EV Fundamentals: Level 3 Charging (DC Charging)
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Level 3 Charging (DC Charging)
07.10
| 00:00 | As any EV owner is probably already aware, besides level 2 charging, which is relatively slow, often taking upwards of 10 hours to fully charge an EV, there's also a faster way to charge your car, which is known as level 3 charging. |
| 00:12 | This type of charging completely bypasses the vehicle's onboard charger and makes a direct connection between an off-board charger and the vehicle's high voltage bus. |
| 00:21 | Basically, instead of using the onboard charger, the car uses a much more powerful off-board charger. |
| 00:27 | This high power unit can supply much more power to the battery than the onboard charger, but the costs associated with a level 3 charger versus a level 2 are orders of magnitude greater. |
| 00:37 | Rather than just a simple box with some relays, there's now a requirement for a lot of expensive power electronics, as well as a need for infrastructure in place to supply 150 to 300 kilowatts per charger. |
| 00:48 | We're talking about a substantial amount of power here, which in rural areas especially, may not exist anywhere nearby. |
| 00:55 | There are three main types of level 3 chargers, and we'll start with a look at the Tesla Supercharger, simply because these were the first chargers to show up in any real volume around the world. |
| 01:04 | In North America, these chargers use the same connector as Tesla's level 2 charging, the NACS connector. |
| 01:10 | Using the same pins for both AC and DC makes the inlet connector very compact and elegant. |
| 01:15 | In other parts of the world, though, Tesla has adopted the connector to meet local standards. |
| 01:20 | For example, in Europe, Tesla now uses the Type 2 standard. |
| 01:23 | The second most popular fast charging standard is called CCS, which stands for Combined Charging System. |
| 01:29 | There are two formats of the CCS, which combine either the J1772 or Type 2 plug with an additional two large contacts below that feed in the DC voltage. |
| 01:39 | The idea with CCS is that it uses the same connector for signaling and simply adds two larger high current contacts for high power charging. |
| 01:47 | The final charging standard, and one that holds a special place in my heart, is called CHAdeMO. |
| 01:51 | This standard was developed in Japan, and was used on some of the first EVs around the world. |
| 01:56 | CHAdeMO uses CAN bus-based signaling, making it relatively easy to integrate into an EV conversion. |
| 02:00 | Unfortunately, this standard seems to be dying out as Europe is sticking with CCS and North America is heading towards the NACS standard. |
| 02:08 | The good news is that there are now aftermarket CCS controllers, which can be purchased and integrated into EV conversion builds, and I'm sure NACS controllers will be on the near horizon. |
| 02:18 | From a high voltage standpoint, all three standards function in a similar way. |
| 02:22 | Communication is established between the charger and the vehicle, then a separate set of contactors inside the vehicle close and an isolation check is performed to ensure that there's no possibility of current leakage to the chassis earth, which is connected with the ground pin. |
| 02:36 | Of important note, in the NACS standard, because the same pins are used for AC and DC charging, special circuitry needs to be implemented if the onboard charger cannot withstand the high DC voltage that will be applied to the input side of that onboard charger. |
| 02:52 | For example, additional contactors may be required between the onboard charger and the charge inlet. |
| 02:58 | If after performing the isolation check all is good, charging will commence. |
| 03:02 | Otherwise, the contactors are quickly opened and an error is declared by both the car and the charger. |
| 03:08 | Once charging has started, the level 3 charger informs the vehicle of how much power it can supply and the vehicle then commands the charger to start outputting current. |
| 03:17 | The charger and vehicle continue to communicate for the duration of the charge. |
| 03:20 | DC fast chargers started out providing only 30 to 50 kilowatts of power just a few years ago. |
| 03:25 | We're now up to about 350 kilowatts at some chargers. |
| 03:28 | At power levels like these, batteries can charge extremely quickly with a ton of current, which produces a huge amount of heat. |
| 03:36 | That means that cooling is extremely important, but so is preheating. |
| 03:39 | As we discussed in the battery module section of this course, for a battery to be able to charge quickly, it must have low internal resistance. |
| 03:46 | This is because there's a maximum saved voltage that each individual battery cell must operate under, which effectively limits how much current you can push into that battery, simply because you need more pressure or voltage to inject more current. |
| 03:58 | So, DC fast chargers are able to charge at very high power levels at low battery state of charge when the difference between the battery's voltage at that charge level and the maximum voltage is very large. |
| 04:10 | Have a look back to the battery section of this course if you want to recap on that, as well as how to calculate how much voltage will change based on the different current levels. |
| 04:19 | What's important here though is that generally a warmer battery will have less internal resistance, which means it will be able to accept more current without exceeding its maximum saved voltage. |
| 04:29 | For those reasons, EVs are now preheated prior to arriving at a high power charging station, so that they're able to continuously absorb these huge amounts of power, which are often in excess of what they'd be able to discharge in a similar amount of time. |
| 04:41 | This concept explains why level 3 chargers taper off the power as the battery state of charge goes up. |
| 04:46 | There's simply not enough difference between the rest voltage of the battery and the maximum saved voltage to jam the electrons in there at the same rate. |
| 04:54 | As we discussed in the earlier onboard charger module, this charging method is called constant current, constant voltage, and it's just a quick way of saying charge at the maximum rate possible until a maximum voltage is hit, at which point ramp down the current to keep from exceeding this target voltage. |
| 05:10 | Most chargers, both level 2 and level 3, will have the constant voltage value set quite high to allow full power charging up to as high as possible, but this is where manufacturers and battery suppliers spend a lot of time learning about what is safe and what could damage the battery excessively in the long term. |
| 05:26 | For EV conversions, it's important to carefully program the charge current limits based on temperature to avoid pushing a battery too hard when it's too cold, and also to ensure that charging rapidly slows down and fully stops if the battery temperature gets too hot. |
| 05:39 | Products are now coming onto the market that are standalone CCS or CHAdeMO controllers, and some onboard chargers even support communicating with these units natively. |
| 05:49 | This means it's becoming easier than ever to integrate level 3 charging into your EV conversion project. |
| 05:55 | To do this, you need an additional two contactors that connect these DC pins on the charge inlet to the DC positive and negative rail on the battery. |
| 06:03 | One last but important point to note here is that level 3 chargers perform isolation checks in a very similar way to those done by onboard isolation devices. |
| 06:11 | These two devices can interfere with each other and cause false low isolation faults, so the onboard isolation device may need to be deactivated when using a level 3 charger. |
| 06:21 | So, to summarize the key points found in this module, level 3 charging bypasses the vehicle's onboard charger and supplies the batteries directly with DC power. |
| 06:29 | There are three main standards of a level 3 charger that, from a high voltage standpoint, all essentially do the same thing, although with different communication protocols. |
| 06:39 | Tesla's NACS, CCS, and CHAdeMO. |
| 06:41 | Modern level 3 chargers are currently providing up to 350 kilowatts of power, and that creates a huge amount of heat, meaning that cooling is very important. |
| 06:49 | On the flip side, the batteries also benefit from being heated initially in order to reduce internal resistance, which allows more current into the battery at the same voltage, reducing charge times. |
| 07:00 | Charging slows down considerably as the battery nears a full charge, as there is less difference between the battery's resting and maximum allowed voltage. |
