 depending on where you're joining us from. Welcome to Engineer for Change, or E4C for short. Today, we're pleased to bring you this month's installment of E4C's 2018 Off Grid Energy webinar series, focusing on battery fundamentals for off-grid electrification. My name is Mariela Machado and I am program manager at Engineer for Change. I will be the moderator for today's webinar. The webinar you're participating in today will be archived on our webinars page and our YouTube channel. Both of those URLs are listed on this slide. Information on upcoming webinars is available on our webinars page. E4C members will receive invitations to upcoming webinars directly. If you have any questions, comments, and recommendations for future topics and speakers, please contact the E4C webinar series team at webinars at engineerforchange.org. If you're following us on Twitter today, please join the conversation with our hashtag E4C webinars, as seen on the slide. 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For more, please visit our website at engineerforchange.org to learn more and sign up. Today's webinar is set the third in an upgrade energy webinar series on energy. Additional topics covered in this series are drawn from the book, Battery Fundamentals for our UpGrid Energy, authored by presenter Dr. Henry Louie. The future webinar in this series are listed on this slide and will be announced via our newsletter. E4C members will receive the information directly into their inbox. Sorry, the book title is UpGrid Electrical System in Developing Countries, as seen on the slide. And this is the fourth webinar called Battery Fundamental for UpGrid Electrification. For reference, you can find examples of UpGrid energy products like the Mobisol Solar Home System on the Solutions Library, like seen on the slide now. There, you can learn more about technical performance, compliance with standards, academic research, and user provision models for these systems. All of the information is sourced by our E4C research fellows and reviewed by our community of experts. And of course, it's available to our E4C members free of charge. So be sure to sign up if you're not already. A few housekeeping items before we get started. Let's practice using the WebEx platform by telling us where you're joining us in the world. In the chat window, which is located at the bottom right of your screen, please type your location. Right now, if the chat is no open on your screen, try clicking the chat icon at the bottom of the screen in the middle of the slide. You can use this window to share remarks during the webinar. And if you have any technical questions, you can send us a private message to engineer for change admin. So let me take a moment right now to see where all of you are joining us from. Just type in the chat window your location. Okay guys, I'm not seeing that. A couple of here, oh, New York, Guatemala, Nashville, who else? 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Great, so I would like to take a moment now to tell you a bit about our presenter today. Dr. Henry Louie is an associate professor and Francis Wood Endauer Research Chair in the Department of Electrical and Computer Engineering at Seattle University. His research areas include electricity access in development communities, renewable energy and appropriate technology. He's the president and co-founder of Kilowitz for Humanity, a nonprofit organization providing electricity access and business opportunities in some Saharan Africa. Dr. Louie served as a Fulbright scholar to Copper Belt University in Kedwa, Sambia. He's recognized as a distinguished lecturer of the IEEE and is an associate editor of the Journal Energy for Sustainable Development. He's the author of the book Off Grid Electrical Systems in Developing Countries published by Springer Nature. Henry, I hope I made you some, I read everything right. I welcome you and I thank you for joining us and over to you. All right, well thank you for that introduction. It's great to be back here for our fourth webinar in our series. And today we're gonna be talking about Battery Fundamentals for Off Grid Systems. So as mentioned, the webinar today is gonna be following the book Off Grid Electrical Systems in Developing Countries and you can access that book. You can buy it on the publisher's website, the publisher Springer, you can buy it through my website and it's also available in hard and electronic copy on Amazon. So the book itself contains much more details. It contains example problems, homework problems and additional references. So think of it as a way of supplementing this webinar. So if you do have the book and are following along at home, today's webinar is gonna map to chapter eight, which is Battery Storage for Off Grid Systems. So today's webinar, we're gonna cover the basic electrochemistry that happens in lead acid and lithium ion batteries. We're going to focus also on how to interpret battery specification sheets. And then also hopefully you'll be armed with the knowledge that you need to select the right battery for your off-grid system. So just so that we all are coming from the same perspective, we use batteries in off-grid systems anytime we want it to decouple when power is produced from when it's consumed. So the classic example of battery storage in an off-grid system is a solar powered system. So during the day, the solar panels are producing excess energy which we store in our battery. And then during the evening when maybe we use some lighting or some other load for entertainment, we will discharge that battery. So I like to think of the battery at inhaling during the day and exhaling during the evening in that scenario. Now there's a lot of different types of batteries that we can use in off-grid systems, but by far the two most common are lead acid and lithium ion. And there are several types of sub-chemistries and configurations within each of these types. And we'll discuss a few of them in today's webinar. Now there's some other chemistries that are out there as well. And we're all optimistically hoping that they are able to be commercialized and are as good as they are promoted to be. But for now it's really lead acid and lithium ion batteries that you see out there in the field. Now I think there's a tendency for