Modules 3/4 Practice

Question 1

What is the intent of a ‘battery passport’ and how does it affect the design of EVs and EV battery packs?

Answer 1

• digital twin of a EV battery (cell, module and pack level) that conveys information about its origin, sustainability and lifecycle.
• intent is to ensure battery packs are design with a closed-loop lifecycle in mind
• EV batteries will need to be designed with safe disassembly in mind (limit use of semi-permanent adhesives/welding)
• battery structure and adhesives should be designed with automated disassembly in mind and minimize the amount of dissassembly steps required.

Question 2

List and explain the relative strengths and weaknesses of the three primary cell formats as they apply to pack design in EVs.

Answer 2

Cylindrical Cells
- low cost (available at high volume, extensive use outside of EV applications, well understood manufacturing and automation techniques - reduces manufacturing costs)
- primary weaknesses are poor energy density, poor packing factor, cooling issues and high cells counts needed.

Prismatic Cells
- high energy density and packing factor, better cooling and often low overall cost due to reduced complexity
- weaknesses are high manufacturing costs, more stringent safety precautions to prevent puncture/mechanical damage and thermal runaway

Pouch Cell
- strengths are the same as prismatic, higher discharge rates and ease of cooling
- drawbacks are reduced mechanical strength and durability (prone to rupture and swelling - not really used in EVs)

Question 3

List and explain 5 top level design requirements for an EV battery pack.

Answer 3

• Safety: electrical safety, mechanical safety, thermal runaway propagation prevention, pressure management, puncture resistance
• Electrical - internal resistance, voltage, discharge/charge rate, SOC, desired series/parallel connections
• Environmental - weather/ingress protection, thermal management, recycling protocol, EOL applications
• Structural - warping, rigidity, mounting to chassis, weight/balance, vibration resistance and integration to vehicle.
• Cost Optimization - production and material costs

Question 4

List and describe the key structural requirements an EV battery must address in its design.

Explain briefly how (i) structural batteries, and (ii) battery swapping interacts with these
requirements.

Answer 4

Structural Requirements
- resist all external forces during vehicle movement (torsion, compression, vibration, etc)
- cells must be fixed
- resistant to mechanical damage (crash/puncture)
- weight distribution and shape that is compatible with vehicle suspension and steering design.

Structural Batteries
- battery serves as a structural element of the car, replacing purely structural components with this that can also store energy
- cell is designed as a composite structure that is mechanically stable
- must meet requirements listed above

Battery Swapping
- designing for a battery that is easily removed and replaced in the car (swap instead of recharge/swap battery at EOL)
- additional structural design constraints (smaller, self contained, mechanically stable)
- battery containment structure required in addition to vehicle chassis - adding weight

Question 5

Explain ‘binning’ and how this relates to EV pack design.

Answer 5

• start of line assembly step where cells are sorted into groups based on a number of measured parameters (internal resistance, capacity, etc.)
• these perimeters have a tolerance range, which must be a line across all cells within the group to prevent early failure/degradation of the pack
• Challenge = when multiple packs must be construction manufacturers will need to intake, inventory, and match an exponentially increasing number of parts
• this heavily motivates designers to minimize Benay needs by choosing cell types with higher capacities and discharge rates per cell (prismatic cell) and by designing the arrangement to minimize the parallel connected counts.

Question 6

The top level manufacturing steps that would be used in creating a cell to module pack. Explain any places where the methods used may vary from one line to another.

Answer 6

• Intake: arrive from manufacture or discharged, tested, bend, and sorted and QC tested.
• Grouping and Fixturing: cells are arranged into groups (typically all parallel connection) and initial fixturing takes place (cells may be placed into a holder or onto a backing plate, and adhesive may be applied depending on the format)
• Interconnection: cells are welded together into parallel groups, first. QC steps monitor the cell temperature. Series connections, usually take place second and mainly involves manual operations.
• Module Closure: remaining electronics installed, module is sealed and tested. Modules may be shipped and assembled on a separate line or continue to pack assembly directly.
• Pack Assembly: modules connected to make larger packs (automated), adhesives applied, connection of cooling, BMS and other internal cabling
• Closure/Testing - pack is sealed, aged and tested

