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The most common version is the cylindrical cell. So, I have just drawn a cross-section of it. So, in principle, you will have a central part that looks like that and then you know you will have a shell which is all around which goes around and that shell basically if you come to the top it look like that ok. So, so inside it all these things are winding up I mean, so from outside this is what you see you see I am showing you know as a cross-section of it as though you are sectioned it vertically down and then you would see something like this. So, here you can see you have a separator, I will just call this the anode, this grey area is the anode, this is a separator and this is the cathode. So, you have anode-cathode separator kind of thing which is rolled. So, you have separator anode-cathode separator I am sorry separator anode separator cathode separator. So, this whole thing is wound up and then you get something like this. So, this is the separator anode separator cathode. So, this is this way we have it then you can sort of you know wind it up and then you can have this battery you make a connection to the anode from one side. So, if you are a positive electrode I mean your negative electrode is the anode. So, you will connect to it to the bottom of the cell the positive electrode is the cathode you will connect to the top of the cell. So, you will have some connection there which sits up there too which you connect up the positive electrode. So, that is the way. So, that’s the cathode. So, this is what the connection would be. So, that cathode will get connected there the anode would get connected at the bottom and that’s how you make this cylindrical cell. So, cylindrical cell as I said is perhaps the most commonly made cell. So, all those batteries that you buy there which is said double-A battery which in Indian parlance we call as the pen torch battery we have a tendency to call this the pen torch battery, pen torch battery because that was one common place where we extended to use this we also have the triple a battery then you have the C cell, D cell all of these have this basic construction of a cylindrical type. cylindrical cells, these are all cylindrical cells. And they are generally I think the manufacturing process is quite simple I mean relatively, is straightforward to do and most easier to get a handle of and therefore, a lot of initial battery manufacturers used to this kind of structure. So, the problem though is that if you look at the I am looking at the vertical cross-section right, if you take a horizontal cross-section what we have is a circle right we have a circle it is a cylinder. So, the horizontal cross-section is a circle. So, now, if I want to, if this is a cell and if I want to make a battery which is a whole bunch of these cells in series or parallel then I am going to have all of them sitting like that right, so let me just put it here. So, for example, I put 1, 2 here, 3 here, 4 here, 5, 6, 7, 8. So, let me say I put a dozen of them in series-parallel some connection. So now, what about all this space? We have all this space here which is wasted space this space here, space here, right, this is all wasted space. So, if you are looking to save space you want it you know held in some kind of a compact way so that because space is a constraint and in you I mean there are many places where space is a constraint. So, for example, if you look at automotive batteries made for the automotive purpose. So, you have to put the battery right now we are already used to an automobile of a certain shape and size right. So, you have to fit the battery within that without affecting the comfort level of the passengers that are who are present. So, therefore, if you look at a car there is already space if you take a car space for passenger and let’s say their baggage. So, this space is already defined. So, any car you go get a find whatever is the current popular car that is out there if you take it you will have some specification saying this much space is available for the passengers this much space is available for luggage. This is already baggage or luggage this much space is given they will open the trunk and show you space and then show you how much space how spacious the seats are and how much legroom you have all these stuff they will tell you. So, now but the external size of the car is fixed. So, if you want to make the same car both as a petrol version as well as a battery version, electric version the batteries have to use only what remaining space is there. So, whatever space was used by engines plus petrol tank ok. So, whatever space was there which was being used by the engine and petrol tank that is the space that is now available for you to put whatever you want to put in terms of electric infrastructure. Now, in space of engine anyway going to have some motor some electric motor is going to appear there right. So, it may be slightly smaller than the engine may be the same size etcetera, so but you cannot depend on a lot of space being available there in the engine area. So, whatever was there in terms of a petrol tank that’s the kind of space that is available for you for the battery pack. But people do a lot of different things, so when you make an electric vehicle you don’t have to start from a petrol vehicle and then necessarily refit the patrol vehicle as a battery vehicle, but many times at least the general overall structure is taken from the regular vehicle that is out there, but significant modification is made. So, they don’t simply use the petrol tank as the battery pack they sometimes lay the batteries below the seats. So, a lot of different things they do, because they would also look at the safety of the battery in the event of a crash in the event of an accident, will the battery get punctured, will the battery get hit by the other vehicle what will happen to the battery. So, these are