BMS AFE

Description

The BMS AFE (Currently at Rev 5 after FSGP 2019) is used to measure cell voltage and temperatures from each individual battery module. The board mostly hosts the LTC6804 (or potentially LTC6811 since it's pin and software compatible) AFE chips that handle all of the measurement and communications. There are 4 BMS AFE boards daisy chained together in MSXII, with 2 each in the master and slave boxes. 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 slave battery box. This means that isoSPI goes out of the master battery box, through the slave battery box, and then back into the master 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 47 ohm 2515 resistors in parallel was chosen as the discharge resistors. This means that at 4.2V, the balancing current is approximately 179 mA. When multiple modules are discharging, the resistor temperatures have been measured to be approximately 40-50 degrees above ambient temperature. 

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.