High-Voltage Motorsports Powertrain

Overview

Realization of a completely from-scratch EV powertrain. Key developments focused on the battery pack design, redundant safety features, and power electronics design.

Key Specifications

• Peak/Nominal Battery Pack Voltage: 600/554 VDC
• Peak Electrical Power Output: 80 kW
• Battery Chemistry and Configuration: 4p132s Molicel 21700 Lithium-Ion Cells
• Custom per-cell fusible links, thermal monitoring, active battery management
• Designed for use with EMRAX 228 axial flux motor and Cascadia PM100DX Inverter

Background

The work done on this system served as part of the completion requirements for my undergraduate capstone credit, however, as a team lead for my university's Formula SAE team, it was also somewhat of a passion project which we had been wanting to do for a while. The primary goal was to create a compact but simple powertrain architecture which could not only meet SAE's extremely stringent safety and operational requirements, but also be adapted to other vehicles within the same class/size.

As such, many of the driving systems engineering requirements for the design were defined by the need to survive the demanding environment of a Formula SAE competition. Some of these requirements stemmed from purely safety-justified concerns directly from SAE, including

• Survival of a 40 g impact without permanent damage or deformation
• Individual battery failure must not cascade to rest of pack
• HV sections must maintain ≥ 500 Ω/V isolation resistance to chassis under all operating and fault conditions
• Residual HV potentials must reduce to <60 V within 5 seconds

Furthermore, additional design goals from our team were identified to maximize performance potential while staying within FSAE's ruleset, maintaining a competitive edge:

• System should use the highest feasible voltage while staying under 600 VDC limit to minimize current loss
• Total stored battery energy should be just sufficient to complete FSAE endurance race event (29 km race) plus safety margin
• Physical construction shall be lightweight but simple, without using exotic or risk-prone fabrication methods
• Selected battery chemistry shall maintain high energy density without introducing thermal runaway or explosion risk

Battery Segment Design

Molicel INR21700-P45B Lithium-Ion cells were chosen for the design, as they offered superb energy density, and had predictable charge/discharge behavior under a wide thermal operation range, mitigating some of the common safety risks of Li-Ion chemistry. Furthermore, these cells were designed for performance vehicles, and offered a staggering 45 A continuous discharge current rating, with an extremely low DC impedance of only 15 mΩ.

OptimumLap, a point-mass vehicle physics simulator, was used with a model of our FSAE vehicle design (with expected EV powertrain mass and gearing) to obtain an estimate for total energy required within the pack. With our selected margin, the simulation revealed an energy requirement of approximately 10 kWh, or 36 MJ. FSAE regulations require the battery system to be segmented into modules containing no more than 6  MJ each; in our case this meant that we were able to design a 6-segment system with each module containing almost exactly the maximal energy allowed per segment.

Air cooling was chosen to maintain the cell temperature within safe (and optimal performance) levels. Although liquid cooling can remove more heat, the increased complexity, mass, and electrical risk were not worth the price. For a 20 km endurance race, the system can largely ride out the thermal inertia of the cell mass itself, and requires minimal cooling. Within each segment, the cells were physically arranged within a custom fire-resistant polycarbonate, machined to position the cells such that airflow from neighboring cells create turbulence and flow separation, preventing stagnation and overheating. The entire battery enclosure was vented to allow for airflow and gas venting in the event of catastrophic failure, but also covered at intake and exhaust with hydrophobic PTFE meshes, allowing 40% of the airflow of an unimpeded duct while not allowing liquids in or out.

Each segment integrated multiple monitoring and safety systems. Groups of four parallel cells were connected in series via custom resistance-welded nickel busbars which also acted as fusible links. Material thickness and width was first estimated then refined empiriclly using a high-current test apparatus, microcontroller, and current clamp, allowing the fusible link to conform to a desired time-current curve.

Supporting Electronics and Safety Systems

Additional systems were developed to manage communication and telemetry between the battery system, BMS, inverter, and CAN network. Numerous safety mechanisms were implemented to protect both the driver and service personnel working on the powertrain. I designed most of these systems directly, and as team lead, reviewed and refined contributions from other members to ensure all prototypes met functional and safety requirements prior to final integration. Most of the boards seen here were designed with CAN-bus functionality, allowing central control and data acquisition/telemetry for the full system.

Per-module temperature and voltage sensing

The circuit board affixed to the top of each battery segment routed and collected thermistor data, voltage sensing taps for each series group of parallel cells, and housed the discharge resistors which allowed the BMS to intelligently balance charge states between cells. All traces exceeded IPC standards for high voltage systems to minimize the chance of internal shorts between parallel groups. The bus bar/fuse apparatus for each group was soldered directly to the square pads on the side of each board, and TE connectivity HV post connectors carried current between full segments. Ideally, future revisions of this board will integrate BMS ICs directly on these boards rather than sending taps/discharge paths off-board.

Precharge-Discharge Circuit

This system allowed safe charging and discharging of the large capacitance present within the motor inverter/controller. Without a system such as this, massive inrush currents risk damaging the system, and the system can stay energized when not in use. An ARM Cortex-M7-based microcontroller facilitated the charge/discharge processes, and this system at all times serves as part of the watchdog system, immediately discharging HV components upon an electrical fault.

Grounded Low-Voltage (GLV) System

In conjunction with a COTS compact Vicor DC-DC converter which provided 24 VDC from the main pack's 600, this board handled all low-voltage components for the vehicle. The system was designed to be fully configurable, with multiple programmable high-power digitally controlled outputs for devices such as water pumps, fans, and data telemetry. Vishay amplifying optoisolators were used to act as N-FET high-side drivers, directly generating V_gs for the power MOSFETs from logic-level voltage, forgoing the need for charge pumps or switching converters.

Isolation and Signal Routing Board

As mentioned previously, this system makes use of COTS BMS, the Orion BMS 2. I plan on integrating BMS hardware directly into the segment boards in future revisions, however for this initial design, we needed a serviceable/removable way to safely interface the high-voltage tap lines from the battery pack (2x 10-pin Molex MX150L) to the Orion inputs (12-pin Molex Mini-Fit). An additional board was created which sat within the battery box, accomplishing three key tasks:

• Route segment 20-pin voltage tap interface to 12-pin BMS input
• Provide a safe, isolated low-voltage supply for monitoring thermistors
• Provide mounting and connection points for a Bender IR155-3203 Insulation monitor

Testing and Conclusion

Despite not having a full vehicle system with motor and controller to test the full system, electronic and passive load systems were used to verify the powertrain could output the expected peak and steady-state power levels without faults, shorts, or thermal issues. In all cases, the included failsafes successfully triggered a safe shutdown procedure when their conditions were encountered, and individual overcurrent events were induced which successfully characterized the fusible link operation under load. The system was considered feature complete and ready to implement in a vehicle, and left in the care of Saginaw Valley State University for future use with their Formula SAE team.