kinds of things that you have to be careful about. And therefore, you may position it differently, but my point is there is some specific set of volume constraints within which you have to fit your battery. So, then it does not help if you have batteries which are upfront wasting the space. Right at the beginning, you know that there is so much space that is going to be wasted ok. So, therefore, this kind of a cylindrical structure even though it is quite popular even though it is quite prevalent and easy to get it is not the structure that is of choice when you want to do space-saving batteries concerning automotive applications or concerning you know toys etcetera. Instantly I spoke about the safety of the battery in an automotive sense that’s all also something that you have to keep in mind because often, in fact, you if the vehicle gets hit and let’s say petrol leaks it can leak away from the vehicle, but here the battery sitting inside the vehicle. So, you are sort of you know keeping all the energy right there. So, a leaky petrol tank may you know eventually lose a lot of fuel and fuel may be lost and it is dangerous it is not that it is not dangerous it is dangerous. But here you have all of the dangers sitting right there, you don’t have a leaky battery so to speak in this case you the whole battery is sitting there in the event of a; of course, you may have a partly discharged battery. So, in that sense, you have a slightly less amount of battery. But you don’t lose any part of the battery it is sitting right there in the vehicle right. So, so anyway, this is that’s something you have to keep in mind. So, therefore, a cylindrical structure is not convenient, it helps much better to have a flatter structure and that is what this prismatic cell is, ok. (Refer Slide Time: 26:42) So, in concept it is I mean again you have the same thing you have anode separator, cathode and separator and this you can actually set up in a slightly different way and then you get yourself this cylindrical cell. So, if you peel out the cell then this is these are the various layers that you will see and with that, you make this cell. So, as I said this is very good for automotive applications mobile phone applications, laptops, and toys these are common places where you will find this. So, this kind of battery, and these days you know even toys its. A lot of people now have fairly elaborate hobby centres and hobby clubs in their universities where people are making specific devices for specific applications. So, at that point, you can select what kind of a battery you want, what configuration you want in what, structure it should have, what capacity it should have what power it should have all those things you can decide and therefore, this kind of information is very useful to you ok. So, this is the prismatic cell. Now, this usually has a hard case and therefore, can have some additional weight because of the casing and then it is in this context that people also came up with something called the pouch cell. (Refer Slide Time: 28:24) Where they figured some interesting ways in which they would make contact with the chemicals inside without having to do the large scale, you know hard case structure which would take up more weight. The only issue with this is often the pouch because it is very soft can swell. So, it can change the dimension. So, that is, so you have to keep that in mind when you use to put it for an application. So, if you have an application where the overall shape of the object cannot change much and there is not much space for something to expand then this can be a bit difficult to put into that situation because this may expand based on these circumstances with considerable use it may swell up. So, you have to be a little careful with it. But this is the pouch cell. (Refer Slide Time: 29:16) So, when we look at batteries we have to look at you know the structure that we just spoke about. We have to look at things like energy density; we will need to look at a specific energy, specific power and so on. So, there are a lot of things. So, you look at the voltage you look at current density, look at cycle life all right. So, you have a wide range of different parameters these are some of the parameters that you like to likely to encounter. So, for example, when you say specifically it is concerning weight. So, you are looking at watt-hours per kilogram, or in this case watts per kilogram and when you say density is concerning volume. So, it is what hours per litre. So, this is the kind of thing you would look at current density is amps per centimetre square cycle life of course, are several cycles ok. So, these are various parameters which you use to compare batteries. So, now, there is no specific, there is no you know a clear hierarchy, we also want you to know safety, environment friendliness. So, these are all factors right. So, so there is no particular order in which you have to look at this it depends on your application what you would like to pick and that is why as I said you know these days when they look at you know hobby club activities in various universities you have the flexibility of picking something that meets your specific requirements. Many times in these hobby clubs, so let’s say suddenly they want to make some you know underwater robot something like that they want to make. So, now, it has to be you have to keep in mind it is underwater, so it may get wet. So, you have to pick a battery that is sealed very well that can operate under underwater conditions, it should not corrode underwater then you have to also look at how long you want the battery to run supposing this vehicle is supposed to run for 1 hour and come back on its own. Then clearly you know 2 hours of one hour up and one hour back two our duration it has to run. So, whatever power it draws the battery pack that you put should provide at least twice enough power, so that it lasts at least 2 hours of application