Through the process of electrochemical oxidation-reduction (redox), a battery is a device that directly transforms chemical energy found in its active components into electric energy. Electrons are transferred from one material to another during this kind of reaction through an electric circuit.

The actual electrochemical unit used to produce or store electric energy is called a cell, even though the term "battery" is frequently used.

To comprehend the distinctions between a cell and a battery, consider that a battery is made up of one or more of these cells connected either in series, parallel, or both, based on the capacity and desired output voltage.

Envision a future devoid of batteries. There would be a lot less of those portable electronics on which we rely so heavily! We would be limited to using our phones and laptops within the reach of their wires, which would render that new app you just downloaded for your phone very meaningless.

Fortunately, batteries are available. The Parthian civilization in Mesopotamia utilized a gadget called the Baghdad battery circa 150 BC. It was composed of copper and iron electrodes and contained vinegar or citric acid. According to archaeologists, these were mostly utilized for religious rituals rather than as batteries.

The first battery was created by the Italian scientist Alessandro Volta in order to refute another Italian scientist, Luigi Galvani, and is credited with creating the battery as we know it today. Galvani demonstrated in 1780 that frogs suspended on brass or iron hooks would twitch their legs when contacted by a probe made of a different kind of metal. He referred to this as "animal electricity" since he thought it was caused by electricity coming from the frogs' tissues.

Although first astonished by Galvani's results, Volta eventually came to the conclusion that the electric current originated from the two distinct types of metal—the probe's varied metal and the hooks the frogs were hanging on—and was only passing through the frogs' tissues. He experimented with stacks of layers of silver and zinc alternated with layers of cloth or paper drenched in seawater, and found that an electric current did in fact flow through a wire applied to both ends of the pile.

Volta also discovered that the voltage could be raised by utilizing various metals in the pile. In a letter to Joseph Banks, the Royal Society of London's president at the time, he reported his findings in 1800. Napoleon was largely impressed, therefore it was a significant event! and he received ongoing acknowledgment for his creation when the "volt," a unit of measurement for electric potential, was called in his honor.

The chemistry of a battery
An apparatus that stores chemical energy and transforms it into electrical energy is a battery. An electrochemical cell is the system that powers a battery. This process is known as electrochemistry. One or more electrochemical cells, as in Volta's original pile, can make up a battery. An electrolyte separates the two electrodes in every electrochemical cell.

So where does the electricity come from in an electrochemical cell? We must first define electricity in order to respond to this question. To put it most simply, electricity is a form of energy that is created as electrons move. In an electrochemical cell, a chemical reaction at one electrode (more on electrodes below!) produces electrons, which subsequently flow to the other electrode to be consumed. We must examine the parts of the cell and their assembly in greater detail in order to fully comprehend this.

In order to generate an electron flow, there must be a source and a destination for the electrons to travel through. The electrodes of the cell are these. From one electrode, known as the anode (or negative electrode), to another, known as the cathode (the positive electrode), electrons move. Usually, these are distinct kinds of metals or other substances.

The zinc in Volta's pile served as the anode, and electrons from it moved to the silver, which served as the battery's cathode, via the wire (if it was attached). To create the entire pile and increase the voltage, he placed many of these cells together.

However, where does the anode initially obtain all of these electrons? And why do they feel so content to be despatched on their way to the cathode? It all boils down to the internal chemistry occurring within the cell.

We must comprehend a few chemical reactions that are occurring. Electrons are produced at the anode by an interaction between the electrode and the electrolyte. At the anode, these electrons build up. Simultaneously, a chemical reaction takes place at the cathode, allowing that electrode to receive electrons.

Redox reactions, also known as reduction-oxidation reactions, are the formal scientific name for reactions involving the exchange of electrons. One half-reaction takes place at the anode and the other at the cathode of an electrochemical cell. The complete reaction can be divided into two half-reactions. The cathode experiences reduction, which is the gain of electrons; during the process, we refer to the cathode as being reduced. We refer to the anode as oxidized oxidation is the loss of electrons.

Every one of these reactions has a standard potential that is unique. Consider this property as the reaction's vigor in an electron tug-of-war—its capacity or efficiency to either produce or absorb electrons.

Since the stronger conducting material can absorb electrons from the weaker one, any two conducting materials that react with differing standard potentials can form an electrochemical cell. However, a material that yields a reaction with a much lower (more negative) standard potential than the one you select for your cathode would be the perfect choice for an anode. In the end, we have electrons from the anode being drawn to the cathode (with the anode not attempting to resist too much), and we can use their energy to power our phone, torch, or other device by giving them a simple method to get there: a conducting wire.

