So, in the context of batteries a few specific terms, I told you earlier that we have the batteries and is an energy storage device and so, what is the significance of it? It means that the fuel and the oxidant are stored within the now within the device. So, the significance of this is that the size of the battery is some indication of how much chemical is there in the battery and therefore, how much how long it can operate. So, for example, you go to the shop and you buy some you know same brand battery, you can buy you know what we call the pen torch battery or you can buy the triple-A battery which is even thinner than that or you can buy D cell or C cell which are bigger batteries. So, for the same brand same kind of chemistry that is there, the thinner battery will last lesser time the thicker battery will last a long time if you use it to power the same device. So, let’s say you connect the thinner battery to power led, that led let us say will run for 10 days if you connect it to a thicker battery which is twice that the volume let us I am assuming if it's twice that volume instead of 10 days it will now run for 20 days. So, the size of the battery decides how long it can operate in energy another kind of device which is the energy conversion device, the chemicals are stored outside of the device they are not stored in the device. So, later when we look at fuel cells that would be an energy conversion device and you know the chemical being stored outside or being stored in the inside may not seem like much of a difference, but operationally it’s a big difference because it means that the size of the device does not decide how long it will last. So, even with a small device, you can have it running for an indefinite period because you are just supplying the chemicals from outside and that’s why those devices are interesting and are more flexible. And in fact, a device that you are used to very commonly which you have not thought of in this in the context of this terminology is the internal combustion engine. So, your internal combustion engine or ICE internal combustion engine which is what is the engine that is there and say your moped or your motorcycle or car whatever no. So, there the patrol is sitting in the petrol tank. So, the size of the engine if you say I have a 1.2-litre engine, that does not tell you how long that engine will run that how long the engine will run or how long the car will run which depends on the size of a petrol tank how much petrol you have got in the petrol tank. So, you fill your petrol and it will last for the duration of that petrol tank, and as it gets close to empty you simply refill petrol and you can keep going. So, that is the convenience of an energy conversion device you simply have to tank up change the tank and keep going whereas, in an energy storage device like a battery you have to do recharging because that chemicals have to be now you know corrected in some form or in other words reversed in some form within that device and that takes time. So, that is the difference ok. (Refer Slide Time: 40:01) Also in this context, we have this idea of a cell and a battery. So, this is something we should at least be aware of I mean we may still use it in commonplace terminology and that’s how I even used it earlier part of this class, but strictly speaking what we refer to as a battery and we say you know we tell somebody in our house saying a go to the shop and buy some battery, and come right that actually at that point we are asking them to go and buy. A cell we are not asking them to go buy a battery is a collection of cells in series or parallel. So, if you take you to let’s say television remote and you open it right. So, inside that you will have two batteries right actually what you have are two cells, and there they may be sometimes pointing in the same direction in which means that two cells are sitting in parallel or you may have them inverted, one-pointed in this way and the other pointed the other way in which case they are in series. So, two cells there are sitting in either series or parallel based on how the remote has been designed, then that combination is called a battery. So, you similarly even a remote control toy, in the toy you will have some few cells in your remote you will have a few cells, you will find in some case it is sitting in series some case it is sitting in parallel this combination is called a battery. Strictly speaking, that is the terminology that is used and each one of them individually is called a cell. So, that’s how the terminologies are used, but we of course, in common usage we tend to simply call it a battery. So, at least we should be aware of this definition we may not use it very strictly, but if you ever run into the word cell and you run into the word battery you should know the difference. So, this is the idea ok. (Refer Slide Time: 41:34) Also in this context, we have something called a primary cell and something called a secondary cell, these are the terms that are used in a more technical sense primary cell versus secondary cell. A primary cell is a single-use power source. So, these are the kinds of cells that cannot be recharged in other words the chemicals that have been used in that the chemistry that is being used in that cell is such that it is not reversible chemistry. So, once the reaction is complete, you cannot simply flow current in the opposite direction and get those reactions to go back to their previous condition the reactants to go back to their previous condition or to make the products go back to being the reactants. So, that is not possible with that. So, we have a lot of those you know cells that we buy in the shop which are single-use cells. So, you use it in your remote, once it is done you throw it away you go buy a new cell right. So, we don’t necessarily keep recharging that. So, that kind of a cell is referred to as a primary cell that is the correct