us to view batteries as kind of mysterious black boxes of chemicals. We're not quite sure what's going on inside. And we maybe have a sense of how that battery is going to perform. And I know that was more or less my interpretation of a battery until I really started working in off-grid electrification and quite frankly started doing a lot of in-depth research for the book that I wrote. So we're gonna try to demystify what is actually going on in those batteries in today's webinar. Now thankfully battery manufacturers provide a specification sheets or data sheets with each battery that they make. And so these specification sheets are really useful for us to understand how that battery is going to perform under certain predefined situation. And often however, and especially in the off-grid context, the situations that we put our batteries in don't exactly map to what the manufacturer has supplied. So you do need to have a understanding of what's going on inside the battery to know how to adapt the spec sheets to your particular case. Now battery spec sheets provide a lot of information but I think the three most important things that they provide are the information about the battery's nominal voltage. And that's usually two volts, six volts, 12 volts or 24 volts. They provide information about the charge capacity which is usually in amp hours. Or if you wanna convert it to kilowatt hours you just multiply it by the battery's nominal voltage. And they also provide some indication of how that battery is going to perform over time as it's charged and discharged. So what happens to the capacity of that battery as it begins to age? So I think a natural question is where does the voltage in a battery come from? So what is actually giving rise to the voltage that we breed when we measure the voltage on a battery's terminals? Well, to really answer this question you have to get into the electrochemistry. And just as a caveat here, I'm not an electrochemist. I'm an electrical engineer by training. So my goal here isn't to get into this in excruciating detail but rather give you sort of a working model that you can use to understand what's going on inside the battery. So to create a battery we need nothing more than a electrolyte which will put in some sort of container and an electrode. The electrode itself just can be sort of any type of metal really. And let's note that before we dip the electrode in the electrolyte, both of these are electrically neutral. They don't need to be pre-charged or anything like that. So when we take that electrode and we dip it into the electrolyte there's gonna be several things that happen. There's gonna be some reactions that cause charge to leave the electrode and enter the electrolyte and reactions that cause the electrolyte to pass some charge into the electrode. So for example, that electrode might dissolve a little bit passing positive ions into the electrolyte. Now these reactions are really driven by something called the chemical potential. And this chemical potential is naturally occurring and it's spontaneous. So we have this force that's fostering this exchange of charge. And this actually will happen between any two dissimilar substances that come in contact. Even two pieces of metal for example you'll have this exchange of charge. Now the rate of charge leaving the electrode and entering the electrolyte and leaving the electrolyte and entering the electrode are not the same. One of them is going to dominate the other. And it really depends upon the chemicals being used. And so what I've done in this picture here is I've shown something called the chemical potential. That's what's giving rise to these reactions. And I've drawn it as an arrow indicating the dominant direction that the positive charges follow. So you can see that we are more likely to be transferring positive charges out of the electrode and into the electrolyte than the other way. But because we have this net flow of charge it means that the electrode and the electrolyte are going to be no longer electrically neutral. So what's going to happen in this particular case is that we're going to have a net negative charge on the electrode and the electrolyte is going to have a net negative charge. And you can see in the picture that there are more negative charges in the electrode than positive charges in vice versa with the electrolyte. But because we have this imbalance of charge that has been caused by the chemical potential we're going to have an electric field that develops. And so for every positive charge that is pushed out by the chemical potential the electrical potential grows and it makes it harder for that next additional charge to enter the electrolyte. So at some point, and this actually happens quite quickly the electrical potential and chemical potentials are going to balance. And what we're going to have then is we're not going to have any more net transfer of charge. In other words, we're going to be at an equilibrium state. So if we were able to we could actually measure a voltage then between the electrode and the electrolyte. And so this to summarize where the voltage comes from then what we do is we take the chemical potential and it gives rise to an electrical potential. And so that's where the voltage in a battery comes from. So this electrochemical cell that I've shown you here is it's not very useful because we have no way of powering a circuit to it. So in order to power a circuit what we need to do is we need to add another electrode. And so this could be a different type of metal. In fact, it needs to be a different type of metal than electrode A. So we put electrode B in there and just like electrode A there's going to be some reactions that are driven by the chemical potential. And in this example I've just shown the chemical potential arrow just being in the opposite way that is positive charges going from the electrolyte to the electrode but that is entirely arbitrary. I could switch it around and then the battery would still work. So now what we use an electrode B is going to have an electrical potential that builds up just like an electrode A and there will be a voltage between electrode B and the electrolyte. So then we can connect these two electrodes with a wire or we can run it through a resistor or some