Battery-pack thermal study for an electric scooter in 45 °C summers

Voltra sells a delivery scooter to food and parcel fleets in Chennai, Hyderabad, Delhi and, since last year, Dubai. The pack is 20S8P: 160 cylindrical 21700 cells, 72 V nominal, 40 Ah, about 2.9 kWh. The cells sit in a sealed ABS housing under the floorboard with no airflow.

Last May the fleet telemetry showed a problem. On afternoon shifts with ambient air at 44–46 °C, 18% of packs hit the BMS derating limit of 58 °C, and the scooter dropped to half power in traffic. Riders lost deliveries and the fleet customer threatened to switch suppliers. Warranty returns for capacity fade doubled on packs from hot cities.

The engineering lead wants a thermal study before anyone changes tooling. You will: - build a duty cycle for a 4-hour delivery shift (average pack current 38 A, peaks of 110 A for 20 s at each start from a signal); - calculate cell heat generation by hand, then build a lumped thermal model of the pack in Python; - extend it to a simple resistance network so you can find the hottest cell, not just the average; - compare three cooling options: an aluminium heat spreader plate bonded to the housing floor, external fins on the housing, and a 4 W fan with inlet and outlet vents; - recommend one, with cost per pack in rupees.

You do not need a scooter or a pack. Everything here is done in Python with NumPy and SciPy, using cell data written into the brief. If you have Fusion 360 student or FreeCAD, an optional thermal simulation of the spreader plate is a bonus, not a requirement.

Done looks like a model the lead can re-run with different cell resistance values, a hottest-cell temperature for each option at 45 °C ambient, and a two-page recommendation. The target is every cell under 55 °C at the end of the shift, for less than ₹900 added cost per pack.