Two-Wheeler Battery Pack Enclosure with Cell Holders and Busbars

The problem

Two-wheeler battery packs must fit a tight space under the floorboard or seat, hold cells firmly, carry current safely and survive road vibration and handling. Many student packs are a box drawn around cells with no thought for busbar sizing, BMS access or how the box is made. A designed pack shows each of these decisions.

Objectives

• Choose an example cell configuration and lay out the cell array.
• Model cell holders, busbars and a BMS bay in an assembly.
• Design a sheet metal enclosure with flanges, mounting brackets and flat patterns.
• Size busbars for current density and check creepage and clearance spacing.
• Check mounting brackets under an inertia load and produce manufacturing drawings.

What you need

Key concepts

Series-parallel configuration

Cells in series add voltage and cells in parallel add capacity. For example, 16S5P means 16 groups in series, each of 5 cells in parallel.

Busbar current density

Current divided by busbar cross-section. Keeping it within a conservative design value limits heating and voltage drop.

K-factor

The position of the neutral axis in a sheet metal bend, used to compute the flat length. It depends on material, thickness and bend radius.

Creepage and clearance

Minimum distances over a surface and through air between conductors at different voltages. They prevent tracking and arcing and are set by the applicable standard.

Step-by-step guide

1. 1

   Size the pack electrically

   Choose an example 60 V nominal pack using 21700 cells, for example 16S5P. In Excel, compute nominal voltage, capacity, energy, peak current from an example motor controller rating and per-cell current. Check that per-cell current stays within the datasheet continuous rating. Record cell mass and dimensions from the datasheet and compute total cell mass. Also compute pack resistance roughly from cell internal resistance to estimate voltage drop and heat at peak current.

   Checkpoint: A sizing sheet with voltage, capacity, energy, per-cell current and cell mass, all within datasheet limits.

2. 2

   Model a cell and the cell array

   Model one cell as a cylinder with its terminal features and assign mass by overriding Mass Properties to the datasheet value. In the assembly, pattern cells with a Linear Pattern or a pattern driven by a sketch so that parallel groups sit side by side and series groups alternate polarity. Keep a gap between cells for the holder walls. Save the cell as a reusable part so other projects can use it.

   Checkpoint: The cell array assembly shows the correct number of cells and alternating polarity in the series direction.

3. 3

   Design the cell holders

   Design top and bottom holders with pockets slightly larger than the cell diameter, for example 0.2–0.3 mm clearance, and windows for busbar welding. Use a Fill Pattern or Linear Pattern for the pockets. Add interlocking features and bolt bosses so holders clamp the cells. Choose a flame-retardant polymer such as a PC or PC-ABS grade with a suitable rating from its datasheet.

   Checkpoint: Holders mate around the cell array without interference and bolt bosses align top to bottom.

4. 4

   Design the busbars

   Model nickel strip or copper busbars connecting each parallel group and linking groups in series. Calculate cross-section from peak current and a conservative current density target, and check that strip width fits the holder windows. Route the main positive and negative busbars to the pack terminals. Add sense-wire tabs for each series group for the BMS. Check total busbar resistance and the heat it would produce at peak current, and widen strips if they would run hot.

   Checkpoint: Busbar cross-section meets the current density target and every series group has a sense-wire tab.

5. 5

   Add the BMS bay and harness

   Place a BMS board block, a fuse or contactor and the main connector at one end of the pack. Model mounting standoffs and a cover. Sketch harness routes with a 3D Sketch and check bend radius. Check creepage and clearance distances between high-voltage parts and the enclosure using the Measure tool, taking target values from the applicable standard and the BMS datasheet.

   Checkpoint: Creepage and clearance distances measured and above the target values in every location.

6. 6

   Design the sheet metal enclosure

   Create the enclosure in Sheet Metal from aluminium sheet of an example 1.5 or 2 mm, using Base Flange, Edge Flange and Hem features. Set bend radius and K-factor from a gauge table or supplier data. Add a lid with a flange for a gasket and fastener holes with the Hole Wizard. Add internal standoffs for the module and keep a gap for insulation sheet between cells and metal.

   Checkpoint: The enclosure flattens without errors and the flat pattern fits a standard sheet size.

7. 7

   Add mounting brackets and check packaging

   Model brackets that bolt the pack to the scooter frame, matching the frame hole pattern. Insert the pack into a simple frame assembly and run Interference Detection. Use Mass Properties for pack mass and centre of gravity and compare with the vehicle layout. Check that the pack can be removed with the floorboard off, if removal is required. Leave space for insulation and harness clips between the pack and the frame.

   Checkpoint: Interference Detection passes and the pack mass and centre of gravity are reported.

8. 8

   Check brackets under load

   In a Static study of the enclosure and brackets, fix the bracket bolt holes and apply the pack mass as a Remote Mass or distributed force under an example 5 g vertical and 3 g longitudinal load. Use a fine mesh at bracket bends. Check factor of safety and deflection. Hand-check one bracket as a cantilever to confirm the order of magnitude.

   Checkpoint: Factor of safety above 2 and the hand cantilever check within about 20 per cent of Simulation.

9. 9

   Drawings and report

   Produce flat pattern drawings with bend lines and bend notes, an assembly drawing with a bill of materials and exploded view, and holder drawings. Add general tolerances to ISO 2768. The report covers sizing, layout, busbar and spacing checks, the bracket study and mass. State that real packs need testing against current standards such as AIS-156 for L-category traction batteries.

   Checkpoint: A drawing pack ready for sheet metal fabrication and a report covering electrical and mechanical checks.

Testing & validation

• Per-cell current within datasheet limits.
• Busbar current density within target.
• Creepage and clearance measured against stated targets.
• Flat patterns generate without errors.
• Bracket factor of safety confirmed by hand calculation.

Expected results

A complete two-wheeler battery pack CAD with sized cells, holders, busbars, BMS bay and a manufacturable sheet metal enclosure, with electrical, spacing and bracket checks and a fabrication drawing pack.

Take it further

• Add a Flow Simulation or Thermal study of cell heating at peak current.
• Run a Frequency study of the pack on its brackets.
• Design an IP-rated gasket and breather for the lid.

Viva and review questions

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