other load and we'll allow electrons then to pass through that wire and they're going to travel from electrode A to electrode B. And the reason why it's in that direction is you can just look at the charges on electrode A and electrode B and you see that electrode A has an excess of negative charges whereas electrode B has an excess of positive charges. So we'd expect the light charges to push each other away and so an electron would flow from A to B. So we've then allowed electrons to flow through the circuit so we've actually have current then that is flowing. So you can imagine an electron traveling from electrode A and entering electrode B and when that electron leaves electrode A it actually is going to weaken the electrical potential at that electrode because there's a less charge in balance then and a similar thing happens at electrode B. And so the chemical, excuse me, so the electrical potential then decrease and because the electrical potential decrease across both electrodes we can have more transfer of positive charge in the direction of the chemical potential. And so this process will continue so long as we facilitate the flow of electrons in the circuit and ions within the electrolyte and that's all it really takes to develop a battery. You need to have electrons flowing externally and you need to have ions that are able to flow internally. So this then describes the basic interworkings of what a battery, of how a battery works. So again, we use chemical potential to generate a voltage and then as long as we can facilitate transfer of charges within the battery the battery will itself work. So then you might ask yourself, well, what voltage actually appears across the battery's terminal? And the answer to this is actually fairly complicated. What we do know is we know that the voltage that appears is an intrinsic property of that battery for that cell, meaning it doesn't matter how much electrolyte or how much the size of the electrode plates. What really depends, what really matters is the chemicals themselves. So what chemicals in particular you are using in the electrode and the electrolyte, the state of those chemicals, like their temperature and pressure and concentration. So those are the things that really dictate the cell voltage. Now, there are hundreds and hundreds of different combinations of electrodes and electrolytes that we could use to create a battery and electrochemists have tabulated the voltages that you would get under certain conditions and those conditions correspond to 25 degrees Celsius of temperature or pressure of one atmosphere and a concentration of one mole per liter. So then all you need to do is consult one of these tables and look up the chemicals and the reactions that are being used in your battery and you can get an idea of the voltage that each cell will give you. And so for a lead acid battery, you get about 2.04 volts when you do this. So that's the voltage that would appear under standard test conditions. So then you might ask, well, what happens when my battery isn't under those standard conditions? Can we somehow estimate or calculate how the voltage would change? And the answer is yes. And to do that, we have to use something called the Nernst equation. So the Nernst equation allows us to look at those tabulated voltages and adjust them if, for example, the temperature changes or the concentration of the chemicals and the electrolyte vary. And so I'm not gonna get into the Nernst equation in detail, but I will point out that there is a natural logarithm in that equation. And that natural logarithm then means we're gonna have some nonlinear behavior. So we can't expect a nice tidy model to be developed. It's gonna be nonlinear. We'll also note that the reaction quotient, that's what that QR is, that really depends upon the so-called activities of the chemicals in the reaction. So most often this has to do directly with the concentration of the chemicals in our battery. So as these things vary, the temperature and the concentration and the general state of the chemicals, we're gonna get different voltages that appear across our battery. So let's take a look at a lead acid battery in particular. Again, lead acid batteries are by far the most common battery that's used in off-grid systems. So lead acid battery has, and then in fact this is actually just a cell of a lead acid battery. So it's just part of a lead acid battery. It has two electrodes, one we designate the cathode and the other the anode. The anode is a spongy lead substance and the cathode is made out of lead dioxide. Now the electrolyte is sulfuric acid that has been diluted with water. And the concentration of the sulfuric acid is about five or six moles per liter, which is different than the standard state in those tabulated values. And so the actual voltage of a fully charged lead acid cell is not 2.04 volts, but closer to 2.1 volts. And we could actually apply the Nernst equation to figure that out. So as we discharge a lead acid battery, the negative terminal there will evolve from lead to PBSO4. So some of that SO4 ions that are in the electrolyte are going to react with the lead and become PBSO4. Now on the positive terminal, the lead dioxide is also going to turn into PBSO4. So again, it's gonna react with some of the ions in the electrolyte. And this oxygen, the oxygen in the PBO2 is actually going to react with the hydrogen ions and form water. So as a lead acid battery discharges then, we will see the plates become more and more coated with PBSO4 and the electrolyte become more and more diluted with water. So here's just a summary of the reactions that occur. The most important thing to note is if you look at the complete reactions for discharging and charging, they're really the same reaction just run in opposite direction. And this is why the lead acid battery then can be recharged as the charge and discharge reactions are the same, but just run in opposite direction. So as we discharge a lead acid battery then both the electrodes tend towards lead sulfate that's PBSO4 and the water, the electrolyte becomes diluted with water. Now that we know from the Nernst equation that as the concentration of the chemicals and the reaction change, so does the voltage. And so this is why the voltage on a battery decreases when