Intro to Powertrain

Powertrain is the “guts” of the vehicle, responsible for distributing and managing electrical power throughout the car. It consists of three subsystems:

• Battery – Designs and packages the high-voltage lithium-ion battery pack. This includes cell selection, pack and bus bar design, battery protection system (BPS) integration, enclosure design, and mounting.

• Thermals – Develops active and passive cooling strategies for power-dense components such as the battery, motor controller, and MPPTs, ensuring reliability and performance under peak load conditions.

• Enclosures – Designs and fabricates sealed, structurally sound housings for all electronic components, with attention to IP ratings, serviceability, and integration.

Beyond subsystem responsibilities, Powertrain plays a critical system-level role, coordinating between electrical and mechanical domains. This makes it the bridge between many systems in the vehicle, and members gain experience across a wide range of mechanical and electrical concepts.

The first two sections of this documentation provide general background in electrical and mechanical design. These serve as context for the subsystem sections, which go into detail about each area of Powertrain.

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Daybreak Powertrain 2025

Table of Contents

Electrical Architecture

This section provides an overview of the vehicle’s electrical architecture. It is especially important for Battery and Enclosures, but less so for Cooling. Since Powertrain revolves around the mechanical integration of the electrical system, members should understand the fundamentals of how our vehicle’s electronics are structured.

Electrical Systems

Each major electrical system is broken into subsystems with defined responsibilities:

• Power Systems

  • LV (Low Voltage) → All low-voltage PCBs (e.g., power board, pump board, BBPDU).

  • HV (High Voltage) → High-voltage PCBs (e.g., VCU, Precharge, Motor Controller).

  • BPS (Battery Protection System) → Protection circuitry for monitoring and safety.

  • Power Generation (Array) → Array hardware and PCBs (e.g., MPPTs, Blackbody).

• Vehicle Controls and Telemetry (VCAT)

  • Controls → Driver input circuitry (dashboards, pedal board, control software).

  • Telemetry → Sensor circuitry and data transmission (battery, dyno, Blackbody).

  • Tracksim → [TODO: description needed].

Fundamentals

General Terminology

• HV (High Voltage) → ~120V system; DC from the battery, AC from the Motor Controller.

• LV (Low Voltage) → ~24V system; powers electronics and PCBs.

• Fuse → Safety device that opens the circuit during a current spike.

PCBs

• PCB (Printed Circuit Board) → Flat board with electronic components (chips, resistors, capacitors).

• Connections → Linked via wires; standardized four-corner M3 screw hole mounts.

• Key signals → Power, Ground, and Signals.

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Daybreak Controls Leader PCB

Wire Harness

A wire harness is a bundle of wires with connectors at both ends.

• Interior harnesses → Connect PCB to PCB inside enclosures.

• Exterior harnesses → Connect panel-mount connectors; typically have multiple insulation layers for protection.

Key Terms:

• Shielding → Grounded conductive insulation to prevent EMI.

• Connector → Plug or receptacle interface.

• Panel Mount → Receiving connector mounted on enclosures (waterproofing).

• Insulation → Protective layer around wires.

CAN (Controller Area Network)

• Protocol for serial communication between PCBs.

• Two-wire system → CAN High & CAN Low.

• Allows node-to-node communication without a host device.

• PCBs are connected on a CAN bus (loop).

SOM System

Each SOM board contains an MCU and CAN interface. SOM Stands for System on a Module

• LSOM (Leader SOM) → Accommodates two CAN lines.

• PSOM (Peripheral SOM) → Accommodates one CAN line.

• SOMs are “hat boards” that mount on top of specialized boards.

HV Power Distribution

Manages battery output to DC-DC converters, Motor Controller, and MPPTs/Solar Array.

Battery Box

• HV+ and HV– contactors.

• Array/Array Precharge Contactors

• BPS circuitry for voltage, current, and temperature monitoring.

• Key boards: BPS Leader PCB, Volt-Temps PCB, Amperes PCB.

• Fuse for overcurrent protection.

