Formula SAE Electric: Battery Management System

The Formula SAE Electric is competition that brings together student teams from all over the country that build formula style racecars from the ground up.

I co-designed the battery management system (BMS) electronics for the team.

This is our car:

An electric car needs a battery. This is our battery:

left: full battery pack | right: battery modules

In 2017 the team decided to build a custom pack along with a custom Battery Management System (BMS) to go along with the pack.

A teammate and I co-deigned the BMS electronics (white and black PCBs in above picture). The black PCB has the main BMS chip (LTC6804) that reads voltages and gpio and performs balancing. The white PCB contains the physical voltage sense connections to the pack, thermistor connections, and a mux for the thermistors.

The temperature of the module is read at 24 spots using a simple resistor divider. An analog mux muxes through each thermistor circuit and the output of the mux is read by the gpio of the LTC6804. There is a redundant thermistor connection for each thermistor in case of poor assembly or thermistor failure.

Balancing is done using external FETs to maximize the balancing current and avoid overheating the main chip.

Each module has a pair of the mother and daughter boards. A master PCB talks to the module mounted PCBs through daisy-chained communication using ISO-SPI, a Linear Technology (Analog Devices) proprietary communication protocol.

The two PCB architecture was developed so that the white PCB, with relatively simple circuitry, can stay permanently bolted to the module. Whereas the more complicated black PCB can be easily taken on and off for debugging and board swaps in case of faulty components. The ease of removal is an added safety feature since we did not need to take out tiny bolts with HV gloves in an HV battery pack. For a student project where time is limited and a working system is critical, using twice the number of PCBs allowed us to stay on target for the completion of the car.