Circuit Design, Power Systems, PCB Design, Digital Logic, Mechanical Design, Manufacturng

Overview

For my senior capstone project, I was a part of a group that converted an internal combustion FSAE racecar to be fully electric. The electrical side of the project consisted of creating a custom high voltage battery pack (accumulator) with a battery management system to power the vehicle’s motor. There was also a low voltage subsystem composed of a device to check for braking and accelerating faults, a circuit to cut off high voltage power in dangerous scenarios, and 12 V bus for miscellaneous electronics. Furthermore, because we were converting an existing internal combustion vehicle, certain mechanical alterations were made. Personally, I was responsible for the low voltage subsystem and acted as team treasurer.

Low Voltage Electrical

The low voltage system is broken up into three main parts: the acceleration and brake fault detection device, the safety shutdown circuit, and the 12 V bus. The purpose of this subsystem is to handle all auxiliary functions of the vehicle. Basically any function of the car outside of the high voltage power train is a part of the low voltage system. Below is the circuit diagram for the vehicles auxiliary systems.

Low Voltage Auxillory System Circuit Diagram

Auxillory System PCBs

Acceleration and Brake Fault Detection Device

One key feature of this project is the acceleration and brake fault detection device. This device detects the following accelerating and braking conditions that could cause harm to the driver:

• Simultaneous pressing of accelerator and brake pedals
• Significant current delivered to motor while driver is hard braking
• Short or open circuits in the accelerator and break pedal sensors

If any of these faults were detected, a trigger signal is sent to a 500 ms timer which would then actuate a relay to open the safety shutdown circuit and cut off high voltage power. The logic is described in the diagram below.

Accelerator and Brake Pedal Fault Detection Logic

Per FSAE rules, the device had to be compeltely analog. Because of this, I chose to detect faults using op-amp comparators and logic gates. If the input signal from the sensor went above a certain value or outside a specified range, the comparator would send a high signal to 500 ms timer. Below is the circuit diagram for the device.

Accelerator and Brake Pedal Fault Detection Circuit Diagram

Using EaglePCB, I created the manufacturing drawings for the board’s PCB. I then ordered the board and soldered the components on myself.

Accelerator and Brake Pedal Fault Detection PCB

Safety Shutdown Circuit

A safety shutdown circuit was created to cut off high powered electricity in unsafe conditions. This circuit connects the 12 V battery to the coils of the two high power relays that are connected to the vehicle’s high power battery or accumulator. The rest of the circuit is a number of switches and relays in series such that if any are disconnected, the circuit is open and no voltage is applied to the accumulator relay coils. When there is no voltage at these coils, they are open and the accumulator is disconnected.

The circuit has 3 emergency stop buttons, 2 master switches, a brake over travel switch, and 2 low voltage relays. The brake over travel switch is a toggle switch located behind the break pedal such that if the brakes are pressed extremely hard, the flip is switched and the circuit is opened. The 2 low voltage relays are connected to the acceleration and brake fault detection device (shown as APPS/BSPD) and the insulation monitoring device (IMD). The IMD measures the insulation between the high voltage bus and the chassis ground. If a short between the two is detected, the device outputs a high signal to open the relay in the shutdown circuit.

Safety Shutdown Circuit

Low Voltage Power

Part of this project was ensuring it could withstand the FSAE endurance event which we calculated would take about 30 minutes. To spec out the battery for this, I found the max current draw of each component on the 12 V bus to be 34.6 A. To complete the endurance event the vehicle required a battery with a capacity of 17.3 Ah.

12 V Bus

High Voltage Electrical

The high voltage powertrain shown below consists of the high voltage battery pack (accumulator), accumulator isolation relays, high voltage disconnects, motor controller, and the motor. The accumulator isolation relays and high voltage disconnects are used to isolate the accumulator when not driving. The high voltage lines are then fed into the motor controller which operates the motor based on the input from the accelerator pedal position sensors.

High Voltage Bus

The accumulator itself was made of 756 18650 Li-ion rechargeable cells broken up into 9 parallel connections of 84 series connections. Because the motor we selected was a 300 V motor, we required a 300 V battery pack. After performing a simulation of the endurance event it was determined that a battery required at least a 6.3 kWh capacity at this voltage to finish. Because 18650 cells have a nominal voltage of 3.6 V, we concluded that 84 cells in series were necessary. Furthermore, to reach the current requirement (21 Ah), we calculated that 9 cells in parallel were needed since the cell’s nominal capacity is 2.5 Ah. We also used an off-the-shelf battery management system to ensure proper cell balancing and charging.

Battery Model

Manufacturing

To assemble the battery pack, series connections were made by spot welding positive and negative terminals together with nickel strips. Parallel connections were made by screwing thicker nickel strips into a copper bus bar.

Battery Segment Manufacturing

Mechanical

The mechanical systems of the vehicle are shown below. Changes to the drivetrain and cooling system were necessary when changing from internal combustion to electric and a battery housing was needed to protect and insulate the battery segments.

Mechanical Systems

The Team

• Douglas Byrd
• Muhammed Al-Hassani
• Esther Yoo
• Michael Hom
• Felix Rodrigue
• Jacob Antie 
• Alejandro Nunez