Question 7

Briefly describe the relative advantages and disadvantages of the cell-to-module and cell-to-pack design topologies as they apply to EVs

Answer 7

Advantages over cell-to-module:
- Improved Energy Density – Layout is more compact, tends to use prismatic cells for better packing
- Reduced Weight, BOM Count – Fewer spacers, binders, separators, insulators, etc.
- Better Thermal Management – Easier to achieve direct thermal contact with cells, also moves
away from cylindrical cells in most applications
- Lower Manufacturing Cost – Fewer steps overall, less human-in-the-loop steps, more easily
automated (multiple reasons), etc.
- Fewer single points of failure as compared to high cell-count cell-to-module designs

Disadvantages:
- Incremental module replacement no longer an option, some second-life avenues closed
- In CTP designs puncture and thermal runaway resistance is complicated, as is pressure
management, crash resistance, and many other safety concerns.

Question 7

Identify the steps which would be needed (and in what order) to disassemble the pack for recycling. Which of these steps would require manual labour or otherwise substantially increase the cost of this process? Suggest one modification that could reduce recycling cost without worsening other design metrics (safety, structural strength, etc.)

Answer 7

The steps needed would likely proceed similar to the following:
- Deep discharge pack to low SOC
- Break external weather seal and remove housing/cover
- Individually test module SOCs and locally discharge as needed
- Remove BMS, cabling, sensors, and any parts of the harness with connectors
- Drain and remove cooling system
- Break, dissolve, or otherwise remove inter-module sealant
- Break the modules free from the base sealant

• most of these steps (other than discharging and fastener removal) would require manual labour
• Moving towards automation could reduce recycling costs
• reduce degree of difficulty of the steps (the ones there the battery is not at low SOC, or potential for mechanical damage is high)
• replacing the adhesives used (last two steps) with alternatives such as fasteners that have similar mechanical durability

Question 8

Explain how the coolant loop for this vehicle works. Be sure to explain any distinct ‘modes’ of operation.

Answer 8

The coolant loop has three distinct modes of operation:

• Mode A: 3-way coolant flow control valve is set to position A, coolant flows in a short loop bypassing all other parts of the system. The 12V coolant pump circulates the coolant through the 360V heater and the battery. This mode is used when the external ambient temperature is low, to maintain a minimum battery temperature and prevent thermal damage. The coolant is not routed through any radiator or long pipe runs to avoid loss of heat.
• Mode B: 3-way valve in position B, coolant flows through the primary battery chiller. In this mode the coolant, and by extension the battery, is actively cooled by the air conditioning
  (A/C) loop consisting of the battery evaporator (cooler), 360V A/C compressor, and A/C condenser – this would likely be a high-pressure loop. The expansion of HP coolant in the evaporator creates
  the temperature gradient needed for refrigeration, and by extension cooling of the battery (low pressure) coolant.
• Mode A, coolant (LP) flows through the external radiator, exchanging heat with the ambient air (likely helped by vehicle movement of air through the radiator and/or a fan). In this mode thecoolant cannot be brought below the temperature of the external free air – there is no refrigeration, only heat exchange with the outside air.

Question 9

Describe five potential end-of-life (EOL) pathways for an EV battery pack. (5 ‘Re’s)

Answer 9

• Repair – Some packs can be repaired and re-enter service. Since cells tend to age relatively
  uniformly, service life of already aged packs is inherently limited – repair is most applicable to
  non-catastrophic short-term damage, such as mechanical damage or electronics failure.
• Remanufacturing – Packs can be refreshed by replacement of cells or modules, allowing for re-use
  in the original application. Savings tends to be minor as the cells are already the major cost of the
  pack.
• Resale – Some packs can be recovered with usable service life remaining and thus resold for use
  in-application.
• Repurpose – Other packs may be too degraded/aged to be of use in the original application, due to
  reduction in capacity and/or discharge ratings. However, these may still be used in lighter-duty or
  long-term-storage applications.
• Recycling – In many cases the above pathways are simply not feasible or not applicable, in which
  case the pack can be broken down into raw materials which re-enter the supply chain.