Precharge

• Circuit limits inrush current to the motor controller.

• Uses two contactors + one resistor.

• Array precharge → Inside BPS compartment.

• Motor precharge → Inside Power Distribution Enclosure.

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Precharge Circuitry

Motor Controller

• Receives DC power from the battery.

• Converts DC → AC for motor.

• Outputs three-phase power to the BLDC Mitsuba motor

MPPT (Maximum Power Point Tracker)

• One MPPT per subarray (3 total).

• Tracks voltage/current from solar array.

• Boosts voltage to maximize array power output.

• Increases array efficiency significantly.

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HV Diagram

Mechanical Architecture

This section will cover a general overview of the mechanical architecture for our vehicle. It is important for ALL subsystems. Powertrain revolves around mechanical integration of the electrical system so fundamental knowledge is required.

Mechanical Systems

• Dynamics

  • Steering

  • Unsprung

  • Suspension

• Body

  • Ergonomics → Driver interaction, ex. pedal box, steering wheel, seat

  • Frame

• Aerodynamics → Aeroshell and its mechanisms

• Composites → Advait Joshi

Fundamentals

Forces

Shear, compression, tension, and bending are fundamental types of mechanical stresses and deformations in materials. Tension is a pulling force that stretches a material, while compression is a pushing force that shortens or compacts it. Shear occurs when parallel forces act in opposite directions, causing layers of a material to slide past each other. Bending is a combination of tension and compression that happens when a force causes a material to curve, placing one side in tension and the opposite side in compression. Together, these stresses describe how structures and components respond to loads. It is important to know these concepts to design a stable part.

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Y'all should know this

3D Printing

Frame

Longhorn Racing Solar implements a space frame, meaning our frame is made up of interlocking 4031 chromoly steel tubes. The frame undergoes rigorous FEA to pass safety regulations. It is welded in house and spray painted to prevent rust. Powertrain’s tabs will have to also be chromoly so it is weldable to the frame.

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2026 Frame

Aeroshell

The aeroshell is created from a gigantic carbon fiber layup. It is split into the bottom, middle, and top shells. The bottom shell is bowl shaped and attaches the frame, the top shell holds the canopy and array and the middle shell provides rigidity. The aeroshell undergoes rigorous fluids simulations with TACC to optimize shape. Bulkheads are composite panels laid through the aeroshell to provide rigidity. Powertrain mounts enclosures on and routes ducts through bulkheads.

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2026 Aeroshell

Composites

In our context, a composite is a three layer material. The two outer layers (called “plies”) are fiber sheets that are typically carbon fiber. The inner layer is the core which mainly determines the mechanical factors such as compressive and tensile strength. The three layers are combined with epoxy and vacuum sealed to compress and cure. The result is a lightweight but very strong material. Powertrain uses rohacell and aluminum honeycomb core composites. Aluminum honeycomb is stronger in out plane tension and compression but the same in bending as rohacell. COMPOSITES ARE COOL BECAUSE WE CAN COMBINE PROPERTIES OF DIFFERENT MATERIALS INTO ONE LIGHTWEIGHT SHEET.

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Carbon Fiber Composite Panel

Enclosure Mounting

Powertrain enclosures are mounted on tabs welded to the frame, or on aeroshell bulkheads. Enclosures must be able to withstand acceleration and braking loads, FEA will be completed accordingly.

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Power Distribution Enclosure Tabs

Battery Subsystem

The battery subsystem is responsible for the design and manufacture of the battery pack and enclosure. They work most closely with composites and BPS systems.

Fundamentals

Cell

A lithium-ion (Li-ion) cell is a rechargeable battery cell that stores and releases electrical energy through the movement of lithium ions between two electrodes — a cathode (positive) and an anode (negative) — across an electrolyte.

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Samsung 50S 21700 Lithium Ion Cells

Cell Form Factor

Form factor refers to the physical shape and packaging of a lithium-ion cell. Each form factor has unique trade-offs in:

• Energy density → How much energy can be stored in a given volume or mass.

