EV Fundamentals: Battery Cooling
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Battery Cooling
05.57
| 00:00 | To be able to properly cool a battery for aggressive driving use, we really need to understand two metrics. |
| 00:05 | How much heat the battery produces and how much heat can be removed from the system. |
| 00:09 | We've already covered a little bit about internal resistance and how to roughly calculate the battery heating that will occur under different use cases in the battery section of the course. |
| 00:17 | But in this module, we're going to look at exactly how to estimate the battery heat load produced. |
| 00:21 | Keep in mind, in many applications, the battery and coolant can be used as a thermal buffer. |
| 00:25 | So, the cooling system doesn't necessarily need to be sized to be able to keep the system at a target temperature under full load. |
| 00:31 | In fact, I don't know of any OEM EVs, which can run at full power for more than 8 or 10 minutes at the absolute maximum and not overheat the battery or some other components. |
| 00:39 | It simply doesn't make sense for OEs to size the cooling systems that large for the 1% of customers who will push their cars that hard on the racetrack. |
| 00:47 | Most road cars have cooling systems sized only for what's required, which is usually just enough to keep the battery temperature stable during repeated fast charging cycles and some light drag race or single lap usage for magazine tests. |
| 00:59 | However, what do we do if we want to build a system to be able to run for longer than a few minutes without overheating? Let's take a look, assuming that the inverter, motor, and other high voltage components are not thermally limited. |
| 01:09 | The first consideration is that the thermal conductivity between the cell and the coolant must be large enough to facilitate the heat transfer required. |
| 01:17 | Many battery designs use cooling only on the bottom of the cells or in the case of Tesla, small strips that just touch a portion of the cell. |
| 01:24 | In these cases, it can be true that it doesn't matter how cold you get the coolant. |
| 01:27 | There simply isn't a direct enough path to the core of the battery to be able to keep things at a stable temperature when driving under sustained load for an extended period of time. |
| 01:35 | The reality is that, unfortunately, many battery designs are simply not compatible with high performance cooling. |
| 01:40 | As a result, many battery suppliers list a maximum pulse time, which indicates the maximum amount of time that a cell can be put under extreme current before a rest period is required. |
| 01:49 | One of the reasons for this is that it allows the heat that's at the core of the cell to conduct out to the cooling medium and to the temperature sensors, allowing the control system to get an accurate picture of what the real state is of the cells. |
| 02:00 | If the system was allowed to run at full power indefinitely, the core of the cell could approach a dangerous temperature well in advance of the surface mounted temperature sensors. |
| 02:08 | In cases where the path between the cell and the coolant will do the job, the heat created in a lap can be calculated fairly accurately by adding up the internal resistance multiplied by the current squared. |
| 02:18 | This is called dissipation or the amount of heat the battery is dissipating, with the majority of it going into the coolant. |
| 02:25 | A smaller proportion of the heat will be conducted through the bus bars and radiated into the surrounding air. |
| 02:30 | In the following example, we've calculated the dissipation of our 350Z race car's 100 kW hybrid battery to be about 5.5 kW on an average throughout a lap. |
| 02:39 | This is a relatively tiny amount of heat compared to the heat produced by an internal combustion engine, but the difficulty is that the heat rejected to the airstream is proportional to the difference in temperature between the fluid and the air, as you'd expect. |
| 02:52 | A properly ducted dual-pass performance racing radiator might dissipate around 40 kW of heat with a temperature gradient of 40°C at typical racetrack speeds. |
| 03:02 | Now, the challenge we face comes when the temperature gradient is small, something you'd see with a high-voltage battery on a hot day. |
| 03:08 | If the inlet air is 35°C and the inlet water temperature is 40°C, that same radiator at the same speed of operation would only dissipate about 8.75 kW of power. |
| 03:19 | Depending on the water pump flow rate and the area of conduction to the battery, it's likely that a very large radiator would be required to cool this relatively small battery, or we'd need to be willing to run the battery at a hotter temperature. |
| 03:30 | Water flow rates aren't the only solution to everything, and while it's true that heat rejection calculations suggest that if we pump water faster, we'll dissipate more heat, there's no magic bullet here, because as you pump the water faster, the temperature gradient gets smaller. |
| 03:45 | Generally, the water only needs to be pumped fast enough so that the temperature gradient across the radiator or components isn't too large. |
| 03:52 | For example, if the water is pumped too slowly, it'll have cooled significantly before reaching the exit of the radiator, rendering a large amount of the radiator ineffective at dissipating heat. |
| 04:01 | If, on the other hand, the water is getting through the radiator with only a few degrees of temperature drop, any further pumping is just wasted energy as the core of the radiator can't get any hotter anyways. |
| 04:11 | With all that said, it is important to note that when there's a large temperature difference between the inlet and outlet of a cooler or a component, pumping the fluid faster will still yield a useful benefit. |
| 04:20 | It's also significant to understand that the power to pump coolant is exponential, so adding a second pump will not get you double the flow rate, and if you're looking for a lot more flow, you'll need a lot more power. |
| 04:30 | Based on the above, a balance needs to be kept. |
| 04:33 | If the inlet and outlet temperatures of the battery are differing by 10 or 20 degrees Celsius, clearly more pump would be beneficial. |
| 04:39 | If the inlet and outlet are only a few degrees apart, wasting energy on more or larger pumps will not be very useful. |
| 04:44 | Summarizing what we've learned in this module, the majority of OEM cooling systems simply aren't designed to deal with sustained performance driving conditions, and that means we're likely to get one, maybe two laps out of an EV road car before it overheats. |
| 04:56 | Due to the way many batteries are cooled, often it doesn't matter how cold you get the coolant flowing through the battery, there just isn't a way to keep temperatures under control if you're using all of the car's performance for any decent length of time. |
| 05:08 | A lot of battery suppliers will list a maximum pulse time, which lets us know the maximum amount of time under extreme current before a rest period is required. |
| 05:16 | With that in mind, if we're putting together an EV conversion project, we need to be careful when selecting donor batteries. |
| 05:21 | Do the research and make sure that whatever you're looking for has the capabilities to match the way you plan to use the vehicle. |
| 05:27 | A drag racing application is much easier to cool compared to a road racing application. |
| 05:31 | A high voltage battery and a very hot day can be challenging for anyone looking to get sustained performance from their vehicle, and these kinds of conditions are going to require a large cooling system due to the small differences in temperature between the air and the coolant. |
| 05:43 | Increasing the water flow rate won't solve all of your problems, the coolant only needs to flow fast enough such that the temperature gradient across the radiator or components isn't too large. |
| 05:52 | Anything faster than that, in most cases, is just wasted energy. |
