EV Fundamentals: Battery Module
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Battery Module
07.00
| 00:00 | An electric vehicle requires large amounts of power in order to function, and that means that the battery cells we covered in the previous module need to either be combined in series or parallel to increase their voltage and capacity, creating what's known as a battery module. |
| 00:14 | These modules are also responsible for other important functions like the cooling of the cells, monitoring of cell voltages and temperatures, as well as safety requirements such as self-using. |
| 00:24 | Modules will also typically have a number of temperature sensors, with the goal of being able to detect if one area is running hotter than others, which is a clear sign that some kind of problem may be developing. |
| 00:35 | Battery modules come in various different form factors, but they're generally assembled into cubic shapes that can then be combined to create a full battery pack. |
| 00:45 | Manufacturers will design the total voltage of the module so that a certain number of modules when they're all connected in series result in the target total capacity and voltage. |
| 00:52 | What this means is that modules from high energy packs are typically lower voltage, and modules from low energy packs like what you'd find in the hybrid are typically higher voltage because there are fewer of them. |
| 01:03 | This is an important consideration if you're looking to do an EV conversion using OEM batteries, as you'll generally need to find modules from a pack that is of similar capacity to the one you're building if your target voltage is the same. |
| 01:14 | Keep in mind you likely won't be able to use the entire pack in its original form, as it's most often the wrong shape and size. |
| 01:20 | Let's now discuss cell configurations, which as we touched on earlier, fall into two methods, series or parallel. |
| 01:28 | When configuring in series, the positive end of one cell is connected to the negative end of another, and so on. |
| 01:34 | And when this is done, the voltage of each of those cells add on to one another. |
| 01:39 | So, taking NMC cells for example, each cell added in series adds another 4.2 volts. |
| 01:44 | So, if we look at the most common voltage for an EV battery pack, which is currently 400 volts, this works out to 96 cells in series. |
| 01:51 | Other voltages are used as well, and 800 volts is becoming more popular, because as we discussed earlier, voltage times current equals power. |
| 01:59 | So, more voltage requires less current, and everything can be smaller and lighter when less current is required. |
| 02:06 | When it comes to running cells in series, getting enough voltage is one thing, but it's only one part of the equation. |
| 02:12 | If for example a pack was being built with 18650 cells, these only have a capacity of around 3 amp hours. |
| 02:18 | That means that if we just stuck 96 of these cells in series, we'd only be able to draw 3 amps for an hour, with a maximum current of around 30 amps, which would be pretty useless for most battery applications. |
| 02:29 | The good news is that the capacity of the battery can be built up by combining cells in parallel. |
| 02:34 | These parallel groups then stack up in series to achieve the target battery voltage and capacity. |
| 02:40 | One thing worth remembering here is that when cells are connected in parallel, they will balance themselves out automatically and they'll always have the same voltage. |
| 02:48 | But cells connected in series can and will drift to different voltages, as there's nothing that naturally balances them out. |
| 02:54 | This means that a voltage tap is required, which the battery management system can then use to monitor the voltage of each series group, and then using bleed resistors can drain down the high cells to ensure that all of these different series cell groups are at the same voltage. |
| 03:07 | In order to have as few of these voltage taps and circuits for the BMS to manage as possible, all of the parallel connected cells are combined in the battery module, which are then stacked on top of each other in series. |
| 03:20 | So, in other words, battery modules are only connected to each other in series, and the reason for that is to reduce the number of BMS connections and external high voltage connections required. |
| 03:30 | So, it's wise if you're shopping for batteries for your project to look for modules, which can stack in series to achieve your target capacity and voltage, rather than combining a number of modules in both parallel and series. |
| 03:42 | However, sometimes this isn't feasible and you can combine modules in parallel if needed. |
| 03:46 | Moving on, battery modules are also in charge of creating a cooling and heating interface to the cells. |
| 03:52 | There are different approaches to this, but the simplest and easiest is a thermally conductive cold plate that all of the cells are in contact with. |
| 03:59 | A coolant passage through this cold plate allows the liquid to absorb heat that's released by the cells and carries that heat away to the vehicle's cooling system. |
| 04:07 | The downside of this cooling design is that heat must travel longitudinally through the cells, and significantly the busbar connections, which are typically a point of high resistance and therefore heat, are the furthest away from the cold plate, which is usually at the bottom. |
| 04:19 | With that in mind, temperature measurement is usually done at the top of the cells in these cases, as that will be the hottest part of the cell. |
| 04:26 | Other modules use coolant ribbons that contact a small part of the side of a cylindrical cell. |
| 04:31 | The benefit of this approach is that the heat has a shorter distance from the core to the coolant, but the downside is that there's a very small contact area per cell along these ribbons, and they're very difficult to manufacture. |
| 04:43 | Lastly, modules can also be air cooled. |
| 04:45 | Looking at my hybrid 350z race car as an example, our old battery used heat sink fins bonded to the module's cold plate connections to extract heat from the module with forced air. |
| 04:55 | It's important to note here that this solution is only going to be good in dry weather conditions and isn't something you'd want to implement in a road-going car. |
| 05:01 | The last consideration when it comes to battery modules is that they also need to be designed to protect the cells, and there are a few ways of doing this. |
| 05:08 | The first is with cell fuses, where each of the cells are connected to one another or to a common bus bar using a connection that has been sized so that it'll blow if a cell shorts out. |
| 05:17 | The fusing is designed to only blow when a cell fails, which would be at a current well above what it would see during normal operation, and importantly well above the current where the main battery fuse would blow, which is also called clearing. |
| 05:28 | However, because these cell fuse connections are relatively small, they are a point of significant heat creation when the battery is operating at high current levels. |
| 05:36 | This reduces the efficiency of the battery and adds more thermal load to the cooling system. |
| 05:40 | Additionally, module level fuses are sometimes added, which will blow if the current through the module is too high. |
| 05:45 | This is a slightly redundant safety consideration, as the main pack fuse should accomplish the same task, but it can be thought of as a backup protection. |
| 05:54 | Cells can also be protected through the use of a fire retardant material injected between them, or by designing an injection ports for a fire suppressant or water, which can be used in the case of an emergency. |
| 06:04 | Before we move on to the battery pack itself, let's quickly recap the main points found in this module. |
| 06:10 | A battery module consists of battery cells connected either in series or parallel, most usually both, in order to combine their voltage and provide enough capacity for an EV application. |
| 06:20 | When cells are connected in parallel, they will automatically balance themselves out and always produce the same voltage. |
| 06:27 | However, cells connected in series will not, which means that the battery management system needs to use voltage taps in order to measure and bleed down the high cells. |
| 06:34 | Battery modules also manage the temperature of the cells and will use either a cold plate or coolant ribbons to ensure that the temperature is kept at acceptable levels. |
| 06:42 | Lastly, it's also in the battery module that you'll find methods of protecting the battery cells if something goes wrong, most commonly with the use of fuses that are designed to blow before serious damage is done. |
| 06:51 | Some modules also use fire retardant materials or injection ports as additional safety measures. |
