Battery Basics Richard Perez A battery stores electrical energy. Batteries are chemical machines. In the battery, chemical energy is converted into electrical energy. Electricity is stored within the battery as potential chemical bonding between the battery's active materials. Batteries are simply chemical engines used to push electrons around. Primary and Secondary Batteries As a battery is charged or discharged its chemical composition changes. In some batteries the chemical reaction is not reversible. This type may only be discharged. It cannot be recharged. Batteries which cannot be recharged are known as "primary" batteries. One example of a primary battery is the disposable zinc-carbon cell used in flashlights. Other types of batteries are rechargeable. The chemical reaction within a rechargeable battery is reversible. Rechargeable batteries are known as "secondary" batteries. They may be emptied and refilled many times. An example of a secondary battery is the lead-acid battery used to start an automobile. How Batteries Store and Transfer Energy The battery converts chemical energy into electrical energy. In rechargeable batteries the conversion process is reversible. Rechargeable batteries can also convert electrical energy into chemical energy. The Cell The conversion and storage processes take place in the basic building block of all batteries Ð the cell. The cell contains the active materials and the electrolyte. Most batteries are composed of many cells because the voltage potential of each chemical cell is quite low ( a few volts at most). The electrical storage capacity of a cell is roughly proportional to its physical size. The larger the cell the more capacity it has. A battery is composed of cells which are assembled together to increase the voltage or the capacity of the battery. INSERT CELL GRAPHIC Active Materials The cell contains two active materials which can react chemically to release free electrons (electrical energy). Such materials are known as "electrochemical couples." The active materials are usually solid. The cell also contains an electrolyte which transfers the electrons between the electrochemical couple. The electrolyte is usually a liquid, a jelly, or a paste. Electrolytes may be either acids or bases (alkaline). In some cells such as lead-acid cells, the electrolyte participates in the chemical reaction in addition to acting as a path for electrons. In other cases, such as nickel-cadmium or nickel-iron cells, the electrolyte does not participate in the cell's chemical reaction, but merely acts as a transfer medium for electrons. During the discharge of a cell, the active materials undergo chemical reactions which release free electrons. During this reaction the chemical compositions of the active materials are changed. The reactants actually become different chemical compounds. When all the original active materials have undergone reaction, the cell will produce no more free electrons. The cell is "dead." In the rechargeable secondary cell the chemical process is reversible. By forcing electrons through the cell in the opposite direction, the active materials can be restored to their original chemical composition. This is know as "recharging" the cell. The cell has polarity: one of the active materials is electron deficient and is positive. The other active material is electron rich and is negative. The flow of electrons while discharging the cell is from the negative pole (cathode) to the positive pole (anode). During recharging the flow is reversed Ð the electrons flow from the anode to the cathode. There are many different chemical compounds which form electrochemical couples. The electrical nature of the cell is determined by the electrochemical couple used. Due to restrictions such as material cost, technical limitations, and material availability, relatively few electrochemical couples are actually used in commercially available cells. Two examples of electrochemical couples commercially produced are the lead-acid reaction and the nickel-cadmium reaction. Energy Storage in Chemical Reactions A charged battery has energy stored within its chemical bonds. The active materials (the electrochemical couple) within the charged battery exist in such a form that the reaction between the materials releases free electrons. These free electrons are available for our use at the battery's output terminals. All elements have electrons revolving around a nucleus of protons and neutrons. Chemical bonding between elements is the exchange or sharing of these electrons. For example, sodium and chlorine are chemical elements. They are distinct materials, each with its own distinct characteristics. When they bond with each other they become salt, which is another totally distinct material. Here is a case of two elements (sodium and chlorine) chemically bonding to form a compound (salt). When this bonding occurs the sodium atom gives up an electron to the chlorine atom. Each atom becomes electrically unstable; they become ions. These ions cling to each other from electrostatic attraction. The resulting compound is more stable than the original elements it is made up of. The entire two-atom system has less energy. Atoms form ionic chemical bonds in order to reach states of greater electrical stability. The science of chemistry deals with the nature of the elements and the myriad forms of bonding which can occur between them. In all chemical reactions which release energy, the materials bond in order to form a more stable structure. The idea is similar to the fact that water runs downhill. It seems that all the materials around us are seeking to form structures of the lowest energy potential-- to become more stable. In batteries the active materials can form more stable structures of lower energy by transferring electrons. The electrochemical couples in batteries may be either elements or compounds. Discharging The addition of a load to the cell's output terminals allows the electrons to be transferred between the active materials. This process is known as discharging. The electrons flow as the materials seek a more stable