we discharge it. The electrolytes just becomes diluted and the concentration decreases. Now if we discharge a battery, a lead acid battery very deeply, we run the risk of the lead sulfate hardening and basically crystallizing, forming a cocoon around the electrodes. And when this happens, that crystallized version of the PBSO4 is no longer reversible. It will not turn back into lead or lead dioxide when we recharge the battery. So in other words, we permanently damage the battery when we discharge it deeply or when we leave it in a discharged state for too long. So you should never do that. So in an off-grid system then, you should carefully plan your battery size so that you don't discharge it too deeply. And if it does become deeply discharged, you're able to recharge it fairly quickly. Now as I said before, there's several types of lead acid batteries. And the most common is a flooded lead acid battery, well also known as a wet cell. So this is a very mature type of lead acid battery. And it's called a wet cell because the electrolyte is accessible. All you need to do is unscrew the caps and you'll see the liquid electrolyte. And this is important because in a flooded lead acid battery, if the battery is overcharged at all, some of the hydrogen will gas and essentially you'll need to add water to the battery periodically over time to rehydrate it. So then there's some maintenance required. Now the upside is it's far cheaper to build a flooded lead acid battery than a sealed lead acid battery. And of course the water that you should add should absolutely be distilled. You should actually never add additional acid to the battery because the sulfuric acid doesn't really go anywhere. So you're just really adding distilled or pure water to the battery. Sealed lead acid batteries on the other hand, their electrolyte is not accessible. And in fact, it's sort of a paste that is impregnated in sort of like a fiberboard or it's a gel. And these batteries are nice because they don't require any maintenance. You don't need to add water to the battery. They basically are entirely sealed. And so that any gas that does evolve can actually recombine and rejoin the electrolyte. I've worked with lead acid batteries and sealed, excuse me, flooded and sealed lead acid batteries. And my preference now is for using sealed lead acid batteries. I think the maintenance in off-grid systems is a big, big concern. And so I would say that it's worth it to pay an extra capital cost to avoid having maintenance. But each situation is a bit different. And so if you have proper training of local people then flooded could be an option. Again, it's cheaper. Okay, so we'll continue on. When you look at a battery specification sheet, the most important thing that's gonna tell you is the battery voltage. And it's gonna be a nominal voltage. So what do I mean by that? So in other words, if you buy a 12-volt battery, would you expect to measure 12 volts across the terminal? First of all, you know, we know that a 12 volt, excuse me, a lead acid cell can only produce, will only produce 2.1 volts when it's fully charged. So to get anywhere close to 12 volts, we actually need to connect six cells in series. So that's what's going on inside a 12-volt battery. You'll have six in series. But even then we would expect then that the voltage to be 12.6, not 12 volts when the battery is fully charged. So you shouldn't make them a sake of assuming that a 12-volt battery will always read 12 volts. So the 12 volts is the nominal voltage. It's not the battery voltage when it's fully charged. And it's also not the battery voltage when fully discharged. It's approximately equal to the average voltage you get when we discharge the battery and then charge it again. So the nominal voltage, the battery may, will often be at a higher voltage than the nominal voltage, but it could also be lower than that. So this question about estimating the state of charge from the battery voltage comes up quite a bit. I know I've had to wrestle with it a few times. So for example, you've installed a system and you've commissioned it and you're trying to make sure that the battery is behaving as you expect. So the battery is just charging during the evening. And you're wondering how much battery capacity is left and you're tempted to just measure the terminal voltage while the battery is discharging. And from that, try to estimate the state of charge. Although this is tempting to do, it's also really, really challenging to do. In fact, estimating the battery state of charge by measuring its voltage is not recommended. You should only do that if the following three things are true. First, the battery needs to be open-circuited. You can't really estimate the charge remaining in a battery if the battery is charging or discharging. The voltage at the terminals is just going to be very different from the open-circuit voltage. Second is the battery has to have been rested for a long time. So meaning you can't just simply disconnect the battery voltage and then measure it open-circuit. You have to let the chemicals in that battery reach an equilibrium. You have to let the concentrations, the local concentrations that might have occurred to go away. And then you also want to do this when the battery is not too hot or not too cold. So under those conditions, then you can consult a chart like the one shown on this page. It'll be slightly different perhaps for each battery. And you can use this then to estimate the state of charge. And the reason why you don't want to estimate the battery state of charge from its voltage while the battery is being charged or discharged is summarizing this plot here. You can see that when we discharge a battery, its voltage is going to drop from 2.1 down to maybe 2. It kind of depends on how much you're discharging it. And it's going to drop from there. And then as soon as you start charging it, it's going to jump up to a really high value and then it will increase non-linearly over time. So the fact, the action of charging and discharging really obscures the battery's true voltage from what you read at the terminal. So the second most important thing that you'll find on the spec sheet is the capacity of the battery. And I think this is the aspect of a battery, the characteristic of a battery