technical term for it. Similarly, the opposite is true for what is a secondary cell, these are the cells that we in common usage refer to as rechargeable batteries. That we say rechargeable battery you know that term that we use is the term that is being used for this technically indicated item as a secondary cell. We what has been indicated technically as a secondary cell that is what we are referring to as a rechargeable battery. So, these are cells which where if you reverse the direction of current you will have the opposite reactions happening or the reactions will happen in opposite direction, and you will get back the original reactants that you started with and that’s a secondary cell. And therefore this, and so you should keep that also in mind that not every cell is reversible so you that is why you know you see those warnings in those you know rechargeable units saying you cannot just put any cell that you want in that, only that specific cell you have to put in there and recharge because there will also something associated with the voltage, first of all, some cells cannot be reversed. So, if you send force current through them, they may just explode or something bad may happen. So, you don’t want that happening, only some cells can be recharged, only they can be they should be recharged and even there you should only put it in a specific recharger and not just you cannot take different brands and just throw them into different rechargers, because the recharging unit is designed to work with this chemistry work correctly with that voltage associated with that chemistry and so, you should not arbitrarily mix them. So, this is the difference. (Refer Slide Time: 43:52) So, now there are there’s another aspect of this battery technology that we should be aware of the voltage that we measured. The standard electrochemical potentials that we measure, you know the potential of that combination that power that you then you know subtract one potential from the other and you get the voltage of the cell etcetera right that is the thermodynamics of the cell that refers to the thermodynamics of the cell. So, that is the theoretical condition which represents the driving force for the cell to operate. It represents the possibility of the reaction to occur you know the drive for the reaction to occur etcetera. When we draw current from the cell that represents the rate at which the reaction is happening that is called. So, the thermodynamics gives a cell voltage ok. So, thermodynamics which represents the ability of that reaction to happen gives us the cell voltage when and so, that ability is something that we measure in typically measure in open circuit conditions. So, we are. So, you just you know put a voltmeter across your cell and you measure a voltage that cell voltage represents the thermodynamics of that system and represents the ability of that cell to perform. On the other hand when you draw current from it that represents the kinetics of the system, that represents how fast that reaction will happen okay and that is the cell current. So, cell current is the kinetics cell voltage is the thermodynamics. So, these are two very important parameters we often do not realize it, but these are the two that decide many things in our in nature. In nature many things are decided with thermodynamics and kinetics, so for example, if you take a piece of wood right you take a piece of wood and you light a fire, the wood burns. So, another why does that happen? Because the wood tends to burn it what is happening when you wood burns, wood burns and then all the carbon in the wood becomes carbon dioxide right. So, and the fact that it is burning or continuing to burn means the carbon prefers to become carbon dioxide. So, carbon dioxide is a more stable condition. So, if you allow carbon to react with oxygen, it will react with oxygen and form carbon dioxide ok. So, this ability of carbon to react with oxygen and form carbon dioxide and this you know the tendency of carbon, this tendency of carbon to react with oxygen and form carbon dioxide is represented by its thermodynamics that tendency is captured by thermodynamics. So, that is why you get delta g for that reaction, the delta g for that reaction would be negative; that means, it is spontaneously it has a direction in which it will tend to go the C will react with O2 and form CO2 ok. So, it a that is there is a driving force for that reaction ok. So, it is trying to happen, but it is also important to understand that when I keep a piece of wood in the air I just keep it on the table or even the table itself is wood why does it not immediately burn?. Now there is a tendency that you know you understand that carbon dioxide you would it would rather be carbon dioxide than carbon why does it not burn you just keep it there it doesn’t just spontaneously burn and disappear into carbon dioxide it does not because you have a driving I mean you have an activation energy barrier, there is an activation energy barrier. So, that is how. So, you will have the reactant and some level, you will have the product at this level and. So, this is the energy difference this is the energy, and therefore, since you have this drop in energy when you go from C2 to O2 a two. From C to CO2 that is a desired state of affairs, but to go to this distance you have to go past an activation energy barrier and it is this activation energy barrier that is what makes the world stable the way we see it. So, that is why know their clothes we wear things that we use etcetera are not just spontaneously catching fire and you know getting oxidized, even though the oxide will be the more stable constate in which it will be there. So, there is an activation energy barrier, which prevents a reaction from happening even though thermodynamically the reaction is trying to happen. So, in the context of things we have to understand the kinetics the rate at which the reaction