Understanding Battery Types and Their Functions
BATTERIES
A battery is a series of electrolytic cells used to generate a continuous electrical current or direct current. There are primary cells and secondary cells. Commonly called primary cells or batteries, they produce electricity through an irreversible chemical process and, when depleted, must be removed and replaced. Secondary cells, commonly called accumulators, operate on a reversible principle and can be recharged by connecting them to an adequate source of electrical power.
Every cell has two electrodes immersed in an electrolyte. The electrolyte is a substance, often liquid, that conducts electricity due to its large number of dissociated ions. These are atoms that have lost or gained electrons and therefore have an electric charge. Known examples of electrolytes are solutions of acids, bases, and salts.
When two proper electrodes are immersed in an electrolyte, an excess of electrons occurs at one electrode (negative), and a deficiency of electrons appears at the other (positive). This difference in electrical charge between electrodes creates a potential difference, which can generate an electric current through an external circuit that links the two electrodes. The flow of electrons occurs from the negative electrode to the positive electrode, but by convention, due to historical reasons (do not forget that electrons were discovered long after the invention of electrolytic cells), it is agreed that the current flows from positive to negative.
The Primary Battery
Some experiments in the eighteenth century led to the discovery that when two dissimilar materials, such as zinc and carbon (or copper, used in place of carbon), are immersed in an acid solution (e.g., dilute sulfuric acid in water), chemical action produces an electromotive force between zinc and carbon. The materials immersed in acid are called electrodes, with zinc being negative (or copper) and carbon being positive. The diluted acid solution is called the electrolyte, and the whole stack is referred to as a battery. It is often confused with the term battery, but a battery is actually a basic unit, while two or more cells constitute a battery.
The cells or batteries may be wet or dry, depending on whether the electrolyte consists of a liquid or a paste. The original wet cell, called a voltaic cell, in homage to the Italian scientist Alessandro Volta (who made the first battery cell made with discs of zinc and copper separated by layers of felt soaked in dilute acid), contains zinc and carbon electrodes (or copper) immersed in an electrolyte of sulfuric acid. The modern dry cell or battery has a zinc negative electrode (which forms the outer coating) and a positive carbon electrode. The electrolyte is a thick paste, consisting of a mixture of graphite, ammonium chloride, and manganese dioxide. This type of cell stack is based on Le Clanché, invented by French scientist Georges Leclanché.
When connected externally, the positive and negative terminals generate electric current. In the battery, the zinc electrode will dissolve slowly while it is running. It can reach a point where the zinc corrodes so much that the battery fails. When it reaches this point, the battery has reached the end of its life. In other words, the primary battery has a limited lifespan, after which its operation cannot return to normal conditions.
Zinc-Carbon Battery (Le Clanché)
The most popular and widely used battery is the zinc-carbon type, sometimes called Le Clanché. In this battery, the positive electrode is carbon (C) and the negative is zinc (Zn). The electrolyte is a chemical known as ammonium chloride (NH4Cl), often called salt of ammonia. The negative electrode is in the shape of the container and contains the entire stack. The positive element is shaped like a carbon rod and is placed in the center of the stack. The electrolyte is mixed with starch or flour into a paste (i.e., a dry cell is not really “dry”). When the electrolyte dries out, the battery stops working.
When the battery is functioning correctly, a potential difference (voltage) of 1.6 volts appears between the positive and negative terminals. When the battery is “running out,” either because the electrolyte has dried up or because it has consumed the zinc, the terminal voltage decreases to about 1.1 volts (discharged).
Such batteries are useless once discharged for most applications and cannot be reloaded; they should be discarded. Different combinations of different metals and electrolytes can lead to different voltages between the terminals.
Operation of the Battery
Connect a conducting wire between the terminals of a dry cell zinc-carbon. The electrolyte (NH4Cl) contains ammonium ions (NH4+) and chloride ions (Cl–). When the zinc electrolyte contacts the zinc, zinc ions (Zn2+) enter the solution, leaving two electrons on the negative electrode. The accumulation of electrons creates a negative charge on the zinc. Ions (Zn2+) in solution repel ammonium ions (NH4+) and positive hydrogen ions (H+), which are collected on the surface of the carbon electrode in the form of gas bubbles. The loss of electrons leaves the carbon electrode with a positive charge. Ions (Zn2+) combine chemically with the ions (Cl–) to form zinc chloride (ZnCl2), a white substance. You can see this substance when the batteries get older. The zinc cover is gradually used to form the (ZnCl2) for the operation of the stack.
Electrons grouped on the zinc electrode repel each other. This repulsion, coupled with the attraction of the positive charge on the carbon electrode, results in the formation of the emf (electromotive force) of the stack. This emf causes a current flow of electrons between the electrodes when an external path is provided.
In the process, hydrogen bubbles accumulate on the carbon surface, which affects the proper functioning of the stack, as it hinders and blocks the chemical action. This is called polarization, and to avoid this, manganese dioxide is added to the electrolyte, which combines with hydrogen to form water.
