The Formula Student Team BFS (BernFormulaStudent) was founded on the premise from students for students. They set themselves the goal to learn as much as possible by constructing an electric racing car every year and compete in international Formula Student races.
In 2019, the BFS has built their fourth racing car and the objective was to implement a new and innovative battery management system in the car. Without external support such a project would be inconceivable. "Computer Controls has been a new but already indispensable partner to us", says the team. Computer Controls provided them with electronic measurement equipment which are crucial for the development of the vehicle’s electronic components.
Use and Necessity of a Battery Management System
Every automobile needs a way to store energy. Because of their energy density, lithium-ion (Li-ion) cells are a very popular choice for electric vehicles to store the energy. But they do have some disadvantages and the biggest one is concerning safety. Because lithium is a highly reactive material Li-ion cells are very sensitive towards environmental factors therefore they are difficult to handle and can even be dangerous if handled incorrectly. If a Li-ion cell gets punctured, over charged, over discharged or gets too hot, the life span of the cell will be reduced and it can even result in a socalled thermal runaway which will result in either spontaneous combustion or explosion. To prevent this, the voltage and temperature of all Li-ion cells need to be monitored which is the job of a battery management system (BMS). The BMS protects the Li-ion cells from overheating or over/undervoltage by disconnecting them from the device they are powering when a problem occurs. In short: turning off the battery so it can cool down and be safely charged/discharged.
The BFS is the only team in the Formula Student to use a wireless BMS that is not relying on infrared communication but on a 2.4GHz gaussian frequency-shift keying (GFSK) transceiver operating in the Bluetooth radio band but on a proprietary protocol. These make the BMS truly wireless since there is no need to include optical fibre anywhere in the design. A GFSK transceiver has the main benefit to physically separate all voltage and temperature sensors from the low voltage system. Thereby increasing the safety of the entire vehicle. Additionally, wireless communication eliminates the need to wire up every cell individually and that makes the entire battery easier to assemble since the only connection is the mounting screw used for the busbar as seen in Figure 1 and a short connector to the temperature sensor.
Figure 1: Mounting of the BMS
Circuit and Testing
The BMS sensors are built in quite a simple way. Central to the sensor board is a microcontroller that is measuring the voltage of a cell stack. This is achieved over a simple analog to digital converter. The temperature sensor is built into the cell stack and sits directly on the negative pole of the cells. The measurement is taking place over an analog to digital converter just as the voltage measurement. On each board is also a balancing function so every cell in the battery can be individually discharged if necessary. To get the measured data to a central controller steering the entire battery management system, the GFSK transceiver is used. This transceiver is on the sensor board itself communicating over a serial peripheral interface (SPI).
Figure 2: Block diagram of the BMS sensor board
To test the communication, the debugger could be used to check if the transmission is correct. What is not possible however, is to test the timing of the transmission with the debugger. To test the timing a Keysight EDUX1002G oscilloscope Download Datasheet [PDF] with a N2142A passive probe was used. Thanks to the impedance of the probe it could be attached to the antenna without interfering with the transmission itself.
Figure 3: Antenna signal of the BMS sensor board
The signal seen in figure 3 is the antenna signal. We see the pulses when a transmission is taking place, but the data cannot be seen. This is because the bandwidth of the oscilloscope with 50MHz is well below the transmission frequency of 2.4GHz. Since the data transmission can be tested over the debugger, it was not the goal of this measurement to analyse data, but to figure out the timing. The rotary switch allows cursors to be moved quickly and with high accuracy so the delay between two transmissions can easily be determined. Thanks to this measurement it could be detected that the delay between transmissions is quite long (4.32ms). To reach the planned reaction times, the delay needs to be 2ms or less. This means that the system requires further software optimisation. To test the accuracy of the voltage and temperature measurement a Keysight U1282A handheld digital multimeter Download Datasheet [PDF] was used for the reference measurement while checking the measured values by the sensor board over the debugger. This enabled us to find the difference between the digitalised and actual voltage of the cell and the temperature sensor. Since the U1282A is capable to measure with an accuracy of +/- 0.025% in the range of up to 6V, it makes a perfect and simple solution for the reference measurement.
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