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1. Fuel cells are different from batteries in that they consume reactant, which must be replenished. While batteries store electrical energy Chemically in a closed system.

2. The major difference between fuel cells and batteries is that batteries carry a limited supply of fuel internally as an electrolytic cells and solid materials (such as the lead acid battery that contains sulfuric acid and lead plates) or as solid dry reactants such as zinc/carbon powders found in a flashlight batteries.

3. Fuel cells have similar have similar reactions: However, the reactants are GASES (hydrogen and oxygen) that are combined in a catalyst process. Since the gas reactants can be fed into the fuel cell and constantly replenished, the unit will never run down like a battery.

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Fuel cells are the most energy efficient devices for extracting power from fuels. Capable of running on a variety of fuels, including hydrogen, natural gas, and biogas, fuel cells can provide clean power for applications ranging from less than a watt to multiple megawatts.

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Batteries are crucial in the global economy transition with their ability to maintain a balance between supply and demand within the power system. The key to decarbonize the world and fight climate change is electrification powered by renewables, which means electrification of cars (e-mobility), buildings and cities.

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The operating temperature of alkaline fuel cells range between room temperature to approximately 250 ˚C and can achieve power-generating efficiencies of up to 70 percent. A diagram of the alkaline fuel cell is shown in Figure 1. The chemical reactions that occur in an AFC are as follows:

Anode: 2H₂ (g) + 4(OH)- (aq) → 4H₂O (l) + 4e-

Cathode: O₂ (g) + 2H₂O (l)+ 4e- → 4(OH)-(aq)

Overall: 2H₂ (g) + O₂ (g) → 2H₂O (l)

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Alkaline fuel cells use an alkaline electrolyte such as potassium hydroxide or an alkaline membrane that conducts hydroxide ions rather than protons. Originally used by the National Aeronautics and Space Administration (NASA) on space missions, alkaline fuel cells are now finding new applications, such as in portable power.

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Polymer electrolyte membrane (PEM) fuel cells, also called proton exchange membrane fuel cells, use a proton-conducting polymer membrane as the electrolyte. Hydrogen is typically used as the fuel. These cells operate at relatively low temperatures and can quickly vary their output to meet shifting power demands. PEM fuel cells are the best candidates for powering automobiles. They can also be used for stationary power production. However, due to their low operating temperature, they cannot directly use hydrocarbon fuels, such as natural gas, liquefied natural gas, or ethanol. These fuels must be converted to hydrogen in a fuel reformer to be able to be used by a PEM fuel cell.

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A nickel-cadmium battery (NiCd or NiCad) is a rechargeable battery used for portable computers, drills, camcorders and other small battery-operated devices requiring an even power discharge. NiCds use electrodes made of nickel oxide hydroxide, metallic cadmium and an alkaline electrolyte of potassium hydroxide.

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Nickel-cadmium battery | Redox reactions and electrochemistry | Chemistry | Khan Academy

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Alkaline battery (zinc manganese oxide, carbon)

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Alkaline battery (zinc manganese oxide, carbon)

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In an alkaline battery, the anode (negative terminal) is made of zinc powder. This allows for a greater surface area which results in an increased rate of reaction and hence a greater rate of electron flow. The cathode (positive terminal) is composed of manganese dioxide. Alkaline batteries are comparable to zinc-carbon batteries, but the difference is that alkaline batteries use potassium hydroxide (KOH) as an electrolyte rather than ammonium chloride or zinc chloride.

They last approximately five times longer than a dry cell, pictured below and have an application in appliances which require intermittent bursts of high current such as a toy car.

The half-reactions are:

Zn (s) + 2OH⁻(aq) → ZnO (s) + H₂O (l) + 2e−

2MnO₂ (s) + H₂O (l) + 2e− →Mn₂O₃ (s) + 2OH− (aq)

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What are the classifications/types of batteries?

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What are the classifications/types of batteries?

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There are two basic types of batteries: primary and secondary. Primary batteries are “single use” and cannot be recharged. Dry cells and (most) alkaline batteries are examples of primary batteries. The second type is rechargeable and is called a secondary battery. Examples of secondary batteries include nickel-cadmium (NiCd), lead acid, and lithium ion batteries. Fuel cells are similar to batteries in that they generate an electrical current, but require continuous addition of fuel and oxidizer. The hydrogen fuel cell uses hydrogen and oxygen from the air to produce water, and is generally more efficient than internal combustion engines.