is happening is an important aspect and so, in the and that rate is often affected by the activation energy barrier. So, the activation energy barrier decides you know how easy or difficult is it is for the same reaction to occur you make the activation energy barrier smaller the reaction will happen faster okay and that’s what the catalyst does. Whenever you use a catalyst that is what it is doing is it is reducing the activation energy barrier; therefore, making it easier for the reaction to happen, but the ability of the reaction to happen is the same. So, this point here and this point here are not changed, only the barrier has changed. So, you may have a smaller barrier or a larger barrier and that. So, if you want. So for example corrosion: corrosion is something is an undesirable reaction. So, we do things to increase the barrier for corrosion. So, we are increasing the activation energy barrier for the corrosion on the other hand. So, if you are talking of a corrosion reaction, you are trying to do this we are trying to increase the barrier. If you are on the other hand you are talking of some you know battery kind of situation, where you want the reaction to occur fast you are trying to do this than to reduce the activation energy barrier. So, that is what you are trying to do concerning the activation energy way. So, this is the point. So, there is thermodynamics and there is kinetics, and it’s a combination of these two that you see eventually as the performance of your cell in terms of voltage and current respectively. (Refer Slide Time: 49:04) And the cell characteristics are captured by a few different terms, we typically only look at voltage. So, we typically keep referring to the voltage in the cell. So, this is the point that we keep referring to that’s the open-circuit voltage, the current is what we draw from the cell, the rate at which the reaction is occurring the rate at which those electrons are being drawn and so, the combination of that those two gives you the power in watts. So, that is simply V into I. So, that is the characteristic of the cell. The capacity of the cell is a term that we use which represents the total charge that is available in the cell, and that charge is in typically in coulombs or ampere-hours. So, that is basically whatever current you are drawing times the duration for which you draw the current. So, that is where if you have the same you know chemistry and you have a smaller battery and you have a larger battery, the capacity of the larger battery is more than the capacity of the smaller battery. So, if you even if you draw the same current, that let’s say it is twice the size the duration for which it will last is twice as long. So, even if you are drawing the same current if one battery lasts one hour, the thinner battery lasts one hour the thicker battery is twice its size let’s say the thicker battery will last two hours. For the same purpose, so if you are lighting some product I mean someplace and you want the light to be on for two hours you should use the thicker battery, and you don’t want to change the battery right. So, the time comes in there. So, the time is associated with this process in this manner and that’s how we get it and the available energy is power into time. So, the current into time gives you the capacity, power into time gives you the available energy and that is in joules or watt-hours ok. So, this is how we get the different parameters that we use to understand what is possible. So, the power the energy that is available also includes you know the fact that it has it is not just the current that is coming out, but the current is coming out at some voltage that is why you have. So, you have V into I into time this is energy and of which this is power right, so V into I into time. So, the energy equals V into I into time, the capacity equals me into time ok. So, that is the capacity how long will it last this is; what is the total available energy. So, this is the. So, this is amperes and that is an hour. So, ampere-hour you will get this is watts and that is an hour, so watt-hours you will get. So, this is the different parameters here ok. (Refer Slide Time: 51:48) So, in conclusion, we have just now briefly looked at all the basic ideas associated with a battery. So, our main conclusions here are the batteries have specific parts that can have dramatically opposite functions, and that is what I meant what I mean here is the of the idea that you have an electrode phase and you have an electrolyte phase and what we are demanding of the electrolyte phase in terms of conductivity is the opposite of what we are demanding from the electrode in terms of conductivity. The electrode should have good electronic conductivity, electrolyte should have good ionic conductivity and they should be in opposition to each other, but as a reaction, they will all assist each other. So, that’s the point. The electrochemical series is the starting point to understand battery voltages, but as I pointed out the electrochemical series is under standard conditions. So, this is standard conditions always please remember that these are standard conditions, but that is the correct place for you to start because otherwise, it's confusing you mean there is just no end to all the conditions you can set these various materials too. So, we set some standard conditions and that is this standard electrochemical series which is the place where we look at the standard electrode potentials which means the electrodes are set under some standard conditions. Any non-standard condition can be derived from starting from the standard condition and that is why this way it is a nice the process, and there you subtract one electrode from the other electrode you will get the open circuit potential and then you can decide which way the reaction is happening. And that potential itself may change so, you have from the standard electrochemical