may be more some factor of safety two and half hours or something by which time it will come back to you. So, a lot of things are there. So, you should do a careful analysis of whatever project you are doing to figure out what is you know parameters that are important to you from the battery perspective and then find out if there is a supplier who can supply you a battery with those specifications. These days those things are available you can get batteries which know particular values of you know all these parameters or some of these parameters maybe the rest of them will get accordingly accommodated based on what you are picking. But that’s how you will get it and then you can get it for your project. So, that is something that you should keep in mind ok. So, the one thing that we didn’t look at here is the chemistry, and that’s effectively you know I actually affect all of these or is affected by all of these and or actually affects all of these and also has a very strong implication concerning environmental friendliness. So, what we will look at now and then this in the remaining part of this class is some of the common battery chemistries that are out there and you know assess them in some general sense ok. (Refer Slide Time: 33:26) So, again as I said that list is not going to be exhaustive because there is there are so many battery types, but it will give you some idea of what we are dealing with ok. So, perhaps the most common form of battery that that is that has been prevalent for quite some time now is the lead-acid battery. So, almost all our automobiles, virtually every automobile that has an electronic start of some sort will have a lead-acid battery unit. So, all our cars, all our automobiles, for a pretty long time have had a lead-acid battery in it and typically it can give you high currents high current density high currents can be drawn from it and that is necessary for when you do the starting of the car. So, it can give you a 100 amps significant amount of current can come out of it which is a lot of currents and it can help you start the car. So, typically it has as the name suggests it has lead and it has acid. So, you have Pb lead and the lead oxide those are the 2 electrodes we have, and the electrolyte is sulfuric acid. So, this is what we have. So, many times these batteries over some time the acid leaks from it and so on it’s in a liquid state. So, liquid electrolytes even though they are very nice in the sense that usually they can support high ionic conductivity and so on and therefore support higher currents. The messy part with liquid electrolytes is they tend to evaporate, they tend to leak from the cell etcetera and if you have acid leaking from the bad cell, you are spoiling the environment wherever you are leaving with these batteries any you know recycling whatever you do any place where you have all these batteries collected in large numbers you have a large amount of acid sitting around which is never a pleasant thing to have. So, in any case when it is an operation you have this lead which is in the solid-state which reacts with sulfuric acid forms lead sulfate 2 protons and 2 electrons. So, these protons go into the external circuit and they do whatever job you are trying to get them to do in this case start the car and they come to the cathode. So, from here the electrons go, they find there they go through the load and they arrive at the cathode right. And they are at the cathode the lead oxide reacts again with sulfuric acid and the 2 protons that came through the external circuit and the 2 electrons are I mean sorry the two protons that came through the electrolyte and its sulfuric acid. So, you have protonic conductivity there and the 2 electrons that came through the external circuit to form that sulfate and water. So, the interesting thing here is that in the anode you start with lead and cathode you start with lead oxide. For both anode and cathode as you discharge the battery, so you start with a charged battery and you start discharging the battery both anode and cathode get converted to lead sulfate, and both of them are consuming the electrolyte both are using sulfuric acid, both are using sulfuric acid, I mean anode use the sulfuric acid cathode use the sulfuric acid. So, when you take recharged battery or a fully charged battery you will have lead and lead oxide and sulfuric acid, this is what you will have. When you have a discharged battery both the anode as well as cathode would be lead sulfate and the electrolyte will be mostly depleted from sulfuric acid ok. So, all the sulfuric acid there would have been consumed more or less you have any dilute sulfuric acid and a lot of water. So, this is what will happen you mostly have diluted sulfuric acid and water in the discharged state. When you recharge it the sulfuric acid will reform and you will get lead and lead oxide. So, this is the way it works. So, of course, this like I said it, it helps you have high currents are possible. So, even now for applications that require high amounts of currents and you know in the relatively cheap manner, this is still the battery of choice. So, the first generation electric vehicles have typically operated using lead-acid batteries and of course, and of course, that is messy as I said because of the lead and acid that is present. So, it is toxic because you have lead and, but the only I would say the saving grace and today’s way of dealing with it are that all the battery manufacturers who make these batteries the recycle the lead actor. So, anytime you are going to change your battery in many of the automobiles today whenever you have to change the battery which maybe you know usually about 2 to 3 years after you buy the car vehicle you will have to change the battery once in about 3 years you will have to charge the battery. If you go to get it changed they will always give you some you know monetary benefit for returning your old battery. So, almost