Primary Battery Zinc-Mercury Oxide
Another type of primary battery is the zinc-mercury oxide, invented in the mid-twentieth century. The battery consists of a negative electrode of zinc powder amalgamated or corrugated sheets, while the positive electrode is a mixture of mercury oxide and graphite, die-cast. Both electrodes are contained in a steel container. The electrolyte is a solution of potassium hydroxide and zinc oxide, and a cellulosic material is used as a separator for filling the electrolyte.
The normal voltage of the battery (when not in use) is 1.34 volts, but with a normal flow of current, the voltage drops to values between 1.31 and 1.24 volts. Compared with most other types of primary batteries, the zinc-mercury oxide battery has advantages, such as a voltage that is practically constant over its lifetime (discharge cycle), and its ability to provide electrons to the negative electrode is higher than that of the zinc-carbon battery. It allows for a relatively higher current, which can be maintained for a considerably long time, and also maintains these advantages even at high temperatures. They are relatively expensive, and their application is mainly where their small size (12 to 25 mm in diameter and a few mm in height) is an advantage.
An interesting advantage of this battery is its ability to maintain a constant voltage throughout its lifetime. In many applications, the voltage of this battery is used as a yardstick against which measurement instruments are adjusted.
Secondary Battery (Lead-Acid)
Storage batteries or accumulators consist of sets of secondary cells. A secondary cell may be discharged and then returned to a full state of charge if it is swept by a current or direct current through it in the opposite direction of the discharge. This process can be repeated hundreds of times before the battery runs out.
The battery of a car, for example, consists of a set of lead batteries. Each contains two plates of lead (Pb) shaped as grids to increase the surface area. The negative grid holes are filled with spongy lead, while the positive grid contains lead dioxide or lead peroxide (PbO2). Lead is the negative electrode, and lead peroxide is the positive. The electrolyte is sulfuric acid (H2SO4) mixed with distilled water (H2O). The secondary set of cells formed by a series of alternating plates of lead and lead dioxide are immersed in a solution of sulfuric acid (H2SO4) in distilled water (H2O), which is the electrolyte. Both lead and lead dioxide react with sulfuric acid to form lead sulfate and water. Hydrogen ions are released as sulfate ions, positive and negative. Lead sulfate is practically insoluble in the electrolyte and forms a white deposit on the plates. When both sets of plates are covered, the battery is discharged because there is no potential difference between the plates. When an external DC power source is plugged in to recharge them, hydrogen ions migrate to the negative plates and sulfate ions to the positive. This returns spongy lead to the negative plates and lead dioxide to the positive.
The nominal voltage of a lead cell is approximately 2.2 volts, and a car battery usually consists of six cells connected in series to achieve twelve volts at the terminals of the battery. The lead-acid battery is capable of extremely high currents of several hundred amperes.
Loading and Unloading in the Lead-Acid Accumulator
Analyze the chemical phenomena in a lead-acid battery. When fully charged, the negative plate (negative electrode) is lead, and the positive plates (positive electrodes) are peroxide. The electrolyte is sulfuric acid and water. If we connect a wire between the positive and negative, current flows, and the battery begins to discharge. During discharge, the acid content of the electrolyte decreases, and lead sulfate (PbSO4) is deposited on both plates, positive and negative. Thus, the amount of water increases. This process continues until both electrodes contain a maximum of lead sulfate and the electrolyte density is very low. At that point, the emf between them is minimal.
The battery can be recharged by reversing the direction of the discharge current. This is done by connecting the positive battery terminal to the positive terminal of the battery charger. During the charging process, lead returns to the negative plate and lead peroxide to the positive. Sulfate returns to the electrolyte, increasing its density. During charging, hydrogen and oxygen are evolved, and water should be added to the electrolyte to replace what was lost. This is why water is added to the car battery two or three times a year.
Alkaline Battery
The alkaline battery can be primary or secondary, so named because it has an alkaline electrolyte of potassium hydroxide (KOH), with a negative electrode of zinc (Zn) and a positive electrode of manganese dioxide (MnO2). It typically generates 1.5 volts.
Nickel-Cadmium Battery
The nickel-cadmium battery is a dry secondary battery whose electrolyte is potassium hydroxide. The negative electrode of the nickel-cadmium battery is nickel hydroxide, while the positive is cadmium oxide.
The working voltage, under normal conditions, is 1.25 volts and has a very high memory effect.
The nickel-cadmium batteries are manufactured in a wide variety of sizes and shapes, the most popular of these types being rectangular seals and cylindrical “button” types. In the synthesized plate type, the plates are arranged in groups and connected by welded strips and separated by spacers. The groups of positive and negative plates are mixed and placed in a plastic container.
During the loading and unloading of a nickel-cadmium battery, there are practically no changes in the density of the electrolyte. It only acts as a conduit to transfer the hydroxide ions from one electrode to another, depending on the condition of charging the battery.