that is most likely to be misinterpreted by users. So let's assume that we have a battery and we're curious as to the charge capacity of it. In other words, how many amp hours can this battery provide? So you can imagine we set up a little test where we connect the battery to a variable resistor and we measure the current in the voltage. And we're going to adjust that variable resistor so that the battery is always providing eight amps, okay? So as the battery starts to discharge, its voltage will decrease, we adjust the variable resistor so that the current is constant. So if we do that and we plot our results, we would get something that perhaps looks like this. So the battery voltage will decrease a little somewhat gradually at first and then very rapidly. But nonetheless, we'll be able to get a constant eight amps of current out of it for a long time. But at some point, the battery voltage will be just too low and no matter how low we adjust that variable resistor, we can't get eight amps anymore. And so we stop our test. So in this example, the battery was able to provide eight amps for a total of 6.5 hours, which would give us an estimated capacity than a 52 amp hours. So the question is, is this a 52 amp hour battery? Could we reasonably say that this is a 52 amp hour battery? Well, the answer is no. So on the one hand, we actually could get more charge out of it provided we reduce the current that we were discharging. So maybe instead of discharging at eight amps, we would discharge it at two amps as soon as we were not able to get eight amps out of it. So we could get more out of it. On the other hand, we note that the battery voltage is quite low by the time we stop the test. It's maybe four volts and whatever we would connect to that battery is probably not going to function properly at four volts if it's expecting a nominal 12. And then finally, there's a good chance this battery has been discharged so far that it's been permanently damaged. That sulfate has built up the electrons. So this is really not a good way of determining the charge of the battery. So what manufacturers do is they do this test but they set a voltage cutoff voltage that they pre-define. And they say as soon as the battery voltage drops below this we end the test. And however much charge we've gotten between the start of the test and when the battery voltage reaches cutoff that's the capacity of the battery. And so in this case, we would stop the test if we use a 1.75 volts per cell cutoff voltage, we'd stop it at 10.5 volts which is about 200 minutes into our test. And so the capacity then that we were able to get the charge that we were able to get out of the battery would simply be eight amps because that's the current we're discharging it times the amount of time which is 3.33 hours. So we would call this a 26.67 amp hour battery. Now we can get more than 26.67 amp hours out of it. It's just the terminal voltage then would be lower than the cutoff voltage. So we picked eight amps sort of arbitrarily. What if we wanted to do this test again but discharged it at a higher current? How would that affect things? Well, if we discharged it at 16 amps the voltage would be shown in that blue trace and we would reach our cutoff voltage much faster. Although we've doubled the current the amount of time it takes to reach the cutoff voltage would actually be less than half the time it took when we discharged it at eight amps. In other words, there's a diminishing return that sets in. The faster we discharge a battery the lower the overall capacity or charge we're able to get out of it. So if you wanna make your battery last longer or get more charge out of it you discharge it at a lower rate. So this actually brings up an interesting question as to which of these capacities should be reported by the manufacturer? The capacity when discharged at eight amps or the capacity when discharged at 16 amps? And so this gets into something called a C-rate. And so what you'll see is you'll see battery manufacturers and their spec sheets will say this battery has a certain capacity you'll say a hundred amp hours at a C-rate. In this case it's 0.05 C. So what does that actually mean? So the C-rate can be known as the capacity rate and it's simply a way of specifying the charge or discharge current as a function of the battery's capacity. So the units of a C-rate are one over hours and you relate the capacity and current by the C-rate. So you take the capacity you multiply by the C-rate and you get the current. So a C-rate of a 0.1 C for a 100 amp hour battery would refer to a current of 10 amps. Now by default the standard for batteries that are used in off-grid systems the C-rate is 0.05, okay? So you can think of the current at which the battery is discharged at 5% per hour whatever current that might be. Now some manufacturers will also report the hour rate which is simply the inverse of a C-rate. So to put this all together if you do have a 100 amp hour battery and the manufacturer tells you that that's mapped to a C-rate of 0.05, then that battery when you discharge it at 5 amps will last 24, excuse me, will last 20 hours before its terminal voltage drops below the cutout voltage. If you discharge that at any current maybe six amps or seven amps then the battery is not going to last 20 hours and it will provide less than 100 amp hours of charge. And if you discharge the battery at less than five amps then it's going to last longer than 20 hours and ultimately provide more than 100 amp hours of charge. So a situation might look like this. This is for three different batteries the manufacturer will provide the capacity at different hour rates and their corresponding C-rates. So if we look at battery C we would call this a 229 amp hour battery again because we'd always default to the 20 hour rate but we see that if we wanted to discharge it at 21 amps we'd only get 211 amp hours out of it. If we discharge it at 2.55 amps we would actually get 255 amp hours out of it. So again as we decrease the rate in which we draw current from the battery the battery is going to be able to provide more charge overall before it's terminal voltage reaches that cutoff value. So if you have an off-grid system then you would figure out the current that you expect to provide to the load and you