cathode will take them in quickly and effectively. The difference in standard potentials is the battery’s electrochemical potential. This potential dictates the battery’s voltage. Higher voltage indicates a more efficient battery because there is less energy loss.

This diagram neatly depicts the electrodes of the battery as well as the flow of ions inside.

Some batteries are one-use, meaning that once the anode is done making electrons, the battery is done, or flat. However, more and more batteries nowadays are rechargeable, especially those found in portable devices like laptops or phones. This means that the electron flow is not one way; instead, when the battery is recharging, the electrons can flow into the anode instead of the cathode. When the battery is dead, its few electrons are in the cathode area. When it is connected to another electricity source, like a power plug, all of the new electrons flow to the cathode area. Soon, the cathode area fills up with electrons. When molecules are crowded together, they find ways to become less crowded. That’s why all of the electrons in the cathode area will pass through the semipermeable barrier to the anode. Once the charging is done, the battery will be as good as new and will be able to provide for its device again. However, after every charging cycle, batteries slowly degrade as their electrodes’ structures are not as perfect as they were at the beginning. This is mostly because of the heat created from charging cycles. As ions are moving through the battery cell, they generate heat. The electrolyte solution containing the anodes and cathodes crystallizes as an effect of the heat, making it more difficult for ions to pass through the semipermeable barrier. These crystals lower the efficiency of the battery. On iPhones, you can even see what health your battery is at through the “Battery Health” feature. A common industry standard for smartphones is 80% battery health, as in 80% of their original capacity, after 800 charging cycles, or about 2 years if you charge your phone almost every day.

Overall, a battery is a very important and interesting component of a device. Technological advancements are being made every day on batteries. Scientists are researching new materials for batteries to make them more energy-dense. Graphene, a material known for being only one atom thick, is being experimented on to see if it can store large amounts of energy. This would revolutionize energy storage around the world. In the nearer future, some companies are transitioning from lithium-ion chargers to gallium nitride (GaN) chargers, which are more efficient in energy transfer because they give off less heat. All in all, batteries have demonstrated their immense versatility and are set to only become more useful in the future.