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In this tutorial, we will learn about one of the important components in Electrical and Electronic Systems: A Battery. We will see some basic information about a battery, take a look at different types of Batteries and also a guide on what Battery Type is suitable for your application.

Introduction

Whether you are an Electrical Engineer or not, you might have come across at least a couple of different types of batteries in your life. Some of the common places where you use batteries are a wall clocks, alarms or smoke detectors, which uses small disposable batteries or cars, trucks or motor cycles, which uses relatively large rechargeable batteries.

Batteries have become a very important source of energy in the last decade or so. Even before that, they were an integral part of our lives in powering several portable devices like transistor radios, Walkman, handheld games, cameras etc.

But with the development in advanced smart phones, tablets, laptops, solar energy and electric vehicles, the research into powerful batteries that can last longer and can deliver the necessary energy has been at its peak.

As a matter of fact, the 2019 Nobel Prize in Chemistry has been awarded to three scientists John B. Goodenough, M. Stanley Whittingham and Akira Yoshino for the development of lithium-ion batteries.

What is a Battery?

A Battery is a chemical device that stores electrical energy in the form of chemicals and by means of electrochemical reaction, it converts the stored chemical energy into direct current (DC) electric energy. Alessandro Volta, an Italian Physicist, invented the first battery in 1800.

The electrochemical reaction in a battery involves transfer of electrons from one material to another (called electrodes) through an electric current.

Cell and Battery

Even though the term battery is often used, the basic electrochemical unit responsible for the actual storage of energy is called a Cell. A Cell, as just mentioned, is the fundamental electrochemical unit that is the source of electrical energy produced by conversion of chemical energy.

In its basic form, a cell typically contains three main components: two electrodes and electrolyte and also consists of terminals, separator and a container. Speaking of electrodes, there are two types of electrodes called the Anode and the Cathode.

The Anode is the negative electrode (also called the Fuel Electrode or the Reducing Electrode). It loses electrons to the external circuit and in the electrochemical reaction, it gets oxidized.

Cathode on the other hand, is the positive electrode (also called the Oxidizing Electrode). It accepts electrons from the eternal circuit and in the electrochemical reaction, it gets reduced. Hence, the energy conversion in a battery is due to electrochemical oxidation-reduction reaction.

The third important component of a cell is the electrolyte. An electrolyte acts as medium for transfer of charge in the form of ions between the two electrodes. Hence, the electrolyte is sometime referred to as Ionic Conductor. An important point to be noted here that the electrolyte is not electrically conductive but just have ionic conductivity.

A battery often consists of one or more "cells" that are electrically connected in either a series or parallel configuration to provide the necessary voltage and current levels.

Different Types of Batteries

Basically, all the electrochemical cells and batteries are classified into two types:

Primary (non-rechargeable)

Secondary (rechargeable)

Even though there are several other classifications within these two types of batteries, these two are the basic types. Simply speaking, Primary Batteries are non-rechargeable batteries i.e., they cannot be recharged electrically while the Secondary Batteries are rechargeable batteries i.e., they can be recharged electrically.

Primary Batteries

A Primary Battery is one of the simple and convenient sources of power for several portable electronic and electrical devices like lights, cameras, watches, toys, radios etc. As they cannot be recharged electrically, they are of "use it and when discharged, discard it" type.

Usually, primary batteries are inexpensive, light weight, small and very convenient to use with relatively no or less maintenance. Majority of the primary batteries that are used in domestic applications are single cell type and usually come in cylindrical configuration (although, it is very easy to produce them in different shapes and sizes).

Common Primary Battery Types

Up until the 1970's, Zinc anode-based batteries were the predominant primary battery types. During the 1940's, the World War II and after the war, Zinc - Carbon based batteries and they have an average capacity of 50 Wh / kg.

Most significant development in the battery technology took place during the 1970 - 1990 period. It is during this time, the famous Zinc / Alkaline Manganese Dioxide batteries were developed and they slowly replaced the older Zinc - Carbon types as the main primary battery.