series you have one that is above which is noble, one that is below that is active. So, if you take the difference then you will find that this is the open circuit potential and this is the. So, what is above is the one that will undergo reduction, what is below is the one that will undergo oxidation. So, this lower electrode is your anode the upper electrode is your cathode. But if you go to non-standard conditions this potential can go low or high this potential can also go high or low. So, supposing this were to go high and that was to go low because you have gone to non-standard conditions, then what originally. So, now, we can see that their relative positions have reversed. So, what originally served as the cathode will be served as the cathode will now become the anode what originally served as the anode can become the cathode. So, you can see that all these things are possible concerning you to know electrochemical combinations that are possible. And I also told you that there are primary and secondary batteries that are and these are all very commonly available and commonly used, primary batteries are single-use batteries secondary batteries are rechargeable batteries. So, that’s our basics that I felt are you know relevant for the discussion on batteries, we will look at some battery systems and how batteries perform and so on and these you know phenomena will all be relevant in that context. So, that’s the discussion and main conclusions for today’s class. Thank you. In this class, we will look at Battery Testing and Performance. In our previous class, we looked at some of the basic concepts associated with batteries as electrochemical devices. So, we will build on it and we will look at battery testing and performance. This is very important to understand because whatever as we mentioned you know whatever renewable energy source we use; often batteries are there as part of the overall scheme to enable you to know more uniform delivery of power or at least delivery as required by the customer. And in this context, you will always find so many manufacturers of batteries who are selling different chemistries of batteries, who are selling you know even with the same chemistry somebody will claim that their battery performs better than somebody else is battery and so, on. So, we need to understand what is the process by which we test a battery and you know what are some parameters; we should look at when we think in terms of performance of the battery. So, that is the kind of issues that we will look at in this class. (Refer Slide Time: 01:14) So, what we will do is we will in this in terms of learning objectives; what we will do in this class is we will first draw a schematic of the typical battery test process. So, the typical battery test process we will draw a schematic and just get an understanding of what happens during a battery test process. And then we there is something called a C-Rate. So, we will try to understand the significance of the C-Rate; what is the C-Rate, how is it indicated and what is the significance of it and how does it matter? We then become familiar with typical discharge and charge curves. So, you can call it charge-discharge curve or discharge charge curve whatever way you want to call it. So, there are such curves that are generated for batteries. So, we will try to understand; what are a typical kind of a curve for it and I mean how do we you know understand it and utilize it? Then we will also since we have learnt about the C-Rate here and we have learnt about this discharge and charge curve, we will try to understand the effect of the C-Rate on the charge-discharge curve. So, we learn C-Rates independently, you will learn charge-discharge curve independently then you understand what is the interaction between them. What happens if you change the C-Rate, what happens to the charge-discharge curve? So, that’s something we look at and then we will also try to understand another parameter which is the polarization curve. So, what is the polarization curve and again how is that used to keep track of what a battery does and understand water battery does. So, these are all our learning objectives schematic of the battery test process, C-Rate, charge-discharge curve, the effect of C-Rate on the charge-discharge curve and the significance of the polarization curve. So, this is what we will cover in today’s class. (Refer Slide Time: 02:59) And so, let’s start with the first point here which is the schematic of a typical battery test process. So, battery, as we saw, has an anode an electrolyte and a cathode. So, that’s the three major parts of battery; so, you can simply draw it that way a battery anode-cathode electrolyte the three rectangles and your all set. So, when we buy a battery and we put it you use in anything, you could put it to use in a toy, you could put it to use in a remote for some you know a television or whatever it is that you are putting it useful. What happens is during the life of the battery; so, you buy it now and then you know let’s say it lasts for a month and then the battery eventually drains out. So, during this time we are making a wide range of different demands on the battery at different points in time. So, there may be a point let’s say you're I mean let’s say it’s a toy that you are using. So, then at some point, you are putting the toy to go through a steady run for some time; at that point the demand from the battery is something. Then suddenly you get it to accelerate to much higher speeds, during the acceleration process the demand from the battery is different. Then you stop the toy for some time and only leave the lights of the toy on. So, if you leave the lights of the toy on; that the demand on the battery is different. So, in this process you have used the battery in different ways for the let’s say the; for the 15 minutes that you have been playing with a toy. Same thing with the remote control; so, ost