P17280: Hot Wheelz Thermal Management System

Caitlin's Work

Table of Contents

Problem Definition Deliverables

Use Case

Use Case: Shutdown State Diagram - File

Customer Requirements

P17280 Customer Requirements - File

Engineering Requirements

P17280 Engineering Requirements - File

House of Quality

P17280 House of Quality - File

Phase 2 Plan

3 Week Plan for Phase 2 - File

System Design Deliverables


Caitlin's Research - File

Circuit Design Interview

Component Benchmarking for Current Draw - File


Testing Method

Motor Controller Temperature Test Plan - File

Motor Controller Temperature Data Sheet - File

public/Systems%20Level%20Design%20Documents/Feasibility_Temperature%20Static%20Graph.PNG public/Systems%20Level%20Design%20Documents/Feasbility_Temperature%20Dynamic%20Graph.PNG

Design Concept Brainstorming

Morphological Chart for "Remove Heat" function - File


Steve Showalter Video Call Notes

9/22/16 Video Call Notes - File

Phase 3 Plan

3 Week Plan for Phase 3 - File

Preliminary Detailed Design Deliverables

BMS Capabilities & Fan Circuitry

Full Report - File

Preliminary Electrical Schematic - File


Battery Box Airflow Design

Full Report - File

Battery Box Cooling Benchmarking & BOM

Benchmarking Table - File

BOM - File


Phase 3 Efficiency
Time Spent Projected Time Efficiency
3000 mins 360 mins 12%

Phase 4 Plan

3 Week Plan for Phase 4 - File

Detailed Design Deliverables

Heat Transfer Analysis

A detailed working document can be viewed here.

The MATLAB script can be viewed here.

The flow over a bank of tubes approach was used to analyze the heat transfer characteristics of the batteries when they are assembled in their structures. All equations, properties, correction factors, and coefficients were directly taken from the Fundamentals of Heat and Mass Transfer, 7th Edition textbook, which can be found here.


  1. Identify the amount of heat a battery structure will generate in an endurance scenario (ER10)
  2. Identify the amount of heat a fan can remove when running at a specified speed
  3. Identify the pressure drop across a battery structure that a fan will have to overcome
  4. Identify the surface temperature of the battery cell that will signal to power on the fans (ER8)


  1. Continuous current through a battery cell is 30A during the endurance event
  2. The battery cell’s internal resistance is 8 milli-ohms;
  3. Each battery structure will generate 20% of the total heat generated by the entire battery pack (5 structures total)
  4. A battery structure will be represented by a staggered bank of tubes consisting of 11 rows of 4 cells each
  5. Inlet air temperature into the battery structure is 18 degrees celsius
  6. Inlet air velocity is equivalent to the fan blade velocity


  1. Assume the inlet air temperature and the battery surface temperature (18C and 55C, respectively).
  2. Find air properties at the inlet air temperature (at 18C).
  3. Compute the heat generated for one battery structure based on the battery cell’s internal resistance and the nominal current at a 2C discharge rate.
  4. Assume the air inlet velocity (0.314 m/s or 1.03 ft/s).
  5. Compute the Reynolds number for the air.
  6. Compute the Nusselt number using the Reynolds number.
  7. Compute the convective heat transfer coefficient using the Nusselt number.
  8. Find the outlet air temperature using the convective heat transfer coefficient.
  9. Find the mean temperature between the inlet and calculated outlet temperature
  10. Find air properties at this mean temperature.
  11. Repeat steps 5 through 8 to find the new Reynolds number, Nusselt number, convective heat transfer coefficient, and outlet air temperature. These are now more accurate with the new air properties.
  12. Find the log-mean temperature difference using the new outlet air temperature.
  13. Compute the heat transfer rate using the log-mean temperature difference.
  14. Compute the pressure drop. Find the correction factor and friction factor using the Reynolds number.



Because the velocity of the air entering the battery structures is not equivalent to the velocity of the air leaving the battery structures through the fans, this analysis does not give insight to what speed the fans should run at. This is an important parameter when picking a fan.