Zinc - Mercuric Oxide and Cadmium - Mercuric Oxide batteries were also used during this period but due to the environmental concerns with respect to the usage of Mercury, these battery types slowly phased out.

It is during this period, where the development of batteries with Lithium as active anode material has been started and is considered a major accomplishment due to the high specific energy and longer shelf life of Lithium batteries over traditional Zinc batteries.

Lithium batteries are manufactured as button and coin cell for a specific range of applications (like watches, memory backup, etc.) while larger cylindrical type batteries are also available.

The following table shows different types of primary batteries along with their characteristics and applications.

Secondary Batteries

A Secondary Battery is also called as Rechargeable Battery as they can be electrically recharged after discharge. The chemical status of the electrochemical cells can be "recharged" to their original status by passing a current through the cells in the opposite direction of their discharge.

Basically, secondary batteries can be used in two ways:

In the first category of applications, the secondary batteries are essentially used as energy storage devices where they are electrically connected to a main energy source and also charged by it and also supplying energy when required. Examples of such applications are Hybrid Electric Vehicles (HEV), Uninterrupted Power Supplies (UPS), etc.

The second category of applications of secondary batteries are those applications where the battery is used and discharged as a primary battery. Once it is completely discharged (or almost completely discharged), instead of discarding it, the battery is recharged with an appropriate charging mechanism. Examples of such applications are all the modern portable electronics like mobiles, laptops, electric vehicles, etc.

Energy Density of secondary batterie are relatively lower than that of primary batteries but have other good characteristics like high power density, flat discharge curves, high discharge rate, low temperature performance.

Common Secondary Battery Types

Two of the oldest batteries are in fact secondary batteries called the Lead - Acid Batteries, which were developed in late 1850's and Nickel - Cadmium Batteries, which were developed in early 1900's. Until recent times, there are only two types of secondary batteries.

The first and the most commonly used rechargeable batteries are called Lead - Acid Batteries. They are based on the Lead - Lead Dioxide (Pb - PbO2) electrochemical couple. The electrolyte used in these types of batteries is the very common Sulfuric Acid.

The second type of the rechargeable batteries are called Nickel - Cadmium Batteries. They are based on Nickel Oxyhydroxide (Nickel Oxide) as the positive electrode and Cadmium metal based negative electrode. Coming to the electrolyte, an alkaline solution of Potassium Hydroxide is used.

In the recent decades, two new types of rechargeable batteries have emerged. They are the Nickel - Metal Hydride Battery and the Lithium - Ion Battery. Of these two, the lithium - ion battery came out to be a game changer and became commercially superior with its high specific energy and energy density figures (150 Wh / kg and 400 Wh / L).

There are some other types of Secondary Batteries but the four major types are:

Lead - Acid Batteries

Nickel - Cadmium Batteries

Nickel - Metal Hydride Batteries

Lithium - Ion Batteries

Let us now briefly see about these battery types individually.

Lead - Acid Batteries

The lead-acid batteries are by far the most popular and most used rechargeable batteries. They have been a successful product for more than a century. Lead-acid batteries are available in several different configurations like small sealed cells with capacity of 1 Ah to large cells with capacity of 12,000 Ah.

One of the major applications od lead-acid batteries are in the automotive industry as they are primarily used as SLI Batteries (Starting, Lighting and Ignition).

Other applications of lead-acid batteries include energy storage, emergency power, electric vehicles (even hybrid vehicles), communication systems, emergency lighting systems, etc.

The wide range of applications of lead-acid batteries are a result of its wide voltage ranges, different shapes and sizes, low cost and relatively easy maintenance. When compared to other secondary battery technologies, lead-acid batteries are the least expensive option for any application and provide very good performance.

Electrical efficiency of lead-acid batteries is between 75 to 80%. This efficiency value them suitable for energy storage (Uninterrupted Power Supplies - UPS) and electric vehicles.

Nickel - Cadmium Batteries

The Nickel - Cadmium Batteries or simply Ni-Cd Batteries are one of the oldest battery types available today along with the lead-acid batteries. They have a very long life and are very reliable and sturdy.

One of the main advantages of Ni-Cd Batteries is that they can be subjected to high discharge rates and they can be operated over a wide range of temperatures. Also, the shelf life of Ni-Cd batteries is very long. The cost of these batteries is higher that lead-acid batteries on per Watt-hour basic but it is less that other type of alkaline batteries.

