FSAE GR24 Battery Pack Design

Electrical Architecture

Key Rule Requirements

  • Max total battery voltage – 600V (EV3.3)
  • Max power output – 80kW (EV3.3)
  • Two NO AIRs (accumulator isolation relays) on each accumulator pole
  • Precharge & discharge circuit to limit TS inrush current and discharge capacitors (EV5.6)
  • TS galvanically isolated from GLV (EV6.5)
  • Shutdown circuit drives one AIR (EV7.1)

Design Decisions:

  • Cylindrical 2170 cells
  • Distributed BMS system
  • 128s3p architecture
  • Max battery voltage – 537.6V
  • Max power output – 72.6kW
  • Nominal energy capacity – 6.2 kWh
  • Power exceeding 60kW not shown to have distinguishable improved placements in dynamic events (EVW)
  • Split into 8 segments in series
  • ACU (Accumulator Control Unit) contains all logic circuitry within accumulator
  • Controls relays, fans, lights 
  • Contains precharge/discharge circuit
  • Contains BMS main board
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Battery Fusing

Power Loads

  • 135A max current (6 seconds during acceleration event)
  • 55A Continuous RMS Current (150 seconds during autocross)

Design Decisions:

  • 63A main fuse (we want this to blow first)
  • 35mm^2 / 2 gauge wire
    • Bend radius is 43mm (½ distance between segments) in design, which is 4 times 11mm (wire OD). This is less than the recommended 8 times wire OD so need to keep an eye on cable kink, strain, and wear
  • 8mm SURLOK connectors
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Motivation – Why Wire Bonding?

  • No heat interaction with Lithium
    • Compared to spot welding
  • Integrated fusing (details in following slide)
  • Reduced contact resistance 
  • Quick and safe automated process
  • Quality Control (Hesse Mechatronics)

Requirements

  • Wire bonds rated for max of 1.75x Max cell discharge current (45A cell, 80A fuse)
    • Spec from FSAE Judge
  • Specify wire diameter, wire length
  • Have Fast and Slow fuse blow out times (160A, 80A)
  • 4N Aluminum Wire (99.99 Purity)
    • Impurities change resistance
 Blow Time
Current Level 1 (80A)30 < t < 300 sec
Current Level 25 sec < t < 15 sec
Current Level 30.2 sec < t < 1 sec
Current Level 4 (160A)0.05 sec < t < 0.15 sec 

MATLAB Simulation of 1mm Round Wire
Lumped Capacitance Model w/ Temperature Dependence:

Fuse Blowout Times for 42mm wire at various diameters

Test setup:

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Battery Thermals

Simulated Power Loads for endurance event pack level adiabatic sim:

  • 35kW RMS power
  • 76A RMS current
  • 135A peak current

Current throttled to 70%:

Battery temps increase 5°C per lap /20 laps

Transient Cell Temperature Simulation:

Assumptions

  • 25°C Initial Temperature
  • 40°C Coolant Temperature
  • 20kW continuous pack power (2.3W each cell)

Hotspot temps w/ 60 min end condition:

Cold Plate Initial Design & Testing:

Key Design Points:

  • No leak paths within of cell enclosure
  • Prioritizing for now on low pressure drop

Accumulator Container

Key Rule Requirements

  • Made from steel, aluminum, or composite (F10.2)
  • Can be welded, fastened, or bonded (F10.2)
  • Each segment separated with walls and lid (F10.3)
  • 10mm holes for air flow or minimum size for cable routing (F10.4)
  • Amount and size of mounts to chassis specified by accumulator weight or mount location (corner attachment) (F10.5)
  • Maintenance plugs accessible immediately after opening accumulator lid and not necessary to move or remove any other components (EV5.3)
    • New rule*

Design Decisions:

  • Accumulator sized to fit & mount into chassis
  • Experiment with welded aluminum by Neal Feay, alternative is fastened steel
    • Prioritizing low tolerance for segment fitment
  • Segments fastened instead of friction fit
  • Second layer dimensioned to not be above maintenance plugs

Overview of Design Decisions:

  • Segments fastened instead of friction fit
  • Two box, three lid design allows easy access to maintenance plugs
  • Adjustable mounting to account for inaccuracies in chassis fabrication
  • Emphasis on ease of fabrication
  • 6061 Aluminum (as per rule F.10.2)
  • Box utilizes a welded (F.10.2) construction
  • Welding to be done by professionals
  • Prioritizing greater tolerance for segment fitment
  • Lid utilizes a bent sheet metal construction 

– Sized to fit and mount to the chassis 

Lid Attachment:

  • ¼”-20 hex head bolts + captive nuts
    • Positively locked with reusable threadlocker
  • Segment box lids are designed for simplest manufacturing
    • Best method for accurate bends will be determined during testing phase
  • Lightweight Fiber Gasket strips to prevent water infiltration

Electronics Tray

Separate compartment within accumulator that houses control electronics.

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Changed to mid-pack fuse design instead of two fuses at each end:

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