The nickel-cadmium battery is the only dry battery that is a real tank with a reversible chemical reaction, which can be recharged many times. Its resistance is degraded and offers reliable service under extreme shock, vibration, and temperature.
Edison Battery
It is a secondary alkaline nickel-iron cell with potassium hydroxide (KOH) as the electrolyte. It is much lighter and more durable than a lead-acid cell. It operates normally at 1.4 volts. It has a positive plate of nickel and nickel hydrate (Ni2O2) and a negative plate of iron (Fe).
Battery Nickel-Metal Hydride (Ni-MH)
This battery uses a positive electrode or anode of nickel hydroxide and a negative electrode or cathode of a metal hydride alloy. Such batteries are less affected by the so-called memory effect. Extreme cold dramatically reduces the effective power that can be delivered. The cell provides 1.2 volts and has a low memory effect.
Lithium-Ion Battery (Li-ion)
The lithium-ion battery (Li-ion) uses a graphite anode and a cathode of cobalt oxide, trifilina (LiFePO4), or manganese oxide. It does not support full discharges or excessive loads. It suffers very little memory effect, so it can be charged without being fully discharged without loss of life. It does not support temperature changes well. It provides 3.2 volts.
Relations in the Output Voltages of the Batteries
An interesting fact about the electromotive force (emf) generated by the batteries is that the output voltage of a battery depends on the type of materials used in it, not its dimensions.
For example, all zinc-carbon batteries with an electrolyte of ammonium chloride give the same voltage supply of 1.6 volts when new. The difference between them is due to the output current that can be supplied. The same is true for lead-acid batteries. A small unit of them has little plates that give an output voltage of 2.2 volts, the same as a large one with a large number of plates.
Relations in the Output Currents of the Batteries
We have seen that if a wire is connected between the negative and positive terminals of a battery, current flows through it. The fact that electrons leave the cell and penetrate the wire is the basis for considering the battery (or battery) as a power source. While continuing the chemical action, the supply of electrons continues. The power of a battery to supply electrons in a certain relation is called current capacity. The maximum number of electrons supplied depends on the amount of active material in the electrodes, the same thing happening with the electrolyte. This explains why a large battery provides more power than a small one.
When expressing the capacity of a battery (a very common practice in accumulators), it is made by the maximum number of amps that can be supplied in an hour. Thus, a battery of 20 amp-hours is a battery that can supply a current of 20 A for one hour, after which it begins to discharge.
If the discharge current is less than the full capacity, then the battery can provide this current for longer than an hour. For example, a 20 Ah battery can provide 1 A for 20 h. Similarly, the current capacity will be proportionately larger for a shorter time, for example, 100 A for 0.2 h, or for 12 minutes. The product of the current in amperes and time in hours cannot exceed the ampere-hour ratio of a given stack.
Finally, the ampere-hour is a basis for correlating the batteries and is used as a measure of the life of the battery before charging again.
Voltage Between the Terminals of a Battery
A battery is a chemical generator of continuous voltage, and all internal components have resistance to current flow. In a chemical cell, the electrolyte resistance between the electrodes is most of the internal resistance of the cell.
When the battery supplies power (because across its terminals is an open circuit), the voltage or potential difference across its terminals is generated at full voltage. This is called open circuit voltage or voltage in vacuum or voltage without load. But if a circuit is closed from the battery across its terminals, the battery delivers current (I) that also circulates inside the battery; its components must go through and overcome internal resistance (Ri). Thus, the voltage (V) at its terminals is diminished by the fall of potential (or voltage drop) that occurs in its internal resistance. Therefore, the voltage at the terminals of a battery or generator is equal to its emf (E) in open circuit (which is the maximum that can be provided) less the voltage drop across its internal resistance (I x Ri).
Connecting Batteries in Series to Form a Battery
Under certain circumstances, the voltage produced by a single battery is enough, but sometimes you need more voltage. This can be achieved by connecting several cells (primary or secondary) in series in a number sufficient to achieve the required voltage.
The emf (E) of a series combination of batteries is the sum of the individual battery emf, and the internal resistance is the total sum of the resistance (Ri) of each stack.
The total voltage of a set of cells connected in series is the sum of the voltages of each cell.
When the batteries are connected in series, a positive terminal is connected to the negative terminal of the other. By doing this, all the individual potentials add up. There is no need for batteries to have the same voltage; batteries of any voltage can be connected in series, but they need to have the same current capacity.
Connecting Batteries in Parallel to Form a Battery
Batteries can also be formed by connecting batteries in parallel, but this can only be done with batteries that have the same output voltage. The purpose of a parallel connection is to increase the current capacity because the parallel connection is equivalent to an increase in the physical size of the electrodes and the amount of electrolyte, which means increasing the available current.
Connecting batteries in parallel does not change the voltage. The final voltage of the batteries in parallel is the same as that of one.
If you connect batteries in parallel with unequal voltages, then current would flow between the batteries due to potential differences and consume electricity.