As mentioned earlier, the Ni-Cd batteries use Nickel Oxyhydroxide (NiOOH) as Cathode and Cadmium metal (Cd) as anode. Typical consumer grade batteries come with an on-line voltage of 1.2V. In industrial applications, Ni-Cd are just second to lead-acid batteries due to their low temperature performances, flat discharge voltage, long life, low maintenance and excellent reliability.

Unfortunately, there is one major characteristic of Ni-Cd batteries called the "memory effect", which is their only disadvantage. When Ni-Cd cells are discharged partially and then recharged, they lose their capacity progressively i.e., cycle-by-cycle. "Conditioning" is the process where the lost capacity of the batteries can be restored.

In this process, the cells are completely discharged to zero volts and then fully recharged.

Nickel - Metal Hydride Batteries

These are relatively new type of batteries are an extended version of Nickel - Hydrogen Electrode Batteries, which were exclusively used in aerospace applications (satellites). The positive electrode is the Nickel Oxyhydroxide (NiOOH) while the negative electrode of the cell is a metal alloy, where hydrogen is stored reversibly.

During charge, the metal alloy absorbs the hydrogen to form metal hydride and while discharge, the metal hydride loses hydrogen.

One main advantage of Nickel-metal hydride batteries over Ni-Cd batterie its higher specific energy and energy density. Sealed Nickel-metal hydride batteries are available commercially as small cylindrical cells and are used in portable electronics.

Lithium - Ion Batteries

The emergence of lithium-ion batteries in the last couple of decades has been quite phenomenal. More than 50% of the consumer market has adopted the use of lithium-ion batteries. Particularly, laptops, mobile phones, cameras, etc. are the largest applications of lithium-ion batteries.

Lithium-ion batteries have significantly high energy density, high specific energy and longer cycle life. Other main advantages of lithium-ion batteries are slow self-discharge rate and wide range of operating temperatures.

Battery Applications

In the last few decades, the usage of small sealed batteries in consumer applications has been exponential. Primary or rechargeable batteries in small form factor are being used in a huge number of appliances. Some of them are mentioned below.

Portable Electronic Devices: Watches, Cameras, Mobile Phones, Laptops, Camcorders, Calculators, testing equipment (Multimeters).

Entertainment: Radios, MP3 players, CD Players, all infrared remote controls, toys, games, keyboards.

Household: Clocks, Alarms, Smoke Detectors, Flash lights, UPS, Emergency lights, tooth brushes, hair trimmers and shavers, Blood Pressure Monitors, Hearing Aids, pacemakers, portable power tools (drills, screw driver).

How to Choose a Battery?

Selecting a battery for your application can be dialed down to just two characteristics: Performance and Cost. But if we dig a little bit deeper, then the following are determining factors in choosing the right battery for your application.

Primary or Secondary

Energy or Power

Shelf Life

Energy Efficiency and Recharge Rate

Battery Life

Battery Temperature

Conclusion

This was a brief introduction to Battery, Different Types of Batteries, Primary and Secondary Batteries, Rechargeable and Non-Rechargeable Batteries and also few common applications of each type of battery.

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What are the Different Types of Batteries? Primary, Rechargeable, Li-Ion

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In this tutorial, we will learn about one of the important components in Electrical and Electronic Systems: A Battery. We will see some basic information about a battery, take a look at different types of Batteries and also a guide on what Battery Type is suitable for your application.

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Hydrogen fuel cells and batteries are both electrochemical cells. They each have two electrodes in contact with a material that can conduct ions, called an electrolyte. One electrode is the anode and the other is the cathode.

With this both also uses the concepts of RedOx reactions.

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What are the differences between batteries and fuel cells.

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What are the differences between batteries and fuel cells.

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What are the similarities between batteries and fuel cells?

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What are the similarities between batteries and fuel cells?