would look at which capacity corresponds to that amount of current. Now this of course is a little impractical because all of these capacities assume a constant current discharge and that's very unlikely in any sort of off-grid system where you might have a peak in the middle of the day or the middle of the night and it might be low consumption overall. So then there's a little bit of an art to figuring out which capacity you would actually which hour rate or C-rate fits your system and so the most conservative way of sizing your battery then is to take the peak current and just assume that that's going to be the current that is discharged at continuously and then look up the capacity of the battery according to that rate. So this idea of the change in capacity versus C-rate is often expressed also in a curve that looks like this. So this is for four different discharge rates. So we would legitimately say that this is a 60 amp hour battery meaning that if you discharge it at three amps it'll last 20 hours but if you weren't wanted to discharge it at say 10.2 amps you would only get 51 amp hours out of it and it would only last five hours then. So sometimes manufacturers provide this information in a graphical form. So battery manufacturers also like to talk about the either the state of charge or the depth of discharge. So what do we mean by those? Well, you can think of it as if you have like a 100 amp hour battery that's a reference to its 20 hour rate and let's assume that you've been discharging it for five amps for six hours. So you've extracted 30 amp hours of charging at that point. We would then describe the state of the battery of having a 30% depth of discharge because 30% of the charges has been used and a 70% state of charge because that's how much charge is left. So of that 100 amp hours the state of charge refers to how much charge is left and the depth of discharge refers to how much charge has been taken again, according to a particular amp hour rating and at a particular discharge rate. So once you've selected your battery or you understand the nominal voltage and about the capacity that you need it's important to ask how long will my battery last? Batteries will degrade over time and with use especially if the temperature is going to be warm it will degrade much, much faster. So battery manufacturers will provide a plot or some data regarding the cycle life. So the number of charges and discharges that's one cycle. So how many cycles will this battery last? Before its capacity drops to 80% of the capacity it had when it was brand new. So it doesn't mean that the battery is dead. It just means that the maximum capacity the battery has been reduced significantly or at least by 20%. And so you would look at a plot like this based upon how deeply you discharge a battery. So if you only lightly discharge a battery maybe 20% or something like that it's gonna last significantly longer than if you discharge it 100%. So there's a bit of trade off here if you only discharge your battery to 20% you're probably gonna need a larger battery than if you discharge it all the way down to 100%. And I think it's also really important to note that you would want to have some sort of device that disconnects the battery when its voltage gets too low. Otherwise you have no real way of enforcing a depth of discharge limit. So most inverters for example have that functionality of disconnecting the battery if the voltage gets too low. So I'll just quickly touch on this on lithium ion batteries. So lithium ion batteries, the electrochemistry is follows the same principles as I showed you earlier and how they work is they shuttle lithium ions between their anode and their cathode depending on whether or not they're charging or discharging and the ratio or the proportion of the anode that is lithium will dictate the voltages that appear. So the anode is usually just a carbon lattice structure where we literally insert positive lithium ions into it and we extract it depending on if we're charging or discharging, so that's the fundamental. You can think of it as just like a rocking chair the ions go in and out of the different electrodes as we charge and discharge it. Lithium ion batteries are becoming more and more popular in off-grid situations as the prices is really starting to decrease. We like lithium ion batteries because they have a lot of energy density. They operate at a higher voltage than a lead acid battery at least on a cell by cell basis. And so to get more power and energy you actually need a smaller battery. The biggest challenge I think with lithium ion batteries in addition to the fact that they're still more expensive is that there is some risk of a thermal runaway happening and the battery is actually catching on fire. So you just need to make sure that the batteries you buy have a built-in protection circuit. But they offer several advantages like a very flat voltage profile as they are charged and discharged and this means that whatever you connect the battery to it will have a more stable voltage. Lithium ion batteries also last or can last much longer than lead acid batteries so their cycle life could be much larger and you can actually cycle them down. You can discharge them down to a much lower depth of discharge without really sacrificing much of the cycle life. So I'm just gonna end here with a comparison of the different types of batteries across several dimensions and I'll note that these are approximate values only. This space is constantly changing and so prices are dropping and maybe efficiencies are increasing. But you would consider a lead acid battery if you are interested in a low capital cost. If you're not worried about toxicity so you have a plan in place to dispose of the batteries and you can handle spills. Lithium ion, especially lithium iron phosphate they're good for longer cycles. So if you don't wanna replace your batteries every few years if you want it to last potentially longer then you would gravitate more towards like a lithium iron phosphate. They are lighter so if you have a portable application like a solar home system or a solar lantern then you should consider a lithium ion battery over a lead acid battery. In addition all lithium batteries are gonna be sealed so you don't have to worry about them