electrical configuration. The chemical nature of the active materials changes to one of a lower energy level. All cells tend to discharge themselves over a period of time. The electrochemical discharge reaction takes place in the absence of an external load to the cell. The path of the electrons during self- discharging is through the electrolyte. Charging The charging process is simply the reverse of discharging. A voltage is applied across the cell's terminals causing electrons to flow through the cell. In order to overcome the cell's internal resistance the charge voltage must be higher than the output voltage of the cell. The direction of the electron flow is the reverse of that during the discharge cycle. The reversal of this electron flow supplies the energy necessary to return the active materials to their charged state. The chemical bonds made during discharge are broken by the charging process. The active materials regain their higher energy state. They become the original chemical compounds found in a charged battery. The electrical energy is converted into chemical energy. How Cells are Assembled into Batteries Most batteries we encounter are composed of more than one cell. In fact, the word battery means any set of devices arranged or used together. The term "flashlight battery" is actually incorrect when referring to a single flashlight cell. The cell is the basic indivisible unit. A battery is a group of cells. Cells are combined in two configurations to increase the power of the battery. The first method of wiring the cells is in "series." A series electrical circuit has only one path available for the electrons. In the series configuration each cell has its positive terminal attached to the negative terminal of another cell. The second configuration is known as "parallel" wiring. In a parallel electrical circuit there is more than one path for the electrons to travel. In parallel configuration, the cells have their positive terminals interconnected and their negative terminals interconnected. In Series for Voltage Increase All commonly used electrochemical cells have low voltage outputs. The lead-acid cell has an output of about 2.1 volts. The nickel- cadmium cell has an output of 1.25 volts. The zinc-carbon flashlight cell has an output voltage of about 1.5 volts. These are absolute limits on cell voltage. These limits are determined by the potential energy of the electrochemical reaction involved. Size is not a factor in the cells output voltage. Making the cell larger simply increases its capacity, while the output voltage remains constant. Electrochemical cells are interconnected to each other in series in order to use their stored energy at higher voltages. A group of interconnected cells is called a battery. If 2 cells are wired in series the resultant battery will have twice the voltage. If 6 cells are wired in series the resultant battery will have 6 times the voltage of a single cell. For example, an automotive starting battery consists of six lead- acid cells (each 2 volts) in series to give a resultant battery of 12 volts. Some batteries contain all their cells in a single battery casing, some do not. Due to weight limitations very large storage batteries are usually cased as single cells. These are wired in series to produce the appropriate voltage. In some large storage batteries, up to three cells may be housed in the same case. Larger batteries are broken down into smaller units for ease of transport and handling. The basic cell in large storage batteries weighs between 20 and 800 pounds. Another example of series use of cells is in the common flashlight. Two flashlight cells, each a zinc-carbon cell at 1.5 volts, are used in series to provide 3 volts to the bulb. If your flashlight takes 4 dry cells in series then the operating voltage of the bulb is about 6 volts. Figure 1 illustrates the series use of flashlight batteries. INSERT SERIES FLASHLIGHT BATTERY ART A battery consisting totally of cells wired in series has one major drawback. The battery is like a chain, it is only as strong as its weakest link. In a series wired battery the electrons must move through each and every cell. If one cell in the series string is discharged, then the entire string is inoperative, regardless of the condition of the rest of the cells. The output power of the entire battery is limited to that of the weakest cell. Let's say that we have two batteries which we wish to combine in series for voltage increase. Assume that they are both 6 volt batteries (each with 3 lead-acid cells in series) which we wish to combine to get an output of 12 volts. Let's assume that one battery has the capacity of 100 ampere-hours and the other has a capacity of 300 ampere-hours. The resultant 12 volt battery formed by the series wiring of the two 6 volt batteries will have a capacity of 100 ampere- hours. The smallest cell within a series wired battery pack determines the capacity of the pack. When the smallest cell is fully discharged it will not conduct any more electrons. In this state the series circuit is broken. The entire battery is dead, regardless of the state of charge of the rest of the cells. Cells in Parallel for Capacity Increase Cells or batteries (collections of cells) may be wired in parallel to increase the capacity of the resultant battery. When the cells are wired in parallel the voltage stays the same, but the capacity of the battery so formed is increased. The capacity of the resultant battery pack is the sum of the capacities of the individual paralleled batteries which make it up. For example, assume that we have two 12 volt automotive batteries we wish to parallel to increase the capacity of the resultant battery pack (remember the voltage will stay the same-- 12 volts). Each 12 volt car battery is cased individually. In each case there are 6 lead- acid cells in series to produce the output voltage of 12 volts. Let's assume one 12 volt battery has a capacity of 100 ampere-hours and the other has a capacity of 60 ampere-hours. The resultant battery formed by paralleling the two 12 volt car batteries will have a capacity of 