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Fuel cells work like batteries, but they do not run down or need recharging. They produce electricity and heat as long as fuel is supplied. A fuel cell consists of two electrodes—a negative electrode (or anode) and a positive electrode (or cathode)—sandwiched around an electrolyte. A fuel, such as hydrogen, is fed to the anode, and air is fed to the cathode. In a hydrogen fuel cell, a catalyst at the anode separates hydrogen molecules into protons and electrons, which take different paths to the cathode. The electrons go through an external circuit, creating a flow of electricity. The protons migrate through the electrolyte to the cathode, where they unite with oxygen and the electrons to produce water and heat.

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fuel cell, any of a class of devices that convert the chemical energy of a fuel directly into electricity by electrochemical reactions. A fuel cell resembles a battery in many respects, but it can supply electrical energy over a much longer period of time. This is because a fuel cell is continuously supplied with fuel and air (or oxygen) from an external source, whereas a battery contains only a limited amount of fuel material and oxidant that are depleted with use.

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1. How a battery works - Curious (science.org.au)
2. Fuel cell | Definition, Types, Applications, & Facts | Britannica
3. Fuel Cells | Department of Energy
4. PPT - Fuel Cell Technology PowerPoint Presentation, free download - ID:540292 (slideserve.com)
5. chemistry-redox-alkaline batteries (dynamicscience.com.au)
6. What is a Nickel-Cadmium Battery? - Definition from Techopedia
7. Fuel Cell Basics | Department of Energy
8. An Introduction to Alkaline Fuel Cells (fuelcellstore.com)

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Imagine a world without batteries. All those portable devices we're so dependent on would be so limited! We'd only be able to take our laptops and phones as far as the reach of their cables, making that new running app you just downloaded onto your phone fairly useless.

Luckily, we do have batteries. Back in 150 BC in Mesopotamia, the Parthian culture used a device known as the Baghdad battery, made of copper and iron electrodes with vinegar or citric acid. Archaeologists believe these were not actually batteries but were used primarily for religious ceremonies.

The invention of the battery as we know it is credited to the Italian scientist Alessandro Volta, who put together the first battery to prove a point to another Italian scientist, Luigi Galvani. In 1780, Galvani had shown that the legs of frogs hanging on iron or brass hooks would twitch when touched with a probe of some other type of metal. He believed that this was caused by electricity from within the frogs' tissues, and called it 'animal electricity'.

Luigi Galvani found that the legs of frogs suspended on brass hooks would twitch when prodded with a probe made of another type of metal. He thought this response was caused by 'animal electricity' from within the frog. Image source: Luigi Galvani / Wikimedia Commons.

Volta, while initially impressed with Galvani's findings, came to believe that the electric current came from the two different types of metal (the hooks on which the frogs were hanging and the different metal of the probe) and was merely being transmitted through, not from, the frogs' tissues. He experimented with stacks of layers of silver and zinc interspersed with layers of cloth or paper soaked in saltwater, and found that an electric current did in fact flow through a wire applied to both ends of the pile.

Alessandro Volta's battery: a pile of zinc and silver sheets interspersed with cloth or paper soaked in saltwater. Imagine using that to power your phone. Image source: Luigi Chiesa / Wikimedia Commons.

Volta also found that by using different metals in the pile, the amount of voltage could be increased. He described his findings in a letter to Joseph Banks, then president of the Royal Society of London, in 1800. It was a pretty big deal (Napoleon was fairly impressed!) and his invention earned him sustained recognition in the honour of the 'volt' (a measure of electric potential) being named after him.

I myself, joking aside, am amazed how my old and new discoveries of ... pure and simple electricity caused by the contact of metals, could have produced so much excitement. Alessandro Volta

So what exactly was happening with those layers of zinc and silver, and indeed, the twitching frogs' legs?

The chemistry of a battery

A battery is a device that stores chemical energy, and converts it to electricity. This is known as electrochemistry and the system that underpins a battery is called an electrochemical cell. A battery can be made up of one or several (like in Volta's original pile) electrochemical cells. Each electrochemical cell consists of two electrodes separated by an electrolyte.

So where does an electrochemical cell get its electricity from? To answer this question, we need to know what electricity is. Most simply, electricity is a type of energy produced by the flow of electrons. In an electrochemical cell, electrons are produced by a chemical reaction that happens at one electrode (more about electrodes below!) and then they flow over to the other electrode where they are used up. To understand this properly, we need to have a closer look at the cell's components, and how they are put together.