• Thermal management → How easily heat can be added, removed, or spread.

• Mechanical design → Structural strength, robustness, and vibration tolerance.

• Integration → How cells are arranged, connected, and packaged into a pack.

These considerations are critical when designing a solar car battery pack, where weight, space, safety, and serviceability are all tightly constrained.

Common Cell Types

• Cylindrical → Metal can packaging, mechanically robust, excellent cycle life, standardized sizes.

• Pouch → Flexible packaging, lightweight, higher packing efficiency, but requires external support.

• Prismatic → Rigid rectangular casing, good space utilization, easier module assembly, but less standardized

Important Specs

Nominal Voltage → The average voltage during normal discharge.

Charge Voltage → The maximum voltage the cell should be charged to.

Cutoff Voltage → The lowest safe voltage before the cell may be damaged.

Capacity → Total charge the cell can store and deliver (Wh = V x Ah)

Temperature Range → Exceeding this range reduces performance or causes permanent damage.

>45C → Cell Degradation, Life Cycle Damage

>80C → Critical Failure Threshold, Irreversible Damage

>100C → Thermal Runaway

Internal Resistance → Resistance to current flow inside the cell, which generates heat

Internal Resistance and Heat

Internal resistance is the opposition to current flow within a battery cell itself. It comes from several sources inside the cell.

Series/Parallel Configurations

In lithium-ion battery packs, cells are connected in series to increase voltage and in parallel to increase capacity. When cells are wired in series, the positive terminal of one cell connects to the negative terminal of the next, adding their voltages together while keeping the capacity (Ah) the same. For example, connecting ten 3.6V cells in series (10s) results in a total voltage of 36V, with the same capacity as a single cell. This configuration is essential in solar car applications to reach the high system voltages required by the motor controller and other high-power electronics.

In contrast, parallel configurations connect all the positive terminals together and all the negative terminals together, increasing the overall capacity of the pack while keeping the voltage constant. If five 3Ah cells are connected in parallel (5p), the total capacity becomes 15Ah, which enables the battery to provide more current or run for longer without recharging.

Our Battery Pack is a 32S9P configuration. Providing a nominal 120V to power the high voltage electronics while giving us as high of a capacity as possible without exceeding the regulated limit. Properly managing these configurations is critical for performance, longevity, and safety.

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Daybreak Segment 30V

State of Charge:

State of Charge (SoC) represents how much energy is left in a battery compared to its full capacity — essentially, it's the battery’s fuel gauge, expressed as a percentage from 0% (empty) to 100% (full). SoC is influenced by the amount of charge stored in the electrodes and changes dynamically as the battery charges or discharges. It does not directly equal voltage, though voltage is one of the indicators often used to estimate it. Accurate SoC estimation is crucial for preventing overcharge or overdischarge, both of which can damage lithium-ion cells or reduce their lifespan.

Design Considerations

The primary considerations for designing a battery back revolves around the capacity, temperature, and packaging.

Important Metrics

• Voltage, current, power, capacity

• Temperature range

Considerations and Constraints

• Frame bounding box

• Weight

• Manufacturability

• Packaging

Cell Selection

• Samsung 50S 21700

  • 3.6V nominal, 4.2V charged, 2.5V discharged

  • 25A Maximum Continuous without temperature cutoff, 45A Maximum Continuous with 80C temperature cutoff

  • Operating temperature (Surface)

    Charge : 0 to 60℃ (recommended recharge release < 45℃)

    Discharge: -20 to 80℃ (must re-discharge release < 60℃)

  • Internal Impedance ≤ 14mΩ

  • 18 Wh capacity (5000mAh * 3.6V / 1000)

• Lithium-ion

  • High power to weight ratio

  • Standard for vast majority of solar car teams

• Cell configuration

  • Conforms to capacity limit regulation with 120V HV to power motor

Spec Sheet:

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Pack Layout

Cell Configuration

• Nine cells in parallel → One Module (or row) (3.6V nominal)

• 4 Modules in series → One Segment (14.4V nominal)

• 8 Segments in series (or 32 Modules in series) → Battery Pack (120V nominal, technically 115.2V)

• Max Charge → 134.4V, Max Discharge → 80V

• Power and Ground are denoted as HV+ and HV-

• 288 total cells, 9P32S layout

• 5.24 kWh Capacity

The battery pack utilizes mono-tab design where + and - are on the same pole (top). The stainless steel casing is connected to the negative terminal. Mono-tab design can be achieve through stripping off the vinyl wrapping and designing bus bars around this configuration.