spilling. So in my opinion it really comes down to cost and cycle life. And if your insulation is going to be very remote then you might prioritize cycle life over cost. So I'll end there and just a short plug about next month's webinar where we're gonna start our discussion on how we design off-grid systems and in particular how we might characterize the load of our off-grid system and how we might estimate it as well as estimating our energy sources. So I'll turn it over to the moderator for questions now. And here are some references that you might find useful as well. So thank you. Great Henry, thank you so much for that webinar. Please be sure to sign up and to receive an invitation for the next webinar. I'm sure all of you guys are interested in continuing the conversation. We have a lot of questions Harry so I will try to be selective. I will start with the first one. Do lead acid batteries have any requirements for ventilation? Well, if you have a flooded lead acid battery so the electrolyte isn't entirely sealed off from the environment then absolutely you need to have some ventilation. The real worry there is that if you charge a battery if you overcharge the battery it can actually give off hydrogen gas which is explosive. So you need to put it in a ventilated area. Sealed lead acid batteries like AGM or cell they will only vent out to the outside environment if a lot of gas has evolved and it's sort of a safety valve. So it doesn't really happen but for flooded lead acid batteries absolutely you need to make sure they're in a ventilated area. Okay, great. Next question. Is it possible to make nuclear batteries? Well, I think that's beyond my pay grade and understanding of electrochemistry so I'm unfortunately going to have to pass on that one. You could certainly, I mean if you think of a nuclear power plant generating electricity then certainly you could store some of the energy from a nuclear power plant. It would probably have to be a very big battery but I don't know about what a nuclear battery is so I'm going to have to pass on that one. Sorry. Okay. The next one is why are the conditions under which the lead acid batteries emit hydrogen and how much concern should be taken about danger which comes with H2 concentration in a technical room for VRLA batteries? Yeah, so I think that goes back to the first question here. The valve regulated lead acid or the sealed lead acid batteries there really isn't much of a ventilation worry. I mean, these are actually used in a lot of or at least some of the portable battery kits or solar home systems where there's really no ventilation inside but for flooded lead acid batteries, yeah, you should be worried because they do emit hydrogen and if that builds up and somebody likes a match or something you could have a bad result. So the rooms that I have flooded lead acid batteries we definitely have ventilation installed. Yeah, holes in the walls might be enough if you have some at the top of the room at the bottom of the room and you have enough of a draft that could be enough. You also want to make sure that whatever you would mount directly above a flooded lead acid battery is itself sealed. So you often don't want to put an inverter directly above a lead acid battery or a flooded lead acid battery because the hydrogen can waft up into its internal circuits and it's possible that you could have a spark that could occur. Okay, great. Next question, should we consider the toxicity of lead when using lead acid battery in off-grid energy systems? Yeah, what a great question. Absolutely you should, but this is maybe trickier to do. So the worry is that the battery is damaged or after five or six years maybe you need to replace the battery and so what do you do with the lead acid battery? What you don't want to happen is through that battery to just be left on the side of the road or buried underground. But the challenge is, and this is tricky, is there's not a lot of services out there that will come and collect those batteries for you. You kind of have to build it into your business plan or the model that you're following to make your off-grid system sustainable. You kind of have to build recovery of lead acid batteries into it because you don't want the lead, you don't want the acid to be accessed by people. So toxicity is a concern. Okay, great. Next question. Can we use fractals in the battery cells to make batteries with bigger capacities? I think this is another question that's maybe beyond my knowledge of battery development. I know what a fractal is, but I personally haven't seen it applied to batteries, but that doesn't mean that it can't be used, but I really can't offer much more insight than that. Okay, another technical question. Any way of getting the crystallization of Pb-acid BTI plates without taking the BTI apart? So I know that there are some chargers that purport that they can get the crystals off. They pulse a high voltage, and they report that they have some success in doing that. I don't know that the charge controllers that I've seen in a lot of Oscar installations have that capability. I don't think so. There's probably some manufacturers that do it, but it is an area. I mean, you can buy a piece of electronic equipment that will try to help refurbish batteries that have some of those crystals on it. It's, I'm sure, more expensive, and you really have to carefully map the battery that you use with the device that does that. Okay. What about lead-air batteries? I think, so there are some battery chemistry that, like, I think there's a zinc-air battery and even an aluminum-air battery. In my opinion, I actually haven't seen these in the field myself, and they're probably pretty rare, at least in off-grid applications. There's a battery, a new battery a company called Nant Energy has been publicizing that they have used in some off-grid installations, and they expect it to be competitive, cost-wise, with lead-acid batteries. And if they're able to get that price down, if that actually is true, then it's going to be a game-changer, but we'll just kind of have to wait and see. So I would actually encourage anyone who is thinking about doing an off-grid system to really stick with more mature technologies rather than brand new ones, unless you want to go out and support it