Electrodes

To produce a flow of electrons, you need to have somewhere for the electrons to flow from, and somewhere for the electrons to flow to. These are the cell's electrodes. The electrons flow from one electrode called the anode (or negative electrode) to another electrode called the cathode (the positive electrode). These are generally different types of metals or other chemical compounds.

In Volta's pile, the anode was the zinc, from which electrons flowed through the wire (when connected) to the silver, which was the battery's cathode. He stacked lots of these cells together to make the total pile and crank up the voltage.

But where does the anode get all these electrons from in the first place? And why are they so happy to be sent off on their merry way over to the cathode? It all comes down to the chemistry that's going on inside the cell.

There are a couple of chemical reactions going on that we need to understand. At the anode, the electrode reacts with the electrolyte in a reaction that produces electrons. These electrons accumulate at the anode. Meanwhile, at the cathode, another chemical reaction occurs simultaneously that enables that electrode to accept electrons.

The technical chemical term for a reaction that involves the exchange of electrons is a reduction-oxidation reaction, more commonly called a redox reaction. The entire reaction can be split into two half-reactions, and in the case of an electrochemical cell, one half-reaction occurs at the anode, the other at the cathode. Reduction is the gain of electrons, and is what occurs at the cathode; we say that the cathode is reduced during the reaction. Oxidation is the loss of electrons, so we say that the anode is oxidised.

Each of these reactions has a particular standard potential. Think of this characteristic as the reaction's ability/efficiency to either produce or suck up electrons-its strength in an electron tug-of-war.

Any two conducting materials that have reactions with different standard potentials can form an electrochemical cell, because the stronger one will be able to take electrons from the weaker one. But the ideal choice for an anode would be a material that produces a reaction with a significantly lower (more negative) standard potential than the material you choose for your cathode. What we end up with is electrons being attracted to the cathode from the anode (and the anode not trying to fight very much), and when provided with an easy pathway to get there-a conducting wire-we can harness their energy to provide electrical power to our torch, phone, or whatever.

The difference in standard potential between the electrodes kind of equates to the force with which electrons will travel between the two electrodes. This is known as the cell's overall electrochemical potential, and it determines the cell's voltage. The greater the difference, the greater the electrochemical potential, and the higher the voltage.

To increase a battery's voltage, we've got two options. We could choose different materials for our electrodes, ones that will give the cell a greater electrochemical potential. Or, we can stack several cells together. When the cells are combined in a particular way (in series), it has an additive effect on the battery's voltage. Essentially, the force at which the electrons move through the battery can be seen as the total force as it moves from the anode of the first cell all the way through however many cells the battery contains to the cathode of the final cell.

When cells are combined in another way (in parallel) it increases the battery's possible current, which can be thought of as the total number of electrons flowing through the cells, but not its voltage.

Electrolyte

But the electrodes are just part of the battery. Remember Volta's bits of paper soaked in salty water? The salty water was the electrolyte, another crucial part of the picture. An electrolyte can be a liquid, gel or a solid substance, but it must be able to allow the movement of charged ions.

Electrons have a negative charge, and as we're sending the flow of negative electrons around through our circuit, we need a way to balance that charge movement. The electrolyte provides a medium through which charge-balancing positive ions can flow.

As the chemical reaction at the anode produces electrons, to maintain a neutral charge balance on the electrode, a matching amount of positively charged ions are also produced. These don't go down the external wire (that's for electrons only!) but are released into the electrolyte.

At the same time, the cathode must also balance the negative charge of the electrons it receives, so the reaction that occurs here must pull in positively charged ions from the electrolyte (alternatively, it may also release negative charged ions from the electrode into the electrolyte).

So, while the external wire provides the pathway for the flow of negatively charged electrons, the electrolyte provides the pathway for the transfer of positively charged ions to balance the negative flow. This flow of positively charged ions is just as important as the electrons that provide the electric current in the external circuit we use to power our devices. The charge balancing role they perform is necessary to keep the entire reaction running.

Now, if all the ions released into the electrolyte were allowed to move completely freely through the electrolyte, they would end up coating the surfaces of the electrodes and clog the whole system up. So the cell generally has some sort of barrier to prevent this from happening.

