EV Fundamentals: BMS
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BMS
08.36
| 00:00 | You can have the best battery cells, modules, and pack ever made, but if you don't have the ability to control it all, it's not going to be of much use. |
| 00:05 | This is where the Battery Management System, or BMS for short, comes in. |
| 00:09 | The BMS can be thought of as the brain of the battery system. |
| 00:12 | Its responsibility is to ensure that the battery never operates in any way that would excessively stress or damage it, and to broadcast to other devices what the safe limits of the battery are. |
| 00:22 | The BMS also precisely calculates the state of charge of the battery, and performs cell balancing, which is the job of keeping all of the cells that are in series at the same voltage. |
| 00:31 | The first task the BMS needs to handle is the operation of the contactors that we learned about in the previous module. |
| 00:37 | This starts with ensuring that it's safe to power up the high voltage bus, and then closing one contactor and the pre-charge relay, pre-charging the bus, and then closing the second contactor and opening the pre-charge relay. |
| 00:48 | To determine if it's safe to close the contactors, the BMS will confirm that there are no open high voltage connections in the loop, aka HVIL connections, and that the vehicle control unit is also not reporting any faults. |
| 01:00 | We will be looking at HVIL connections later in the course, so for now, just know that these are a safety component that protect us from shock danger when working with a high voltage system. |
| 01:09 | The BMS, or in some cases the VCU, will often have an isolation module. |
| 01:13 | This module measures the electrical resistance between the high voltage positive and chassis ground, as well as the high voltage negative and chassis ground. |
| 01:21 | If there's ever a breakdown of isolation, the BMS can open the contactors and alert the driver that there may be a shock hazard. |
| 01:27 | The BMS can also ensure that there's required isolation prior to closing the contactors. |
| 01:32 | If the VCU is commanding the battery to come online and everything looks good, the first step will be to close the contactor opposite the pre-charge. |
| 01:39 | By this, I mean if the pre-charge is on the positive terminal, the negative contactor would close first. |
| 01:43 | This now creates a path for current to flow when the pre-charge relay closes. |
| 01:47 | At that point, the high voltage bus will slowly start charging up the battery voltage through this pre-charge resistor. |
| 01:52 | Because the motor inverters have very large capacitors, the pre-charge resistor is required to prevent a massive current inrush, which would result in arcing inside the contactors and excessively large current. |
| 02:02 | This arcing would wear out the contactors and could even weld them closed. |
| 02:06 | For that reason, the contactors always open and close under zero current conditions whenever that's possible. |
| 02:11 | It is true that these components are designed to be able to break high currents, but that's not something they should be doing on a regular basis. |
| 02:17 | The rate of climb of the high voltage bus depends on the bus capacitance, voltage, and the size of the resistor. |
| 02:23 | And this is why sizing the resistor correctly is so important. |
| 02:26 | A lower resistance will charge up the bus faster, but dissipate more heat and thus require a more powerful resistor, or limit the amount of time it's on. |
| 02:33 | Generally, a few tenths of a second is an acceptable pre-charge time. |
| 02:37 | As a real-world example, if we have a 1000-ohm resistor at 400 volts, it will dissipate 160 watts of power. |
| 02:43 | But since the pre-charge is only used for a few seconds at a time at most, we can generally get away with a resistor that's slightly undersized for the job. |
| 02:49 | For the most part, once the high voltage bus is within a close tolerance of the battery voltage, the second contactor is safe to close. |
| 02:56 | Once both contactors are closed, the pre-charge relay will open, and the battery will be online, and will broadcast this to other high voltage devices, along with the charge and discharge limits, based on the battery's current state of charge and temperature. |
| 03:08 | Now, it's important to understand that BMS can't directly control how much current outside components draw or put back into the battery. |
| 03:15 | So, the BMS's only method of defense here is the contactors, which can break open to disconnect the battery from the bus. |
| 03:21 | To be clear though, this is very much only something that's done in emergency situations. |
| 03:26 | The BMS relies on CAN or some other communication method to instruct the inverter, charger, DC-DC converter, and other components what the maximum current and power is that can be discharged or charged into the battery at any time. |
| 03:39 | The BMS will calculate these limits based on a number of factors, such as the maximum or minimum cell temperature, the temperature of other ancillary battery components, like the contactors or the current sensor, as well as the battery's current state of charge. |
| 03:51 | We'll be diving into battery dynamics in much more detail in the next module, but for now, just know that the BMS needs to be able to rapidly respond and broadcast its limits so that all of the devices that rely on the battery's information can respond accordingly. |
| 04:04 | The BMS may also reduce the allowed current based on time. |
| 04:07 | So, for example, if there's a large pulse of discharge current for a few seconds, the BMS may start reducing the allowable discharge current based on the battery chemistry in order to protect the cells from being overstressed or damaged. |