all of us are going to return the old battery because there is absolutely nothing we are going to do with that old battery. After all, it is anyway finished it’s not going to function for any practical application. So, it makes no sense to keep it this one massive heavy piece of equipment with it which is just going to sit in your house or sit in your garage and it’s there’s absolutely no purpose in keeping it. So, most of us gladly hand it off because first of all it gets rid of this big a massive piece of thing that we are stuck with and we also get some monetary benefit from it some cashback will be there in the process of doing the battery installation and this is an active process that people follow. So, that’s the lead-acid battery. It’s a rechargeable battery ok. So, that’s the rechargeable battery. (Refer Slide Time: 38:35) So, the other rechargeable battery that is also been around very commonly for of fair fairly long period this other rechargeable battery is the nickel-cadmium battery or NiCad battery as it is called. It has high cycle life, very large number of cycles it can do NiMH we will see in just a moment, high cycle life and its very reliable. So, a lot of initial you know the usage of rechargeable batteries for smaller applications you know the handheld application, handheld devices, small toys, etcetera have all typically used nickel-cadmium batteries it has some lower capacity than nickel-metal this NiMH nickel-metal hydride we will see, see that in a bit and it is toxic because cadmium is toxic, cadmium is toxic so it is toxic. So, if it is disposed of by throwing into the trash that is bad because cadmium we are just distributing a toxic material into the ecosystem. It has an interesting thing called the memory effect what it is that you know if you buy a battery of some capacity and let’s say you discharged it only half the way and then you recharge it and let’s say you keep doing this a few times because you know let’s say your typical usage is morning to evening every evening you put it in the charger. And let’s say in your normal usage or using only 50 to 60 per cent of that battery capacity and evening you want to recharge it as soon as you come home you put it on the charge if that’s your habit then what happens is after several such cycles the battery remembers only the 60 per cent of its capacity it does not remember that it has you know another 40 per cent available in it this is actually. So, this is called a memory effect because it seems it seems like the battery has forgotten that it has extra capacity. In reality, what is happening is that you know structure inside the battery the particle sizes and structure inside the battery is changing in such a way that it becomes easier to recharge only that portion of the battery that got used. You can still recharge the rest of it, but you will have to force it in some way, it does not happen easily it becomes easier and easier to recharge only that part of the battery that has been used repeatedly and therefore, it sort of since it is finding it difficult to recharge the rest of it, it gives you a false signal that it is fully charged. So, you stop with the false signal that it is fully charged even though there is still some capacity left which you did not access ok. So, I mean when you step back and you describe it as, though there is some memory effect. So, that is what this memory effect is about. So, what reactions we have? We have cadmium reacting with 2 OH minus which gives you Cd OH twice and 2 electrons. So, these then go into the external circuit and then come to the cathode where NiO OH reacts with water and 2 e minus gets you Ni OH twice and 2 OH minus. So, these are the two reactions anode and cathode and you can, of course, reverse these. Of course, so the nickel-cadmium system was very convenient a lot of people used it, but they recognized that the cadmium was toxic. So, they tried to think of some other chemistry which would at least keep some of these benefits and then lose the toxicity. So, they came up with this NiMH which is what we have here, nickel-metal hydride. (Refer Slide Time: 41:35) So, there are a lot of rechargeable batteries out there which are called nickel-metal hydride batteries. This is non-toxic it can replace alkaline as well as nickel-cadmium batteries. This does not have the memory effect we just, so happens that the chemistry of the battery and the way the materials work are such that this effect is not visible. It has high capacity and high energy density and its energy density approaches that of lithium-ion. So, energy density would be what have watthour plus a per litre. So, that energy density is approaching that of lithium-ion. Can self discharge meaning it will slowly discharge if you don’t use it, the basic point is instead of cadmium you have this metal hydride? This metal is itself typically an intermetallic kind of material it has more than one metal there and then this the metal alloy is you know reacts with OH minus gets you this M and H 2 O plus e minus which this e minus goes to your external circuit NiO OH plus H 2 O plus e minus reacts with Ni OH twice and OH minus and crates you OH minus. So, this cathode reaction if you see looks essentially the same as what you had here right, what you have here and what you have here are the same except that this has been balanced for two electrons and this has been balanced for one electron keeping in mind that the anode is generating 1 electron here anode is generating 2 electrons here. That’s the only difference and otherwise, the cathode reaction is the same. So, that’s the nickel-metal hydride battery this is also rechargeable. (Refer Slide Time: 43:15) And of course, there is a lithium-ion battery which is you know perhaps the most popular battery at this point in common literature and also in fact, even in scientific circles, people work very heavily on lithium-ion batteries. It is even lighter than the