Electric Car Battery Tray in Sheet Metal with Mounting Points

The problem

Electric car battery trays carry several hundred kilograms of cells, must bolt rigidly to the body and must be made economically in volume. Sheet metal steel trays are common because they can be pressed and welded with existing tools, but they need careful forming design and stiffening to avoid sagging and cracking at mounts.

Objectives

• Package battery modules and mounting points in an example car floor.
• Design a pressed tray with formed ribs, side members and cross members.
• Generate flat patterns with correct bend allowances and check formability limits.
• Check tray deflection and mount stresses under vertical and longitudinal inertia loads.
• Produce sheet metal drawings with flat patterns and a weld map.

What you need

Key concepts

Bend allowance

The length of material in a bend, used to compute flat length. SOLIDWORKS uses K-factor, bend allowance tables or gauge tables to calculate it.

Forming tool

A SOLIDWORKS sheet metal feature that presses shapes such as ribs, louvres or embosses into a sheet, adding stiffness without extra parts.

Shell model

Sheet parts meshed as shells with thickness, which captures bending of thin panels efficiently.

Inertia load

The battery mass multiplied by an acceleration, such as vertical bump or braking, applied as a force that the tray and mounts must carry.

Step-by-step guide

1. 1

   Set packaging and loads

   Define an example floor space between the axles of a compact car, such as 1600 × 1200 mm, and an example 300 kg of battery modules in a 4 × 3 grid. List body mounting points along the sills and at cross members. Define inertia load cases, such as 3 g vertical, 2 g longitudinal and 1.5 g lateral, clearly marked as examples, and a deflection limit for the tray floor.

   Checkpoint: A requirement sheet with floor envelope, module layout, mounting points, load cases and deflection limit.

2. 2

   Lay out modules and mounts

   In an assembly, place module blocks with their mass, a floor envelope part and body-mount blocks. Sketch the tray outline with mounting flanges reaching the sill points. Decide where cross members go: between module rows, so they also serve as module mounting rails. Link key dimensions to Global Variables so the layout can change with module size. Leave space for a coolant plate under modules if the pack needs liquid cooling.

   Checkpoint: Modules and mounts sit within the envelope with a defined gap between modules and tray walls.

3. 3

   Model the tray pan

   Create the tray pan in Sheet Metal from an example 1.5 mm steel using Base Flange for the floor, Edge Flanges for the walls with bend relief and a perimeter flange for the lid seal and mounting. Set bend radius from supplier data and use a gauge table or K-factor. Add corner relief and closed corners so the tray can be welded watertight.

   Checkpoint: The tray pan flattens without errors and the flat pattern fits within a standard coil width.

4. 4

   Add stiffening ribs

   Use Forming tools or Sketched Bends to add ribs and embosses in the tray floor between cross members, oriented along the longest unsupported span. Check that ribs do not clash with module feet. Compare panel stiffness with and without ribs in a quick shell study, so the effect of ribs is measured, not assumed. Keep rib depth and radius within supplier forming limits to avoid splitting the sheet during pressing.

   Checkpoint: A quick shell study showing the reduction in floor deflection from the ribs.

5. 5

   Model cross members and mounts

   Model cross members as hat sections in Sheet Metal, with flanges for spot welding to the tray floor and threaded inserts or weld nuts for module bolts. Model mounting brackets at the sill points with doubler plates where bolts pass through. Use the Hole Wizard for all bolt holes. Add a weld map sketch showing spot or seam weld positions.

   Checkpoint: Every module and body bolt has a matching hole and every part flattens correctly.

6. 6

   Check packaging

   Run Interference Detection on the full assembly with modules, tray, cross members and body blocks. Check ground clearance below the tray and the gap between modules and walls. Use Mass Properties for tray mass and pack centre of gravity. If the tray mass is high, review gauge selection before analysis rather than after. Check that every bolt can be reached with a standard tool during assembly and service.

   Checkpoint: Interference Detection passes and tray mass and centre of gravity are recorded.

7. 7

   Set up the shell model

   In Simulation, use sheet metal bodies as shells with their thickness. Set bonded contact along flanges to represent welds, with a note that spot welds are discrete in reality. Use Bolt connectors at body mounts with fixed far ends. Apply modules as Remote Mass on their bolt locations, then apply the inertia cases with Gravity scaled or as acceleration loads.

   Checkpoint: The model runs and reaction forces equal the battery and tray mass times the applied acceleration.

8. 8

   Run and check load cases

   Run the vertical, longitudinal and lateral cases. Plot floor deflection and von Mises stress, focusing on mount brackets and cross member ends. Refine mesh at the highest-stress mount and repeat until peak stress changes by less than about 5 per cent. Hand-check floor deflection by treating a panel between cross members as a plate or beam strip. Compare reaction forces at each mount to see whether load is shared evenly.

   Checkpoint: Floor deflection below the limit, factor of safety above 2 at mounts, and mesh convergence at the peak location.

9. 9

   Iterate

   If deflection is too high, add a cross member or a rib rather than increasing the whole floor gauge. If a mount is overstressed, add a doubler or increase the bracket flange width. Record mass and results for each change in an iteration table, and choose the lightest design that meets every target. Re-run all load cases after each change, not just the one that failed, because a stiffer region can move load elsewhere.

   Checkpoint: An iteration table and a final design meeting all targets.

10. 10

    Drawings and report

    Produce drawings of each sheet metal part with formed views and flat patterns, bend notes and a bend table, an assembly drawing with weld map and a bill of materials. The report covers packaging, design, rib study, load cases, convergence and iterations. Note that crash and battery safety requirements, such as those in AIS-038 Rev 2, need separate study and testing.

    Checkpoint: A complete sheet metal drawing pack and a report with limitations stated.

Testing & validation

• Flat patterns generate for every part.
• Reaction forces equal applied inertia loads.
• Mesh convergence at the peak stress mount.
• Floor deflection checked by hand.
• Interference Detection passes for the full assembly.

Expected results

A manufacturable sheet metal steel battery tray for an example electric car with ribs, cross members and mounts, flat patterns and weld map, and verified deflection and mount stresses under inertia load cases.

Take it further

• Run a Buckling study of thin tray walls under a side load.
• Compare a steel and an aluminium tray for mass at equal stiffness.
• Add a Frequency study of the tray with modules mounted.

Viva and review questions

Learn it at DIYguru

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