| 04:18 | If the BMS detects that its limits are not being respected, it will typically respond with a fault, which will command all the devices on the bus to shut down. |
| 04:26 | The BMS will then wait a few moments before popping the contactors. |
| 04:29 | This will take the battery totally offline. |
| 04:31 | Once the high voltage bus goes offline, the vehicle typically is rendered disabled until the fault is cleared. |
| 04:36 | So, these types of faults are really a worst-case scenario, and for people who rely on their cars to get from A to B, these types of faults need to be avoided at all costs, which is why a lot of work has been put into avoiding them by OEMs, and they're very rarely seen. |
| 04:48 | The third and final main job of the BMS is to balance the cells. |
| 04:52 | As we discussed earlier in this section of the course, when cells are connected in parallel, they self-balance. |
| 04:57 | A cell with a higher charge will simply charge up a lower cell, and all the cells will even out naturally. |
| 05:01 | However, when cells are connected in series, there's no natural balancing force. |
| 05:05 | The higher charge cell below or above the lower charge cell doesn't know or care about the lower charge cell in the middle. |
| 05:11 | They simply add on top of each other, without directly interacting. |
| 05:15 | This means it's the job of the BMS to balance everything out, ensuring that every cell is as close as possible to the others. |
| 05:21 | This brings us to one key point that's always worth remembering. |
| 05:24 | It doesn't matter how many cells and modules you have in a battery pack, the limits of a battery apply to a single cell. |
| 05:30 | So, if you have one cell that's overcharged, the entire battery will have to stop charging when it reaches its maximum allowable voltage. |
| 05:36 | If the rest of the cells are only at 70% charge, that means the capacity of the battery has been reduced by nearly 30%. |
| 05:41 | Ensuring that the pack is properly balanced isn't just for capacity either. |
| 05:44 | It's also for power and chargeability. |
| 05:47 | If a single cell is at 20%, while the rest are at 40%, the amount of charge the battery will be able to output will be significantly reduced as well, as that undercharged cell will drop below its minimum allowed voltage well before the others, requiring the battery management system to quickly reduce the allowed discharge current. |
| 06:03 | In order to balance the cells, the BMS has small voltage taps between each individual cell. |
| 06:08 | There are typically sub-BMS boards in each module to reduce the length of these voltage tap wires for safety and cost reasons. |
| 06:14 | These sub-BMS boards then create a circuit across each individual battery cell with a resistor that can be activated to slightly discharge it. |
| 06:22 | This only goes one way, meaning it's not possible to charge up a discharged cell. |
| 06:26 | Rather, we can only bleed down an overcharged cell. |
| 06:29 | These sub-BMS boards will communicate with the main BMS controller either with CAN or SPI, giving the main BMS insight into the voltage of each individual cell as well as a multitude of temperature measurements. |
| 06:40 | The main BMS determines when to start balancing based on a number of factors, but generally it's common to top balance a battery, which essentially is bleeding down the highest charged cells near the top of their state of charge. |
| 06:51 | If you remember back to the battery cells module, you'll recall that near the top of a cell's charge, the voltage starts to rise very quickly with a relatively small amount of charge. |
| 07:00 | The BMS can also observe this occurring and bleed down the cells that start to increase voltage faster than the others. |
| 07:06 | Lastly, the BMS can calculate the state of health of each cell by calculating its capacity. |
| 07:10 | If the voltage of a specific cell starts rapidly dropping before others, this indicates that its capacity is lower than the rest. |
| 07:17 | Remember, the total energy that can be used from a battery is limited to the weakest cell group, which we define as a group of cells in parallel. |
| 07:23 | So, if one cell group drops below the critical minimum voltage, then the entire battery can no longer discharge, even if all the other cells still have plenty of energy to offer. |
| 07:33 | A balanced pack ensures that all of the cells are worked evenly and that the maximum amount of energy can be used from the battery. |
| 07:39 | The BMS is really the brain of the battery system, and having a good understanding of its operation and tasks is an important step in properly understanding the workings of an electric vehicle. |
| 07:48 | Before moving on, let's quickly go over a few key points covered in this module to finish up. |
| 07:52 | The BMS is responsible for ensuring that the battery never operates in any way that would excessively stress or damage it. |
| 07:58 | This is done primarily through the safe operation of the contactors, ensuring that they open and close correctly with the least amount of current possible. |
| 08:05 | It'll also pop the contactors in the event of a fault to protect the battery pack as well as to mitigate any potential hazards to the driver. |
| 08:12 | Through constant monitoring, the BMS is always aware of the health of every battery cell and is making sure that the safe limits are adhered to and communicated to other devices on the high-voltage system via CAN or some other communication protocol. |
| 08:24 | The BMS is also able to calculate the state of charge of each battery cell and balance out the cells that are linked in series, keeping them all at the same voltage. |