of the time the remote control is sitting idle except maybe doing some internal you know testing or internal electronics you know some kind of you know charging up that’s going on internally. You press some buttons at that point some signal goes. So, at that point it is you know more actively consuming battery power, you are continuously changing channels; so, then again battery power is getting consumed. Then you stop and then nothing is happening from the battery, again you press to increase the volume or decrease the; again you demand something different. So, the point being whether you are using a remote or using a toy; typically when we use it as you know as an end-user using the device which has batteries in it, we are not ensuring any kind of steady usage of the battery; we just have the moments when the battery usage is high other moments when the battery usage is slow and it completely varies. Maybe you played with your toy for 15 minutes, a friend of yours had the same kind of a toy and they used it only for 10 minutes; somebody else played with it for an hour etcetera. So, there is an extremely wide variation between our user profile from person to person even within a person from day to day and so, on. So, when somebody tells you; I bought those batteries and I put it into whatever they were playing, let’s say it's you know some radio that they were playing; they put it in portable radio that they were putting it in and then they say that it lasted a month. That information is not useful to you because we don’t know how they used it for that month right. So, they might have used it very sparingly for a month in which case the month is not a significant number, they may have used it extensively for the month; in which case the month may be a significant number. So, even somebody says that I; I put this battery and it lasted me a month; that does not convey any information to you about what would happen if you bought the same battery and put it in there the same device in your house. Because you may use it differently than the other person right. So, therefore, this kind of comparison although it is called anecdotal comparison; I mean because they are just giving their experience to you and so on. This kind of comparison is not particularly useful; unless you then you know enquire how much did you use? And they tell you to know every day I have been using from morning to evening and something like that then; then it starts conveying to you something. If they use it only half an hour a day doesn’t convey much to you, it means for 30 days they have used it only for 15 hours. If they used it for 10 hours a day; then in 30 days they have used it 300 hours. So, there’s a huge difference between 15 hours and 300 hours. So, telling you that it lasted a month doesn’t mean anything. So, therefore, we need to test batteries under somewhat standard conditions. Or at least we should be able to report it in some standard way so, that somebody else can look at it and make a decision whether or not that is acceptable to them; this is what a battery manufacturer would have to do. They would have to give you this kind of information so, that you can make a judgment if whether that particular battery is useful to you or not. So, to do that what they do is essentially you have a battery and anode, cathode and an electrolyte. So, we make a connection; we make a connection to an external circuit where there is something important here called the load. Normally the load is whatever end-use you are putting the battery too. So, if it’s a toy then the load is the toy, but in a battery test situation, you will have a test you know test station so, to speak. So, it will be one electronic structure I mean an instrument which has a lot of electronics in it, where the specific purpose is you can specify some current and it will draw that current from the battery steadily. So, you can start the test in the morning and say you know draw 0.2 amps, it will draw 0.2 amps from the battery continuously. So, the question of you know you using it for 15 minutes, somebody else using it for 30 minutes is not there for; it will draw 0.2 amps continuously unless you give another instruction saying now change it to 0.3 amps or something like that. So, it will steadily draw the current. So, this load this electronic load is typically programmable ok; so, can be programmed. So, it can be programmed you can change you know the amount of current you are drawing from the cell to whatever value you want to do. So, you can set different levels of load and it will follow that load etcetera and in series with it you have. So, the current which you are drawing sort of does this. So, electrons are going this way the; so, you know the conventional current is going the other way, you have an ammeter. So, you have some ammeter here which tells you what is the current ok. So, in an in a rudimentary setup, you can have an ammeter there which you can read and you know note down the values from, but in the modern-day setups there are automatic you know data acquisition systems, which are present inside this test station which will record the current. So, whatever load you set to you know you say you say 0.2 amps it will draw 0.2 amps from the battery and it will also record the 0.2 amps in the test station; in the data acquisition system. So, you will have a value of current; current i is being recorded and you also have a voltmeter attached in parallel which now tells you the voltage. So, you record current I; you record voltage V and of course, you have a duration for the test which is time t. So, these three are the important parameters that we record; current, the voltage and the time. And the test setup will usually also have some safety features thrown in saying let’s say you are drawing constant current 0.2 amps. You will also set a voltage window you will say that this battery supposed to operate at 1.5 volts.