When the battery is being used, we have a situation where there is a continuous flow of electrons (through the external circuit) and positively charged ions (through the electrolyte). If this continuous flow is halted-if the circuit is open, like when your torch is turned off-the flow of electrons is halted. The charges will pile/build up and the chemical reactions driving the battery will stop.

As the battery is used, and the reactions at both electrodes chug along, new chemical products are made. These reaction products can create a kind of resistance that can prevent the reaction from continuing with the same efficiency. When this resistance becomes too great, the reaction slows down. The electron tug-of-war between the cathode and anode also loses its strength and the electrons stop flowing. The battery slowly goes flat.

Recharging a battery

Some common batteries are single use only (known as primary or disposable batteries). The trip the electrons take from the anode over to the cathode is one-way. Either their electrodes become depleted as they release their positive or negative ions into the electrolyte, or the build-up of reaction products on the electrodes prevents the reaction from continuing, and it's done and dusted. The battery ends up in the bin (or hopefully the recycling, but that's a whole other Nova topic).

But. The nifty thing about that flow of ions and electrons as it takes place in some types of batteries that have appropriate electrode materials, is that it can also go backwards, taking our battery back to its starting point and giving it a whole new lease on life. Just as batteries transformed the way we've been able to use various electrical devices, rechargeable batteries have further transformed those devices' utility and lifespans.

When we connect an almost flat battery to an external electricity source, and send energy back in to the battery, it reverses the chemical reaction that occurred during discharge. This sends the positive ions released from the anode into the electrolyte back to the anode, and the electrons that the cathode took in also back to the anode. The return of both the positive ions and electrons back into the anode primes the system so it's ready to run again: your battery is recharged.

The process isn't perfect, however. The replacement of the negative and positive ions from the electrolyte back on to the relevant electrode as the battery is recharged isn't as neat or as nicely structured as the electrode was in the first place. Each charge cycle degrades the electrodes just a little bit more, meaning the battery loses performance over time, which is why even rechargeable batteries don't keep on working forever.

Over the course of several charge and discharge cycles, the shape of the battery's crystals becomes less ordered. This is exacerbated when a battery is discharged/recharged at a high rate-for example, if you drive your electric car in big bursts of speed rather than steadily. High-rate cycling leads to the crystal structure becoming more disordered, with a less efficient battery as a result.

Memory effect and self discharge

The almost-but-not-quite completely reversible discharge and recharge reactions also contribute to something called the 'memory effect'. When you recharge some types of rechargeable batteries without sufficiently discharging them first, they 'remember' where they were up to in earlier discharge cycles and don't recharge properly.

In some cells, it is caused by the way the metal and the electrolyte react to form a salt (and the way that salt then dissolves again and metal is replaced on the electrodes when you recharge it). We want our cells to have nice, uniform, small crystals of salt coating a perfect metal surface, but that's not what we get in the real world! The way some crystals form is very complex, and the way some metals deposit during recharge is also surprisingly complex, which is why some battery types have a bigger memory effect than others. The imperfections mainly depend on the charge state of the battery to start with, the temperature, charge voltage and charging current. Over time, the imperfections in one charge cycle can cause the same in the next charge cycle, and so on, and our battery picks up some bad memories. The memory effect is strong for some types of cells, such as nickel-based batteries. Other types, like lithium-ion, don't suffer from this problem.

Another aspect of rechargeable batteries is that the chemistry that makes them rechargeable also means they have a higher tendency towards self-discharge. This is when internal reactions occur within the battery cell even when the electrodes are not connected via the external circuit. This results in the cell losing some of its chemical energy over time. A high self-discharge rate seriously limits the life of the battery-and makes them die during storage.

The lithium-ion batteries in our mobile phones have a pretty good self-discharge rate of around 2-3 per cent per month, and our lead-acid car batteries are also pretty reasonable-they tend to lose 4-6 per cent per month. Nickel-based batteries lose around 10-15 per cent of their charge per month, which is not very good if you plan to store a torch for a whole season when you don't need it! A non-rechargeable alkaline battery only loses around 2-3 per cent of its charge per year.

Voltage, current, power, capacity ... what's the difference?

All these words basically describe the strength of a battery, right? Well, sort of. But they're all subtly different.