However, the analysis does give insight on how much pressure drop occurs within the structures. Another parameter used when picking a fan is its static pressure. The fan should be rated so that it can overcome the pressure drop within the structure. This allows the air to flow into and out of the structures. To be safe, we are assuming that the calculated pressure drop within the structures is a low value. All fans will be sourced to have a static pressure of at least 25 Pa to ensure that it can overcome the pressure within the structure.

The analysis assumed that the temperature of the cells was at 55 degrees celsius. We will be using this temperature as the point when the fans should turn on. This is because the amount of heat transferred is much greater when there is a larger temperature difference. Future testing will determine is this is an acceptable tripping temperature. At this moment, we are comfortable with this temperature and have updated engineering requirement ER8 to reflect this.

Based on the heat generation calculations done in this analysis, engineering requirement ER10 has been updated. The thermal management system should be expected to remove at least the amount of heat that the batteries generate, if not more.

Deliverable Efficiency
Time Spent Projected Time Efficiency
660 mins 120 mins 18%

Prototype Build & Test

Purpose: Observe the airflow within a battery structure with the proposed design for the battery box.

I assisted with the construction of the current battery structure prototype and pictured below. This construction ensured that all "batteries" were sealed into the wooden panels through the use of hot glue. The outside enclosure was also constructed to ensure minimal air leakage for testing.



I assisted with the prototype testing. Two trials were conducted: one with no paper barrier and one with a paper barrier. The first trial showed that there was stagnant air in on section of the structure, which prompted the use of the paper barrier. With this addition, the problem of the stagnant air was resolved.


No Barrier Trial - Video

With Barrier Trial - Video

Conclusions: The proposed battery box cooling design will create the airflow desired within the battery box. A barrier must be added to achieve this. This design can move forward as our final design.

Deliverable Efficiency
Time Spent Projected Time Efficiency
540 mins 240 mins 44%

System Design & Model

Purpose: To have the battery box cooling system modeled in Solidworks so that the Hot Wheelz team can incorporate the design in their master model. These files will also aid in the manufacturing of the components.

The finalized design of the battery box cooling consists of 5 fans mounted on the battery box, one for each battery structure, and two locations on the battery box for vents to allow air to enter the box. This design also required additional battery structure paneling.


I assisted with determining the locations for these components. I was also responsible for modeling all of the battery box cooling components and making drawings for these components.

CAD Files


Main Pack Back Panel Drawing

Rear Pack Front Panel Drawing

Rear Pack Bottom Panel Drawing

Rear Pack Back Panel Drawing

Rear Pack Bracket Drawing

Battery Box Top Plate Drawing

Battery Box Back Plate Drawing

Battery Box Bottom Plate Drawing

Conclusions: The Hot Wheelz team can fully utilize these files and manufacturing can begin on these components once the necessary materials arrive.

Deliverable Efficiency
Time Spent Projected Time Efficiency
1380 mins 390 mins 28%

Component Sourcing

Purpose: To have all necessary components sourced so that materials can be purchased. The plan is to have materials arrive in time for the MSD team to begin building and testing the design.

Originally, I was going to be responsible for sourcing all battery box cooling related components including the fans, vents, mounting hardware, etc. Because a lot of my time was devoted to the model of the system, the fan and vent sourcing were taken by Missy and Kristin, respectively. However, I did source the mounting hardware needed for the battery structure panels.

Because this hardware is located inside the battery box, it was important to source a plastic material. This is why polycarbonate screws were sourced. If metal hardware had been sourced, we would have had to find a way to isolate and insulate them according to the Formula Hybrid rules.


Sourced material highlighted in green.

Conclusions: Mounting hardware and all other necessary components have been sourced for the battery box cooling design. Multiple parts have been purchased and we should begin receiving them in time to begin building in January.

Deliverable Efficiency
Time Spent Projected Time Efficiency
20 mins 5 mins 25%

Efficiency Tracking

Full efficiency tracking can be viewed here.


Reasoning: Some of the work completed in Phase 3 was useful for Phase 4 and did not have to be reworked. This helped decrease the amount of time spent on the phase. The estimated amount of time needed if I were to do it again increased because of the nature of the work that was completed in Phase 4. Things such as constructing the prototype still would take several hours because it takes time to machine and seal the pipe batteries. Modeling also takes time and, therefore, still would take several hours even if I were to do it over again. This is why I believe my efficiency rose in Phase 4 from Phase 3.