How Rechargeable Batteries Work

Instructor: Alison Gu
In this lesson you will learn how rechargeable batteries, as found in your cell phones, work. You will understand the chemistry behind charging and discharging a battery.

Batteries Everywhere

Let's say that it's a Monday morning. When Fred woke up, he quickly realized that the battery in his alarm clock has died and that he's going to be late to work. He decides to call his boss to inform him. Just as the connection was about to establish, the battery of his phone also gives up. He quickly plugs the phone into a charger and is thankfully able to connect to his boss, but fifteen minutes later, Fred is in his electric car trying to start it… but again, the battery lets him down. For whatever reason, he forgot to charge it the night before. Fred finally manages to reach work on his bike, but that doesn't make him any less stressed or tired.

Just like Fred, our daily lives have become very dependent on many battery-powered devices such as cell phones, laptops, flashlights, electric cars, and even medical devices, just to name a few. Batteries, especially rechargeable batteries, have become our everyday companions. Gone are the days when we had to buy new batteries every time it stopped working. Just imagine buying a new battery every time your cell phone stops working. A rechargeable battery is a battery that can be recharged and used many times and is known as a secondary cell. Gaston Planté, a French physician, developed the world's first rechargeable battery in the year 1859.

Components of a Battery

A battery is an electrochemical cell or series of cells that involve electrochemical redox reactions called oxidation and reduction. Oxidation is a process in which a substance taking part in a chemical reaction loses one or more electrons. This oxidation process results in an increase in the overall charge of the substance. On the other hand, reduction is a process in which a substance taking part in a chemical reaction gains one or more electrons. This reduction process results in a decrease in the overall charge of the substance. A mnemonic that's often used to remember oxidation and reduction is OIL RIG:

Oxidation Is about Loss of electrons and Reduction Is about Gain of electrons

An electrochemical cell consists of three main parts that include two electrodes (the anode and the cathode) and an electrolyte. The anode, or negative electrode, is generally a metal or an alloy of some kind. The cathode, or positive electrode, is usually a metallic oxide or a sulfide. The electrodes are conductors of electricity but are never made from the same conducting materials. The electrolyte is an ionic conductor that separates the two electrodes. It's a medium through which ion transfer occurs between the anode and the cathode. Sulphuric acid is a common electrolyte found in rechargeable batteries.

Representation of a battery

The Discharge Process

To return to our opening example, Fred couldn't speak to his boss on his first attempt because the battery of his phone was already discharged. During discharge, the battery functions as a galvanic cell, where chemical energy is converted into electrical energy.

The discharge process in a battery
The discharging of a battery.

Ions in the electrolyte react with atoms in the anode, and this results in the build-up of electrons, causing the anode to be negatively charged. At the cathode, chemical reactions with electrolytes cause electrons to be used up and which result in the cathode to become positive. We therefore have too many electrons on the anode and a lack of electrons on the cathode. Electrons will tend to move from the higher potential negative site to the lower potential positive site, which is from anode to cathode. We'll be tempted to say that the movement of electrons will occur directly from anode to cathode within the cell, but the electrolyte acts as a barrier. So, how will the electrons move? This can only be done using an external circuit by connecting an electric wire externally between the anode and cathode. The anode undergoes oxidation, since there is loss of electrons, whereas the cathode undergoes reduction, where there is a gain of electrons. There is also a flow of negative ions from the site of reduction to the site of oxidation, through the electrolyte.

The Recharging Process

Remember from our early example that Fred was finally able to speak to his boss after charging the battery of his phone by plugging it in? Here, the oxidation and reduction processes that occurred during discharge are now reversed so that electrical energy is converted back to chemical energy.

The charging process in a battery

During charging, the battery functions as an electrolytic cell. When the cell is connected to an outside energy source, the electrons in the cathode are forced to flow back to the anode. There is movement of electrons from an external power source to the anode. On the other side, electrons are removed from the cathode.

Once again, electrons bond to the ion in the anode, thereby allowing the battery to be recharged. When the battery is fully charged, there is a surplus of electrons on the anode, giving it a negative charge, and a deficit on the cathode, giving it a positive charge, resulting in a potential difference across the cell.

Factors Affecting Battery Life

Let's look at the three main factors that can affect the life of a rechargeable battery.

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