The force that electrons will use to move between the two electrodes is roughly equal to the difference in standard potential between them. The voltage of the cell is determined by this, which is referred to as its overall electrochemical potential. The voltage increases with the electrochemical potential, which increases with the difference.

There are two ways we can raise the voltage of a battery. To increase the electrochemical potential of the cell, we might select different materials for our electrodes. Or, we may arrange many cells in a stack. A specific combination of cells (in series) affects the battery's voltage in an additive manner. In essence, the force that drives electrons through the battery can be understood as the total force that the electrons experience from the anode of the first cell to the cathode of the last cell, or however many cells the battery has.

The possible current of the battery, or the total number of electrons passing through the cells, is increased when cells are joined in a different manner (in parallel), but not its voltage.

But the battery consists of more than just the electrodes. Do you recall the bits of paper that Volta dipped in briny water? Another important component of the equation was the electrolyte, which was the salty water. Any material that permits the passage of charged ions can function as an electrolyte, whether it be a liquid, gel, or solid.

Since electrons have a negative charge, we must find a means to counteract the charge shift that occurs when negative electrons circulate through our circuit. The medium that the electrolyte offers allows positive ions that balance charges to move through.

In order to keep the electron's charge balance neutral, an equal amount of positively charged ions must be created in addition to the electrons produced by the chemical process at the anode. These are released into the electrolyte instead of going down the external wire, which is just for electrons.

The process that takes place here must draw positively charged ions from the electrolyte in order for the cathode to balance the negative charge of the electrons it receives. Alternatively, it may release negative charged ions from the electrode into the electrolyte.

Therefore, the electrolyte provides a conduit for the transfer of positively charged ions to balance the negative flow, while the external wire provides a pathway for the flow of negatively charged electrons. In the external circuit that powers our electronics, this flow of positively charged ions is equally as vital as the electrons that provide the electric current. They play a crucial part in charge balancing, which keeps the entire response going.

Now, if every ion that was discharged into the electrolyte was permitted to flow through it without restriction, it would eventually coat the electrode surfaces and block the entire system. Therefore, the cell usually has a barrier to stop this from occurring.

During battery operation, electrons are continuously flowing through the external circuit and positively charged ions are flowing through the electrolyte. The flow of electrons is stopped if this continuous flow is interrupted—if the circuit is open, like when you turn off your torch. The chemical reactions powering the battery will cease when the charges accumulate.

New chemical products are produced when the battery is depleted and the reactions at both electrodes continue apace. These reaction products have the potential to provide a resistance that will stop the reaction from proceeding as efficiently. The response slows down when the resistance increases too much. Additionally, the cathode-anode electron tug-of-war weakens and the electron flow ceases. The battery gradually runs out of power.

Recharging a battery
Certain common batteries—also referred to as primary or disposable batteries—are meant just for one usage. The electrons go in a single direction from the anode to the cathode. Their positive or negative ions are released into the electrolyte, depleting their electrodes, or the reaction is stopped by the accumulation of reaction products on the electrodes, ending the reaction completely. The battery is disposed of in the trash (or, ideally, recycled; however, it is a topic for another Nova article).

However, The interesting thing about the ion-electron flow that occurs in some types of batteries with the right electrode materials is that it may also flow in the opposite direction, reviving our battery and returning it to its initial state. Rechargeable batteries have further improved the usefulness and lifespan of many electrical products, just as batteries have changed the way we may utilize them.

The chemical process that happens during discharge is reversed when we connect an almost empty battery to an external power source and inject energy back into the battery. As a result, the electrons that the cathode absorbed as well as the positive ions that were released from the anode into the electrolyte are returned to the anode. Your battery is recharged when positive ions and electrons flow back into the anode, priming the system and making it operational once again.

But the procedure isn't flawless. As the battery is recharged, the negative and positive ions from the electrolyte are replaced back onto the appropriate electrode, although this process isn't as orderly or well-structured as it was initially. Even rechargeable batteries eventually lose their ability to function since each charge cycle causes the electrodes to deteriorate slightly more.

Several charge and discharge cycles cause the crystals in the battery to lose some of their order. This is made worse by fast rates of battery discharge and recharging, such as when driving an electric automobile in abrupt acceleration or deceleration rather than continuously. High-rate cycling causes the crystal structure to become more disorganized, which results in a battery that is less effective.

Memory effect and self discharge
The "memory effect" is another phenomenon that is influenced by the almost but not entirely reversible discharge and recharge reactions. Certain rechargeable battery types "remember" their previous discharge cycles and exhibit improper recharging when they are recharged without being fully discharged beforehand.

