EV Fundamentals: Battery Pack
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Battery Pack
06.09
| 00:00 | Once our battery cells have been combined into modules, these modules are then combined in series to create a complete battery pack. |
| 00:06 | In more recent years, most OEMs have been including other components in the pack and even using their packs as structural members of the vehicle. |
| 00:13 | However, at the simplest level, the battery pack is just an enclosure for the battery modules. |
| 00:18 | As an example, a pack containing six 67-volt series connected modules would have a total voltage of 402 volts, so that would be 96 cells in series with each module containing 16 cells in series. |
| 00:31 | Most battery packs contain the required high voltage switching and monitoring components, but it's also possible for these to be mounted in their own power distribution box. |
| 00:40 | The majority of EV conversion vehicles tend to have these power distribution components external to the pack because we don't have the luxury of designing a pack and chassis to fit together with each other like an OEM does. |
| 00:51 | However, it's always best to keep all those power distribution components together inside the pack when possible. |
| 00:57 | The battery pack will also contain the main fuse for the complete high voltage battery. |
| 01:01 | It's always best for this fuse to be placed in between modules rather than at the end of the system. |
| 01:06 | The reason for this is that when the fuse is placed between modules, it's guaranteed that the circuit is protected. |
| 01:12 | If the fuse is mounted externally, on the other hand, there's always going to be a certain amount of high voltage cable that's unfused. |
| 01:18 | At the pack positive terminal of the battery, which will be the positive terminal of the highest voltage module, a high voltage switch, which we call contactor, will be connected. |
| 01:26 | This is the main switch to disconnect and connect the positive terminal of the high voltage from the rest of the vehicle. |
| 01:33 | Similarly, on the pack negative, which is the negative terminal of the opposite end of the series string, another contactor is connected to the battery. |
| 01:40 | When both contactors are closed, the high voltage bus is live and has high voltage. |
| 01:45 | Two contactors are used for redundancy so that if one fails, there's still a secondary device to break the circuit. |
| 01:51 | In addition to the two main contactors, another small relay or contactor is used, which is called the pre-charge relay. |
| 01:57 | The purpose of this relay is to slowly bring the high voltage bus up to battery voltage. |
| 02:03 | This bus has certain components on it, like inverters, and these inverters have large capacitors. |
| 02:08 | If the contactors were just to close, there would effectively be a momentary short circuit, which would result in a large rush of current out of the battery. |
| 02:15 | And this large rush of current would cause arcing through the contactors, which would wear them out and potentially weld them closed. |
| 02:21 | The pre-charge circuit instead runs the current through this relay and a resistor, along with one of the other main contactors, to slowly charge the high voltage bus. |
| 02:28 | Once the BMS detects that the bus voltage is close enough to the battery pack voltage, it will close the other main contactor and then open the pre-charge relay. |
| 02:35 | If during pre-charge the voltage doesn't come up at all, the BMS will throw a fault and reopen the contactor and pre-charge relay. |
| 02:41 | Often, the pre-charge resistors aren't designed to carry current for extended periods of time, so it's important that the BMS be configured correctly to prevent blowing this resistor. |
| 02:49 | Next, between just one of the contactors in the battery, we'll find a current sensor. |
| 02:53 | This is the main feedback the BMS has in order to determine how much charge is entering and leaving the battery. |
| 02:58 | And this sensor is used to determine the state of charge, calculate the power, and ensure that the battery is operating within its required limits. |
| 03:05 | The battery pack may also contain a set of contactors for DC fast charging. |
| 03:10 | When the vehicle detects that it's connected to a DC fast charger, these contactors will close after they do some tests and allow the charger direct access to the battery. |
| 03:17 | In addition to making the high voltage connections between the battery modules and outside components, the pack also connects all of the coolant pathways to each module, with different packs handling this difficult task differently. |
| 03:29 | I say difficult because it's a challenge to achieve even cooling throughout the entire pack and generally the cells at the inlet will be cooler than the ones at the outlet. |
| 03:38 | Physically, a battery pack will have steel or carbon enclosure that protects the modules from the elements as well as for crash protection. |
| 03:44 | New battery packs are even being designed as structural members of the vehicle, which kills two birds with one stone by increasing the chassis strength and stiffness while also reducing the weight. |
| 03:54 | While the battery pack is almost entirely sealed, some ventilation is required to allow pressure inside and out of the pack to normalize, which is particularly important when the vehicle experiences large elevation changes like driving up and down mountain roads. |
| 04:06 | OE designed battery packs will also include liquid sensors, which are used to detect water intrusion or if an internal leak in the battery's coolant system occurs. |
| 04:14 | Due to the various exposed high voltage connections inside the pack, this can be pretty important information to have. |
| 04:19 | Finally, battery packs also have high voltage connections to the outside world. |
| 04:23 | A well-designed OE level battery pack will include all of the high voltage connections required negating the need for any external high voltage junction boxes external to the pack. |
| 04:32 | For example, an OE pack might have two connections on the front, one for a heat pump and one for the front drive unit, and then two connections on the back, one for a DC fast charging port and one for the rear drive unit. |
| 04:43 | This is the cleanest and most secure method and it keeps all the external high voltage cable lengths to a minimum. |
| 04:48 | Unfortunately, that isn't very realistic for an EV conversion battery, so these usually just have two connections from the battery , which then go to a power distribution box, which contains the contactors, current sensor, and pre-charged circuitry, as well as the connectors for the various high voltage devices. |
| 05:04 | This concept is exactly the same though, we just have two separate enclosures rather than having everything all in one. |
| 05:09 | The exception here is that the fuse should always be in the battery pack, in between modules ideally as we discussed earlier. |
| 05:15 | This ensures that there are no unfused cables leaving the battery. |
| 05:18 | That covers just about everything we need to know about battery packs, so let's quickly run over the main points again before moving on. |
| 05:24 | Battery packs are usually housed in a carbon or steel enclosure for production and contain multiple battery modules connected in series to form one very large battery. |
| 05:32 | OEM packs are usually integrated with various other components for packaging and simplicity's sake, but this isn't usually the case with packs used for EV conversion projects where you're much more likely to see these components in a separate power distribution box. |
| 05:46 | The battery pack will also contain the main fuse for the complete high voltage battery as well as a contactor at each end , which is the main switch to disconnect the high voltage battery from the rest of the vehicle. |
| 05:56 | Two are used for redundancy's sake. |
| 05:59 | While these packs are mostly sealed from the elements, we will usually find a ventilation port that ensures pressure inside and outside the pack is equalized. |
