EV Fundamentals: Battery Cell
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Battery Cell
09.20
| 00:00 | At the heart of any electric vehicle system is the battery. |
| 00:02 | And so in this section of the course, we'll be breaking these power storage systems down into their individual components in order to get a full and proper understanding of how they work and what they do. |
| 00:12 | So, let's start at the beginning with the battery cell. |
| 00:15 | All EV batteries are made up of hundreds or even thousands of cells arranged in a combination of series and parallel to achieve a target capacity and voltage. |
| 00:23 | The lithium-ion battery is a rapidly evolving technology. |
| 00:26 | So, with that in mind, let's look at more of the overall design and layout of batteries and how they work rather than focusing too much on any one particular chemistry. |
| 00:36 | Currently, the two most popular types of lithium-ion batteries used in EVs are lithium-iron phosphate, LFP for short, and a mixture of lithium, nickel, manganese, and cobalt known as NMC. |
| 00:47 | Both types of cells are similar, although there are a few notable differences to keep track of. |
| 00:52 | We'll briefly cover these later in this module and then in the upcoming battery dynamics module, we'll discuss how the two types react to different dynamic environments such as fast charging and discharging and how they differ at extreme temperatures. |
| 01:05 | Battery cells are rated in terms of their capacity in amp hours or in other words how many hours they can discharge at one amp of current. |
| 01:11 | They're also typically listed with their energy, but if they aren't, the energy can be roughly calculated as a cell's capacity multiplied by its nominal voltage. |
| 01:20 | For example, a 4.2 amp hour rated battery with a nominal voltage of 3.6 volts would have an energy capacity of 15.1 watt hours or similar to the amp hour unit, the battery can discharge at a power of 15.1 watts for one hour. |
| 01:34 | Cells are also typically sold with a data sheet listing the maximum and the minimum allowable voltages, which must be respected at all times for the safety and longevity of the cell. |
| 01:44 | The charge and discharge rating is often expressed in C rating, which indicates the maximum rate, which a cell can be discharged and charged. |
| 01:52 | So, a C rating of 1 correlates to one hour. |
| 01:55 | A cell that can be charged at 5C on the other hand means that it can be charged at a rate of 5 times its capacity so a 4 amp hour battery could be charged at 20 amps. |
| 02:03 | C rating is useful when comparing different size cells because it takes the cell's capacity into account. |
| 02:09 | So, instead of saying this cell can charge at 50 amps and this cell can charge at 20 amps, which is kind of meaningless without knowing the size of the cell, the C rating tells us how hard we can push the cell relative to its capacity. |
| 02:19 | Higher C ratings mean you can get more power into and out of the cell. |
| 02:23 | Battery cell data sheets will usually also list the maximum charging and discharging temperatures, which is critical to pay attention to as preheating the battery may be required to allow battery charging or regen in very cold temperatures and similarly for power reduction when the cell gets too hot. |
| 02:37 | Another interesting bit of information provided in the data sheet is that cell suppliers will generally let you know how the cell reacts when things go sideways. |
| 02:45 | Cells are tested in extreme heat, low ambient pressure, short circuit, shock, over voltage and even cell punctures. |
| 02:52 | This information is very helpful in determining how safe the cells are in these extreme circumstances. |
| 02:58 | Before we get into the chemistry of battery cells we should first discuss the different form factors that lithium-ion cells take. |
| 03:05 | There are three types that are used in the EV market. |
| 03:08 | Cylindrical cells, which are similar to your AA or AAA batteries, prismatic cells , which are encased in a rectangular housing and pouch cells, which look like little ziplock bags full of energy. |
| 03:18 | Each form factor has its advantages and disadvantages but the EV market seems to be headed in the direction of the cylindrical cell as these seem to be the cheapest to manufacture and automate the production of. |
| 03:29 | These cylindrical cells are available in a few different sizes with 18650 having been the most popular. |
| 03:34 | That's 18 millimeters in diameter and 65 millimeters tall. |
| 03:37 | 21700 cells on the other hand are 21 millimeters in diameter and 70 millimeters tall. |
| 03:43 | The relatively small increase in diameter yields a significant increase in volume though making the cells more efficient as there's less casing material per unit volume and as you'd expect it's easier to build a battery with fewer cells. |
| 03:55 | These are now taking over the 18650 as a preferred form factor. |
| 03:59 | Lastly a 4680 cell type was recently announced by Tesla. |
| 04:02 | This takes the size up to 46 millimeters in diameter and the length up to 80 millimeters long. |
| 04:07 | The challenge here is that there's a greater distance between the center of the cell and the surface and since the cells are surface cooled that may mean more difficulty in keeping the temperatures under control. |
| 04:17 | There are of course downsides to the cylindrical style cell when comparing it to prismatic cells. |
| 04:21 | The main ones being that they require more work when building a battery module, they're harder to cool and there's wasted space between the cells as the cylindrical shape doesn't take up all the available space in the battery. |