Voltage = force at which the reaction driving the battery pushes electrons through the cell. This is also known as electrical potential, and depends on the difference in potential between the reactions that occur at each of the electrodes, that is, how strongly the cathode will pull the electrons (through the circuit) from the anode. The higher the voltage, the more work the same number of electrons can do.

Current = the number of electrons that happen to be passing through any one point of a circuit at a given time. The higher the current, the more work it can do at the same voltage. Within the cell, you can also think of current as the number of ions moving through the electrolyte, times the charge of those ions.

Power = voltage x current. The higher the power, the quicker the rate at which a battery can do work-this relationship shows how voltage and current are both important for working out what a battery is suitable for.

Capacity = the power of the battery as a function of time, which is used to describe the length of time a battery will be able to power a device for. A high-capacity battery will be able to keep going for a longer period before going flat/running out of current. Some batteries have a sad little quirk-if you try and draw too much from them too quickly, the chemical reactions involved can't keep up and the capacity is less! So, we always have to be careful when we talk about battery capacity and remember what the battery is going to be used for.

Another popular term is 'energy density'. This is the amount of energy a device can hold per unit volume, in other words, how much bang you get for your buck in terms of power vs. size. With a battery, generally the higher the energy density the better, as it means the battery can be smaller and more compact, which is always a plus when you need it to power something you want to keep in your pocket. It's even a plus for electric cars-the battery has to fit in the car somehow!

For some applications, such as storing electricity at a renewable power plant like a wind or solar farm, a high energy density isn't so much of a problem, as they will most likely have ample space to store the batteries. The main goal for this use would be to simply store as much electricity as possible, as safely and cheaply as possible.

Why so many types?

A range of materials (it used to be just metals) can be used as the electrodes in a battery. Over the years, many, many different combinations have been tried out, but there are only a few that have really gone the distance. But why use different combinations of metals anyway? If you've got a pair of metals that work well together as electrodes, why bother messing around with others?

Different materials have different electrochemical properties, and so they produce different results when you put them together in a battery cell. For example, some combinations will produce a high voltage, very quickly, but then drop off rapidly, unable to sustain that voltage for long. This is good if you need to produce, say, a sudden flash of light like a camera flash.

Other combinations will only produce a trickle of current, but they'll keep that trickle going for ages. We don't need a huge amount of current to power a smoke detector, for example, but we do want our smoke detectors to keep going for a long time.

Another reason to use different combinations of metals is that often two or more battery cells need to be stacked to obtain the required voltage, and it turns out that some electrode combinations stack together much more happily than other combinations. For example, the lithium iron phosphate batteries (a type of lithium-ion battery) used in electric cars stack together to make high voltage systems (100 or even more volts), but you'd never do that with those NiCad Walkman batteries that get hot!

Our different needs over time have led to the development of a huge array of battery types. To read more about them, and what the future holds for battery power, check out our other Nova topics.

This topic is part of our four-part series on batteries. For further reading see types of batteries, lithium-ion batteries and batteries of the future.

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Imagine a world without batteries. All those portable devices we're so dependent on would be so limited! We'd only be able to take our laptops and phones as far as the reach of their cables, making that new running app you just downloaded onto your phone fairly useless.

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But where does the anode get all these electrons from in the first place?

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But where does the anode get all these electrons from in the first place?

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Simply it uses the concepts of "REDOX REACTIONS"

The technical chemical term for a reaction that involves the exchange of electrons is a reduction-oxidation reaction, more commonly called a redox reaction. The entire reaction can be split into two half-reactions, and in the case of an electrochemical cell, one half-reaction occurs at the anode, the other at the cathode. Reduction is the gain of electrons and is what occurs at the cathode; we say that the cathode is reduced during the reaction. Oxidation is the loss of electrons, so we say that the anode is oxidized.

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To produce a flow of electrons, you need to have somewhere for the electrons to flow from, and somewhere for the electrons to flow to. These are the cell’s electrodes. The electrons flow from one electrode called the anode (or negative electrode) to another electrode called the cathode (the positive electrode). These are generally different types of metals or other chemical compounds.

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A battery is a device that stores chemical energy, and converts it to electricity. This is known as electrochemistry and the system that underpins a battery is called an electrochemical cell. A battery can be made up of one or several (like in Volta's original pile) electrochemical cells