To store enough energy to complete the endurance event without carrying weight that never gets used, and to make every cell voltage and a rules-compliant share of cell temperatures visible to the battery management system.
Sony VTC5A 18650 lithium NMC cells in ENEPAQ 1s6p prepackaged submodules, arranged 105 series by 6 parallel. Maximum voltage 446 V, maximum current 270 A, 15.6 Ah, 6.96 kWh total with roughly 5.64 kWh assumed usable. The submodules arrive with two fuses per parallel group for overcurrent protection, built-in temperature sensors, UL94-V0 fire-retardant plastic, copper terminals installed and copper busbars included.
Buy the module, do not build the holder. The build-versus-buy comparison was explicit: building gives full customisation, cheaper cells and the option of liquid cooling, but requires sourcing cells, temperature sensors, cell holders, terminals and busbars, needs a spot welder, has longer lead times, and puts untested hardware in the highest-consequence part of the car. Buying gives prepackaged, rules-compliant, modular assemblies with quicker lead times at a cost premium and slightly less customisation freedom. For a first-year EV program, buying wins. Size the pack from the simulation, not from a round number. Pack mass and energy consumption are coupled, so the OptimumLap iteration was run to convergence: run at a given final drive, record energy spent per lap, feed the resulting battery mass back in, repeat until mass asymptotes. 1. Run the lap simulation to get energy per lap and the required capacity, iterating on pack mass until it converges. 2. Derive initial pack parameters: nominal 420 V, nominal 130 A, 210 A peak for 120 s, preferred capacity 6.552 kWh, maximum 80 kW power draw as capped by EV.4.1.1. 3. Take vendor options against those parameters and select the closest cell. 4. Check each candidate against the rules: EV.8.4.1 cell voltage measurement, EV.6.2.3 nonflammable inter-segment insulation, EV.7.6.1 overcurrent protection, EV.8.5.3-5.7 temperature measurement location, coverage and isolation. 5. Hand the segment geometry to EV Structures so the container can be designed around it.
- Achieve the energy goal with as few cells as possible. - Monitor the temperature of the cells; the intent is every cell, the rule is a minimum of 20% equally distributed. - Allow the BMS to monitor every cell voltage. - Package a layout that can actually be fitted in the car.
- The original layout was extremely difficult to package in the car. - Temperature sensing connectors are bulky and difficult to separate. - Voltage taps are not fused. - Internal resistance of the pack has not been measured, and there is no thermal model of the batteries under load yet.
OptimumLap point-mass simulation for pack sizing (accepted as roughly 75-80% accurate; it ignores weight transfer, suspension effects, inertia, tire grip characteristics, slip, temperature, yaw, CG and driver ability). Orion BMS 2. Vendor datasheets and discharge characteristic curves for cell selection.