and replace it if the new technology isn't as good as it's promoted or marketed to be. So that's me. I'm certainly on the more conservative side of things, but I would really discourage people from using one-off technologies. The people in these communities, they deserve better than to be treated as guinea pigs. I agree completely. Next question. Does EPA regulate lead-acid battery? Oh, well, I can't. I'm not a policymaker. I don't know for sure. And of course, the EPA would only have any sort of authority in the United States, and I haven't done any installations in the United States, so I don't know. But again, if you're going to be using lead-acid batteries, you should be mindful of the fact that at some point you're going to need to get rid of them, and it's also possible for them to leak and you need to plan accordingly. So unfortunately, I can't get into the policies or the regulations by the EPA on that. Okay. Next question. Impact of sub-zero freezing of batteries? Yeah. So batteries are kind of funny. They're powered by these chemical reactions, and like any chemical reaction, the higher the temperature, the more favorable the reaction basically promotes that reaction happening. So when you freeze a battery or you drop it into low temperatures, you actually are going to lose some of the capacity of the battery. The reactions just aren't going to occur as frequently or as readily. In addition, the terminal voltage that you would expect is going to be different than under room temperature, and it's usually going to be less. The voltage will decrease, but that temperature coefficient, if you get the battery cold enough, will actually flip, and you'll see the voltage rise a little bit. That's kind of more of an academic detail. So the best advice then is to prevent your battery from being really cold. You might only consider keeping your battery cool if you're going to not be using it for a while, and you want to limit itself discharge. So that self-discharge in a battery, which we didn't talk about today, but it is an important thing to consider, is the battery will gradually lose charge due to internal reactions over time, and you can slow those reactions by reducing the temperature of the battery. And I would say another concern about really cold battery temperature operation is you don't want the water in the electrolyte to actually freeze. That can actually damage the battery mechanically. So avoid really low temperature operation, but also avoid really high temperature operation as well. Okay, excellent. I have one very specific question. So as far as cost difference between lead acid and leadium, Ion will sell voltages of 2.4 voltages due to 4. It looks like you would need 6 cells for one and only 3 cells for the other. Is the leadium Ion battery still more expensive? Yeah, so if you look at how most people calculate the cost of batteries, they do it not necessarily by the voltage but by the energy it can provide. And so they are already taking into account that lithium ion cells have a higher voltage than lead acid. Okay. They do it at a battery level, yeah. Okay. One other question. I think we have a few minutes left. For typical 60 cell, 120 voltage battery, how often should we equalize charge on the battery? Is it different for the two types of batteries? Yeah, so my default answer to this is you should follow what the manufacturer suggests. I would be... I think I'd be wrong in trying to give you some advice other than that because each battery is a little bit different. And when you equalize charge, it's a certain protocol that needs to be followed. So I would just look at what the manufacturer recommended and follow that. Sorry that I can't give you any more insight, but definitely look at what the battery manufacturer recommended and follow that. Okay, this is also very specific, so this might have the same answer, but I'm going to throw it at you anyway. At what temperature does the typical lead acid battery freeze? Like following up on another question. Well, I guess I would suppose, my guess would be around zero degrees Celsius. I mean, the water is really what's freezing in the battery. I don't think that sulfuric acid freezes at a higher temperature than battery. I suppose I could be wrong, but I would just... I know that the water will freeze at zero degrees, so I'm going to keep the battery certainly warmer than that. I mean, this is why, or a reason why we have trouble starting a car when it's really cold is the battery, all of its reaction is slow when it's cold, and the voltage is going to be suppressed, and you won't get as much current out of it when you connect the battery to the starter. Okay, great. Next question, and then we close out. Are use electric vehicle batteries suitable for off-grade applications? I suppose this is a pathway because batteries in electric vehicles are really designed for performance. They want to be able to extract a lot of power out of them very quickly, and so when the battery is no longer able to do that, it doesn't mean that the battery is entirely dead. It just means that it's not suitable for a mobile application, so I suppose if we could come up with a way of getting these batteries quality checked them and get them into off-grid systems, I think there's some potential there. I mean, this has been discussed in different applications, like even just in homes, and for example, the United States, can we repurpose Tesla batteries or LEAF batteries and use them for home energy storage? So I would say that there's an opportunity there. I would also refer to my previous remarks that if this is something that you're interested in doing, I would be reluctant to do it in an off-grid remote community on my own. I would wait for there to be a more established supply chain to be able to get some sort of warranty on the battery, and so forth, and much, of course, you're willing to do that yourself, and that takes quite a commitment. Okay, great, Henry. I think this webinar was very useful. Thank you so much, everyone, for joining. Thank you, Henry, for another great webinar. I invite everyone to sign up to our membership so that you receive the next invitation for this series, and thank you. Have a great day. Thank you, Henry. Thank you, everyone. Yeah, see you in February. See you. Bye.