nickel-metal hydride battery because lithium is the lightest metal that you can find, therefore, much better than energy density it is rechargeable it is the ion part that makes it rechargeable and it can also discharge over some time its basically uses lithium in carbon as the anode and so when it discharges that lithium leaves that anode and releases the electron this which goes into the external circuit. And then the lithium-ion which comes through the electrolyte and the electrons which come through the external circuit react with cobalt oxide. And cobalt oxide is just an example, there are various materials that they use for the cathode cobalt oxide is one of those example materials and in this process, they form lithium cobalt oxide LiCoO 2. So, this is what is done and you can reverse this reaction. So, this is the rechargeable version of the lithium-ion battery. So, if you look at rechargeable batteries these are then maybe the 4 common ones that we see from you know very visibly in the public space. The lead-acid battery, nickel-cadmium battery, nickel-metal hydride battery and the lithium-ion battery. These are the 4 that you see most commonly in, you know public space where most of us go to the shops and buy batteries ok. So, this is what we see. (Refer Slide Time: 44:45) There are a couple of non-rechargeable batteries that we should also at least briefly be aware of, alkaline batteries we keep hearing about this quite a bit alkaline batteries. They are very inexpensive, but they don’t did deliver as much current, but they are inexpensive, so for many you know applications associated with say remotes, remote controls toys etcetera, this may be ok. So, it has zinc and manganese dioxide as the 2 materials in the 2 electrodes. So, zinc becomes zinc oxide and releases 2 electrons and the manganese MnO 2 becomes Mn 2 O 3 and consumes those 2 electrons. So, this is your alkaline cell and it is typically not rechargeable. There are some versions of alkaline cells which are rechargeable, but it has to specifically say that doesn’t just take an alkaline cell and try to recharge it, it is dangerous it can blow up. So, that is not it is not meant to be like that there are some versions which are rechargeable it will specifically say that there will be specific recharger for it if we have a limited cycle life, but you have to use only in that combination you cannot just randomly do something. All batteries always are careful with that there are some which are rechargeable some which are not rechargeable. You cannot randomly recharge arbitrarily recharge a non-rechargeable battery I mean you cannot even try it, you will not be able to do it because the chemistry will not permit it. But more than that you should not even try because since it cannot reverse the reaction it will do some other reaction. After all, you are forcing current into it and then that can be a dangerous reaction you could be really. You could be releasing something toxic you may release some toxic fumes, you may blow up the battery, a lot of things are possible. So, you should not risk you know recharging a battery which has been designated as a non-rechargeable battery or a single-use battery. Even recharges, you should use the correct recharger device in which you plug in that battery. You should not just take some battery and plug it into some other recharger because they have specific voltages that they operate at and specific ways in which they push the current into the battery and there is a specific cutoff voltage at which they will stop charging the battery. So, if you randomly put some battery into some reach other it is running at some setting that the battery is not designed for. So, it will overcharge the battery and at over at the overcharge point it may explode. So, that is a dangerous thing to do. So, you should always be careful about those things. So, anyway alkaline is a non-rechargeable battery. (Refer Slide Time: 46:57) And the carbon-zinc battery, this has also been very common extremely inexpensive, but very low energy density and not very useful in that sense. So, only for limited applications associated with said remote control or something like that you can use it. It also has zinc and manganese dioxide and creates Mn 2 O 3 and so on. There is a carbon comes actually like the sort of the current collector you can say which connects to the MnO 2 and that is how the name carbon comes into this name and it’s a non-rechargeable battery ok. So, these are the common battery types that are there, rechargeable as well as non-rechargeable. (Refer Slide Time: 47:36) And so in terms of overall conclusions for what we have discussed here, there is a wide range of battery types which we saw. We saw at least a few rechargeable ones a few non-rechargeable ones, and these batteries differ from each other in terms of capacity, environmental friendliness, current densities, that they support cycle life, energy density, specific power, specific energy, a wide range of parameters in which they differ. And that is why as I said if you are creating a new project if you, of course, you buy a device they would already specify what battery has to be used there or they may even provide you with that battery. But if you are making a specific project then you make a battery to your specification and you can get it made to your specification and use it. So, for example in fact, if you go back here to those structures that we were looking at this prismatic cell. This is the same kind of cell that you see essentially in your mobile phones and so on. And you can see that they have been made specifically for that phone. So, if you take you know a bunch of different phones and you open them and look at the batteries they are all of the different sizes. You don’t see as a standard you know the double size or a triple a size, you don’t see any standard everything is the same manufacturer. You pick you to know anyone common manufacturer for