| 04:32 | As you can see the prismatic cells we've already touched on almost look like complete batteries. |
| 04:37 | They sometimes even come with threaded holes for the positive and negative terminals. |
| 04:41 | The pros here are that they're relatively easy to build up into modules, they're available in larger capacities meaning that fewer cells are required to build up a complete pack and they use up all the available space. |
| 04:52 | The big downside is the difficulty in cooling this type of cell. |
| 04:56 | They're typically only cooled using a cold plate on the bottom of the cell and that means there's always going to be a large thermal gradient between the top and bottom. |
| 05:03 | Lastly we have the pouch cell. |
| 05:06 | These are a very minimalist format and can be extremely powerful, lightweight, although not something you're going to see very often because they're difficult to produce in high volume. |
| 05:14 | An enclosure is also required to both mechanically support the cells and to limit the amount of swelling that occurs when the cell is being used. |
| 05:21 | Now, that we have a good understanding of the different styles of cells used in electric vehicles, let's touch on the chemistry. |
| 05:27 | NMC cells, which if you remember from earlier are a mix of lithium, nickel, manganese, and cobalt, can have their chemistry designed to favor either energy density or power. |
| 05:37 | Because the majority of EVs are focused primarily on energy density for range, you'll generally see energy focused cells in large EVs. |
| 05:44 | However, in hybrids, where we tend to see the use of relatively small batteries that need to be able to drive the car at highway speeds, cells need to be able to output enough power to accelerate these vehicles at an acceptable rate at highway speeds. |
| 05:58 | Batteries from the Chevrolet Volt for example have been very popular in EV conversions, because they contain extremely powerful cells with a pack half the weight experiencing significantly less voltage sag than an energy focused Tesla battery. |
| 06:10 | Just for clarity's sake, voltage sag refers to the drop in voltage when the cells are discharging at a high C rate. |
| 06:17 | Don't worry too much about this right now, we'll be covering it in more detail soon. |
| 06:20 | NMC cells typically have a maximum charge voltage of between 4.2 and 4.25 volts and a minimum voltage of around 2.8 to 3 volts. |
| 06:29 | The nominal voltage is usually around 3.7 volts. Let's look at this chart to get a better understanding. |
| 06:35 | Here you can see the typical discharge curve of an NMC cell, which shows how the voltage of the cell changes as it's discharged. |
| 06:42 | You'll notice that the cell quickly drops from 4.2 to 4 volts as it loses the top of its charge. |
| 06:47 | From there, the drop-off is fairly linear to around 3.2 volts at which point the voltage drops off quickly as a cell is no longer able to push those electrons with any urgency. |
| 06:57 | Every cell chemistry will be a little bit different but this gives you a rough idea of what to expect. |
| 07:03 | Lithium iron phosphate cells or LFP cells are cheaper to manufacture and don't require rare earth minerals like cobalt so they are becoming more popular. |
| 07:12 | The downside of LFP cells is that they're not as energy dense meaning that they're generally suited to lower cost, lower capacity battery packs like something you'd see in the Tesla Model 3 standard range for example. |
| 07:24 | This cell chemistry also has a lower voltage of only 3.6 volts maximum. |
| 07:28 | One difficulty with LFP cells is that they have a very flat discharge curve. |
| 07:33 | That makes it difficult to estimate the cell's state of charge because the battery will pretty much always show the same voltage of around 3.2 at rest unless its charge level is over 90% or under 10%. |
| 07:45 | For this reason the battery needs to be regularly fully charged to recalibrate the management system's energy estimation. |
| 07:50 | In both chemistry types the cells want to be kept at a similar temperature to what us humans like to be at. |
| 07:56 | At temperatures much below 20 degrees celsius the cells start to have significantly more internal resistance and experience more sag the colder they get. |
| 08:04 | Again we'll be covering voltage sag soon. |
| 08:06 | Cells can also be permanently damaged if they are worked too hard at cold temperatures. |
| 08:11 | As the temperature goes up the cell's internal resistance continues to go down and they're generally able to output more power and produce less heat. |
| 08:18 | However, run them too hot and they can be damaged permanently and there's a risk of a thermal event. |
| 08:24 | Keep in mind the temperature monitoring of the cell is not at the core of the cell so the actual temperature is likely significantly higher than what's recorded during rapid dynamic moments. But more on that later. |
| 08:34 | Now, that you have a good understanding of the individual cell in the next module we'll discuss how they're built up into useful modules but first let's quickly run over what we've learned in this module before moving on. |
| 08:45 | The two commonly used types of lithium ion batteries used in EVs are LFP and NMC. |
| 08:50 | Both are similar but they do have key differences. |
| 08:54 | LFP is cheaper to produce but less energy dense while the more energy dense NMC is commonly used in higher end applications and is able to be designed to favor either higher energy density or higher power depending on what's required. |
| 09:06 | Cells usually take three different forms cylindrical, prismatic and pouch. |
| 09:10 | All three have their pros and cons but the cylindrical and prismatic seem to be the most popular at this time in production EVs. |