160 ampere-hours. In a parallel wiring configuration, all the anodes of the paralleled batteries are connected together, as are all the cathodes. Figure #2 demonstrates the paralleling of a number of car batteries to produce battery packs of larger capacity. INSERT CAR BATTERIES IN PARALLEL ART Series and Parallel Interconnection Used Together In renewable energy applications, the entire battery pack may contain both series and parallel cell interconnection. Since renewable energy battery systems usually run on voltages between 12 and 48 volts, there is always series interconnection between cells. In some cases, the batteries which have been used in series (for voltage increase) are then connected in parallel to increase the capacity of the entire battery system. Figure 3 The basic battery used as a building block in the lead-acid illustration is the Trojan L-16. This is a 6 volt, 350 ampere-hour, high antimony, deep cycle, lead-acid battery. Each L-16 has 3 lead-acid cells in series, all enclosed within a single battery case. Each individual cell has a capacity of 350 ampere-hours. Figure shows how these batteries are configured for increased voltage and capacity. INSERT L-16 ART Figure 4 illustrates some alkaline cell configurations used in home power systems. Although I used a NIFE HIP-10 nickel-cadmium cell as a basic building block, these configurations will work for any sized nicad or nickel-iron cell. The HIP-10 is a high discharge rate, pocket plate nicad cell with a capacity of 100 Ampere-hours. Note that ten series cells are used for 12 Volt operation and twenty series cells for 24 Volt operation. Some 24 Volt systems have upper voltage limits and use nineteen series connected alkaline cells. INSERT HIP-10 ART These same wiring techniques can be used to assemble battery packs of any desired voltage and capacity. The wiring method is the same no matter what type of cell or battery is used. It is important to maintain a balance within the battery pack. It is highly desirable that all the individual cells making up a series string be the same size, type, and age. Capacity A battery is like a bucket. When it is full, it's full and will hold no more. When it is empty, it's empty and will deliver no more. In the case of the bucket, the content is water or whatever. In the case of a battery, the content is electrical energy. Capacity is how much electrical energy the battery will contain. The unit of capacity is the ampere-hour. Ampere-hour is often abbreviated as follows: amp-hr., A-h., and Amp-H. The larger the ampere-hour rating of the battery the larger its capacity. The ampere-hour is the product of the amount of current a battery will deliver and the time over which it will deliver this current. For example, a battery with a capacity of 100 ampere-hours will deliver 1 ampere for 100 hours. The same battery will deliver 10 amperes for 10 hours, or 100 amperes for 1 hour. Batteries come in many sizes to suit many differing applications. Automobile batteries have capacities between 50 to 100 ampere- hours. Large storage batteries in renewable energy systems have many thousands of ampere-hours. Flashlight batteries vary in capacity from .5 ampere-hours to 10 ampere-hours. The physical size and weight of a battery is roughly equivalent to its capacity. State of Charge The state of charge of a battery tells how much of the battery's electric power is available for use. State of charge is like asking, "How full is the bucket?" A battery which has its entire capacity available is said to be at a 100% state of charge. A battery which has had half its capacity removed is said to be at a 50% state of charge. A battery which has had its entire capacity withdrawn is at 0% state of charge. The state of charge of a battery is important because it tells us when it is discharged and needs recharging. It also tells us when the battery is full and when to stop recharging. Rate of Charge or Discharge. The rate of charge or discharge of a battery is expressed in terms of the battery's capacity. This is done even though the rate of charge or discharge is a current which is actually measured in amperes. This is important and confusing. The charge or discharge rate is expressed in amperes as the battery's rated capacity divided by a time factor. This time factor is the amount of time during which the battery is cycled. As an equation it looks like this: I = C / T where: I = Rate of charge or discharge expressed in amperes C = Battery's rated capacity expressed in ampere hours T = Cycle time period expressed in hours For example, consider a fully charged battery with a capacity of 100 ampere-hours. If this battery is totally discharged within a 10 hour period, then the rate of discharge is 10 amperes. Such a rate of discharge is known as a C/10 rate. If the same battery is discharged within a 50 hour period, then the rate of discharge is 2 amperes, or C/50. The same format refers to the charge portion of the cycle. A battery which was fully discharged and is refilled during a period of 10 hours is being recharged at a C/10 rate. Rates of charge and discharge in batteries are commonly referred to as ratios between battery capacity (in A-h) and time. The actual amount of current used in each particular case is dependent on the battery's capacity. This allow us to express rates of charge and discharge in general terms rather than as specific quantities of current. For lead-acids, consider C/5 to be a maximum rate of discharge or recharge. For pocket-plate nickel-cadmium cells, consider C/2 to be a maximum rate of discharge or recharge. And from the lowly cellÉ The more we understand about the electrochemical cell, the more we understand about our battery. This subject can be as deeply demented doo-doo as the nature of the chemical bond, or as simple as, "A Battery is Like a Bucket." I hope you have enjoyed this short trip into the electrochemical cell. With a little encouragement, I'm sure we can delve into exactly how to operate each type of cell. I welcome your feedback. Access Author: Richard Perez, c/o Home Power, POB 130, Hornbrook, CA 96044 ¥ 916-475-3179