danrobren
commented
5 months ago General Notes from the Datasheet
- The LTC6811 has two variants, LTC6811-1 and LTC6811-2. We are using LTC6811-1.
- Each LTC6811 has 12 cell-sensing inputs. If we have 96 cell banks to measure, we need 8 of these chips.
- The LTC6811-1 allows for "daisy chaining" where one chip's data is passed to the next, which then passes both its data and the data it recieved from the previous chip to the next chip, on and on until all every chip's data is passed to the BMS master at the end of the chain.
- There are many different measurement frequency modes for the LTC6811. The tradeoff made here is higher accuracy and less noise requires lower measurement frequency. The lowest frequency, most accurate measurement is 26Hz. This is still much faster than the sensor read task's frequency, so this mode seems best.
- Noise level for 26Hz is maximum 50µV peak-to-peak (16 bits ADC resolution). shutdown thresholds must take this into account.
- ADSTAT command measures internal device parameters and has a fault code. Use this in initial startup checks. ADSTATD does the same thing with "digital reduncancy"
- A watchdog timer resets the LTC6811 if it has not recieved a valid command for over two seconds. The WDT pin is internally pulled up when this happens
The BMS Slave chip is the LTC6811.
LTC6811 Data Sheet: https://www.analog.com/media/en/technical-documentation/data-sheets/LTC6811-1-6811-2.pdf
The battery is 96S 5P lithium ion. This means there are 96 cells in series and 5 in parallel. Therefore the battery votlage is 96 times the individual cell voltage and the capacity in amp*hours (Ah) is five times that of an individual cell. The cells we use have a minimum votlage of 3.1 volts when fully empty and 4.2 volts when full. Therefore the battery is 297.6 V empty and 403.2 V fully charged.
We cannot allow the cells to go above or below these maximum/mimimum voltages for safety reasons. It is the BMS master's job to measure the voltage of the entire battery, but the BMS slaves measure the votlage of the individual cells. Not every cell of the 96S 5P battery is being measured; each slave channel will connect to a bank of 5 cells in parallel and measure their temperature and voltage collectively.
Not every cell has the same internal resistance due to manufacturing variance, therefore they will discharge at different rates. Over multiple charge cycles, this effect can compound such that cell voltages are very different from eachother, even though the total battey voltage appears normal.
This is why we do balance charging. Every charge cycle, it is the BMS slaves' job to make sure all the cell voltages are equal. The BMS slaves report these cell votlages (and temperatures) back to the BMS master such that it can make decisions about whether to trigger an HV shutdown.
The BMS salve board is not finished as of 23/05/2024, but its functionality can be emulated much like the ADS7044 ADC which measures the total battery voltage This ADC communicates over SPI and its functionality was emulated using an SPI module on the launchpad itself. The goal is to do the same for the BMS slaves such that the code can be tested and ready for integration when the BMS slave board is complete.