The way the metal and electrolyte combine to produce a salt and how that salt dissolves and replaces the metal on the electrodes during cell recharging are the reasons behind it in some cells. We want the ideal metal surface in our cells to be covered in tiny, homogenous salt crystals, but that's not how things actually work in the real world! Some battery types have a greater memory effect than others because of the very intricate ways that some metals deposit during recharge and how some crystals form. The temperature, charge voltage, charging current, and initial battery charge status all have a major role in the flaws. Our battery accumulates a bad memory over time as a result of flaws in one charge cycle that can lead to the same in subsequent cycles, and so on. For some cell types, such nickel-based batteries, the memory effect is strong. Other varieties, like lithium-ion, are not affected by this issue.

Rechargeable batteries also have a higher inclination towards self-discharge due to the chemistry that makes them rechargeable. This is the point at which internal battery reactions take place even in the absence of an external circuit connecting the electrodes. Over time, the cell loses some of its chemical energy as a result of this. A high rate of self-discharge severely reduces battery life and causes batteries to die while being stored.

Our lead-acid automobile batteries also have a quite reasonable self-discharge rate—they typically lose 4–6% per month—while the lithium-ion batteries in our phones have a pretty decent self-discharge rate of about 2-3% per month. If you plan to store a torch for a whole season when you're not using it, nickel-based batteries tend to lose 10–15% of their charge each month! The annual loss of a non-rechargeable alkaline battery's charge is just about 2-3%.

Voltage, current, power, capacity … What's the difference?

I take it that all these terms essentially define a battery's strength? Sort of, anyway. However, they are all slightly unique.

The force at which the battery's reaction forces electrons through the cell is known as voltage. This is often referred to as electrical potential, and it is determined by the potential difference between the reactions that take place at each electrode, or, more specifically, by the strength with which the cathode will draw electrons from the anode (through the circuit). The same amount of electrons can do more work at a higher voltage.

The quantity of electrons that are presently flowing through a circuit at any particular time is known as the current. It can accomplish more work at the same voltage with a larger current. Another way to conceptualize current inside a cell is as the quantity of ions flowing through the electrolyte multiplied by their charge.

Voltage times current equals power. A battery may operate at a faster rate the more power it has; this relationship illustrates the importance of both voltage and current in determining the appropriate use of a battery.

The power of a battery expressed as a function of time is called capacity, and it indicates how long a battery will be able to power a device. When a battery has a higher capacity, it can last longer before draining completely or running out of electricity. A sad little quirk of some batteries is that their capacity decreases if you try to draw too much from them too rapidly since the chemical reactions can't keep up! Thus, we must always exercise caution while discussing capacity and keep in mind the intended application of the battery.

An additional widely used phrase is "energy density." This is the most energy a gadget can store in a unit of volume, or how much power is available for the size of the device. When it comes to batteries, the higher the energy density, the more compact and smaller the battery may be, which is always a benefit when you need to power anything you want to carry around in your pocket. For electric vehicles, it's even a benefit—the battery needs to fit inside the vehicle somewhere!

A high energy density isn't always a concern for some applications, including energy storage at renewable power plants like wind or solar farms, where there will probably be plenty of room for the batteries to be stored. Simply said, the major objective for this purpose would be to store as much electricity as possible in an economical, safe manner.

Why so many types?
The electrodes in a battery can be made of a variety of materials; formerly, they could only be metals. Though countless combinations have been tried over the years, very few have truly stuck with one another. However, why not just utilize other metal combinations? Why tamper with other metals when you have two that function nicely as electrodes together?

When different materials are combined in a battery cell, their electrochemical characteristics cause them to behave differently. Certain combinations, for instance, can swiftly produce a high voltage but are unable to maintain it for very long. This works well when you need to create a sudden light burst, similar to what a camera flash does.

Some combinations will only generate a very small amount of current, but they will do so for a very long time. For example, we don't require a lot of current to operate a smoke detector, but we do want them to last for a long period.

It turns out that some electrode combinations stack together much more happily than other combinations, which is another reason to employ different combinations of metals when obtaining the necessary voltage, which often requires stacking two or more battery cells. For instance, you could never create a high voltage system (100 or more volts) using the lithium iron phosphate (Li-ion) batteries used in electric cars; yet, you could use those hot NiCad Walkman batteries!

Over time, a vast variety of battery types have been developed in response to our changing needs. See our other Nova themes to learn more about them and the prospects for battery power in the future.