Bus Bars

Bus bars are the metal connections between cells of the battery, they are essentially “wires.” For our cell to cell and module to module connections we are using a material called sigmaclad. Our segment to segment connections and collectors use 1/8” 6061 aluminum. For manufacturing bus bars, we waterjet our aluminum and laser cut the sigmaclad (although it is very tricky).

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Aluminum Bus Bars

Sigmaclad is a composite metal made of five layers. The outer two nickel layers are so we can spot-weld the bus bars to our cells. The stainless steel layers provide rigidity to add bends and creases to the bus bars which nickel is weak in. Finally, the center copper layer provides higher conductivity which reduces power loss. We use sigmaclad 60.

Spec Sheet - Designer's Guide

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Sigmaclad Layers

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Sigmaclad 60 Bus Bars

Segment

The battery segment is a collection of 36 cells or four modules in series. The body and handles are 3D printed with polycarbonate which is heat resistant and rigid. SLS printing was considered but the rubbing caused by vibration can release powder from the prints. There is also an acrylic cover which the volt temp board mounts onto. The bottom pole of the cells protrude from under the body to contact the bottom plate of the battery box for cooling. A layer of non-conformal coating and thermal padding will be placed between the poles and the plate. Contact between the poles and the plate will make the battery short and explode.

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Battery Segment with Volt-Temp

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Battery Top View

Important Regulations

• 8.2.A.1 → Single-Occupant solar cars are limited to 5.25 kWh of storage capacity

  • Our battery is 5.24kWh

• 8.3 → Battery terminology and protection circuitry

• 8.11 → Battery Impound

Battery Protection System

The Battery Protection System is a subsystem under Power Systems which monitors the state of the battery and controls HV circuitry. The BPS will be located in a separate compartment from the pack within the battery enclosure, with the exception of the volt-temps which will each be mounted on top of a segment. The BPS has an independent BPS CAN for communication between PCBs, with the leaderboard connected to CarCAN. The contactors and precharge are mounted as part of a power distribution module. The battery is harnessed with 6 gauge wire.

Fault Conditions

• Overtemperature

• Undertemperature

• Overvoltage

• Undervoltage

• Overcurrent

• Undercurrent

Components

• BPS Leaderboard PCB

  • Controls the HV+ and HV- Contactor

  • Controls the array and array precharge contactor

  • Controls up to 4 independent fans

  • Has 2 connectors for e stops (driver and external)

  • Connects to CarCAN

  • The first node of BPS CAN

  • Powers all the boards in bps can

  • Interfaces with the array precharge board

• Amperes PCB

  • Current Sensing of the Battery

  • Connected inline of HV loop prior to HV-

  • Connected to BPS CAN

• Volt-Temp PCB

  • Monitors voltage of each module with voltage taps

  • Monitors temperature of each module through thermistors.

  • Thermistors are resistors that can measure temperature because their resistance changes based on temperature. ADCs can read the voltage drop through them and therefore determine temperature.

  • Thermistors are thermal epoxied to the surface of a cell. The putty like substance bonds the components and allows significant heat transfer.

  • Connected to BPS CAN

• Scrutineering PCB

  • For easy access to inspectors during BPS scrutineering to test the BPS

  • Contains test points for voltage and temperature to test fault conditions

• Contactors

  • Essentially a switch for HV, physically similar to relays

  • 4x → Two for array pre-charge, one for HV+ and one for HV-