BMS AFE: MS 14
- Arshan Khanifar
- Liam Hawkins
Description
The BMS AFE is used to measure cell voltage and temperatures from each individual battery module. The board mostly hosts the LTC6811 AFE chips that handle all of the measurement and communications. There are 3 BMS AFE boards daisy chained together in MSXIV. AFE boards communicate over LT's proprietary isoSPI protocol. This is a 2-wire differential signal that is transparent to the microcontroller through the LTC6820 interface IC. The first (closest to carrier) is AFE 0 connected to the most negative end of the pack in the battery box.
Voltage Sensing
On MSXII, ring terminals were used to connect the AFEs to the bus bars on the modules. One thing to note about the voltage taps is that since we had 18 modules in each box, 1 AFE has 12 cells connected to it and the other AFE has 6 cells connected to it. In the LTC6804 data sheet, an example of 8 cells connected is shown. Basically, you want to connect your unused cell inputs to a used cell input and not leave anything floating.
Temperature Sensing
On MSXII, the module temperatures are sensed using a single 10k NTC thermistor glued to the middle of each module using thermal epoxy (BondaTherm BT-101-50M). On MSXIV, a similar approach can likely be used. On each AFE, there are also connectors for 3 ambient thermistors and 1 PCB mounted thermistor. These can either be used to monitor the discharge resistor temperatures, or to monitor various temperatures (e.g. inlet temperature, ambient temperature, etc) in the battery box.
Discharge
To avoid the complexity of active discharging, passive discharging was chosen for the battery modules. The idea is that we can balance our battery as much as we can during assembly through binning/testing modules and then only balance using the AFEs if absolutely needed during the race. To avoid generating too much heat, 2x 33 ohm 2515 resistors in parallel was chosen as the discharge resistors. This means that at 4.2V, the balancing current is approximately 250 mA.
Isolation
Each AFE board is isolated both from other AFEs and to the carrier board through the use of isoSPI. isoSPI creates this isolation through the use of Ethernet transformers. Since each AFE is powered through the cells that it's measuring, the only isolation needed is for the communication lines. This also means that despite there being only being less than 5V across the thermistors, they should still be considered to be high voltage since they're floating above the high voltage battery ground.