Battery Enclosure Side-Impact Protection Concept

The problem

In a side pole impact, a narrow object crushes into the side of the car close to the battery. Cells must not be crushed, so sills and battery side structures must absorb energy and spread the load before it reaches modules. Full crash simulation needs specialist solvers, but much of the concept work on section shape and load paths can be done earlier with simpler tools.

Objectives

• Define a pole impact concept scenario and an intrusion limit to the cell zone.
• Design several sill and battery side-rail section concepts.
• Compare stiffness and load spreading with linear static studies.
• Compare crush force and energy absorption with nonlinear quasi-static studies of a section length.
• Select a concept and document the limits of the method.

What you need

Key concepts

Pole impact

A side impact against a narrow rigid pole, which concentrates load and causes deep local intrusion. Regulations and assessment programmes include such tests; check the current requirements.

Energy absorption

The area under the force–displacement curve when a structure crushes. Side structures must absorb impact energy within the available crush space.

Load spreading

Cross members and stiff rails distribute a local load across the floor, so no single module sees a concentrated force.

Quasi-static crush

Crushing a structure slowly in a nonlinear static analysis. It ignores strain-rate and inertia effects but compares section concepts well.

Step-by-step guide

1. 1

   Define the concept scenario

   Define an example compact EV floor with a battery between the sills and a crush gap between the outer sill face and the nearest module. Set an example pole diameter and impact position at the battery mid-length. Set an intrusion limit as a fraction of the crush gap. State that this is a concept study and that regulatory side and pole impact requirements must be checked in the current standards.

   Checkpoint: A scenario sheet with geometry, crush gap, impact location and intrusion limit.

2. 2

   Create section concepts

   Design four concepts for the sill plus battery side rail: a single-cell steel sill, a multi-cell aluminium extrusion, a steel sill with an internal aluminium insert and a sill with a separate crushable battery-frame rail. Drive wall thickness and web count with a Design Table. Record mass per metre with the Section Properties tool and Mass Properties. Keep outer dimensions within the same envelope for a fair comparison.

   Checkpoint: Four concepts with section properties and mass per metre tabulated.

3. 3

   Model the side structure assembly

   Build an assembly of a floor section about 1 m long containing the sill concept, the battery side rail, two cross members and module blocks. Add a rigid pole as a cylinder. Use bonded contact for joints with a note on joining assumptions and Bolt connectors where the battery bolts to the body. Restrain the far side of the section to represent the rest of the car.

   Checkpoint: The assembly is fully constrained and passes Interference Detection with the pole just touching the sill.

4. 4

   Compare linear stiffness

   In Static studies, apply a force through the pole to the sill and compare displacement at the pole and at the nearest module, and the force carried by each cross member. Run all four concepts. Check that results change by less than about 5 per cent after mesh refinement at the impact zone. This ranks concepts for stiffness and load spreading only.

   Checkpoint: A table of stiffness and cross member load share for every concept, with a mesh check.

5. 5

   Set up the nonlinear crush study

   For the two most promising concepts, set up a Nonlinear Static study with large displacement, plasticity using true stress–plastic strain curves and contact between pole and sill and between deforming walls. Drive the pole with prescribed displacement up to the crush gap. Use the arc-length or displacement control and check convergence at each step. Use a mesh with several elements across each wall fold length, since folding needs resolution. Save results at enough steps to plot a smooth force curve.

   Checkpoint: The nonlinear study runs to the full crush displacement without convergence failures.

6. 6

   Extract force–displacement and energy

   Export reaction force on the pole against pole displacement for each concept. Integrate in Excel or MATLAB to get energy absorbed. Record peak force, mean crush force and the displacement at which the module zone is reached. Compare with the intrusion limit and with a target energy for the concept study. Compute energy per kilogram of side structure so heavier concepts do not win simply by adding material. Note where force peaks occur, since high peaks can damage the battery mounts.

   Checkpoint: Force–displacement curves and energy values for both concepts, with intrusion relative to the limit.

7. 7

   Check model sensitivity

   Repeat the better concept with a finer mesh in folding regions and with a smaller load step. Compare mean crush force and energy. Check whether results depend strongly on contact settings. Note that quasi-static analysis ignores strain-rate effects and that real welds or rivets may fail, which this model does not include. Record the run time for each setting so the cost of accuracy is clear.

   Checkpoint: Mean crush force and energy change by less than about 10 per cent with refinement, or the sensitivity is reported.

8. 8

   Hand-check crush behaviour

   Use a published analytical formula for the mean crush force of a thin-walled square or multi-cell tube under axial or lateral crush, from the textbook literature, and compare its order of magnitude with Simulation. Large differences point to setup errors or to a different collapse mode, which you should explain. Check also that the deformation mode in Simulation, such as progressive folding or global bending, is physically plausible for the section and load.

   Checkpoint: Analytical estimate and Simulation agree within a reasonable factor, or the difference is explained.

9. 9

   Score and select

   Score concepts on mass per metre, linear stiffness, load spreading, energy absorbed within the crush gap, intrusion margin and estimated manufacturing complexity. Use a weighted table with weights stated and a check on how sensitive the ranking is to them. Select a concept and describe changes for the next design loop. Include a manufacturing note for each concept, such as whether a new extrusion die is needed, since this affects cost and lead time.

   Checkpoint: A scored table and a selected concept with reasons.

10. 10

    Report

    Report the scenario, concepts, linear and nonlinear results, sensitivity, hand check, scoring and limitations. State clearly that full vehicle pole impact requires explicit crash simulation in a dedicated crash solver and a physical test, and that this study supports early concept choice only. Include force–displacement curves of both nonlinear concepts on one plot, contour plots of plastic strain at full crush and the scoring table. Recommend which further analyses and tests should follow.

    Checkpoint: A report with clear method limits and a recommendation for crash simulation.

Testing & validation

• Mesh check on linear results.
• Nonlinear convergence and refinement sensitivity reported.
• Analytical crush force comparison.
• Energy computed from force–displacement curves.
• Method limits stated.

Expected results

A concept comparison of battery side-impact structures for an example EV, with linear stiffness and load-spreading results, nonlinear crush curves and energy for two concepts, a sensitivity check and a selected concept.

Take it further

• Run the selected concept in an explicit crash solver and compare with the quasi-static result.
• Crush a physical section in a universal testing machine and compare force–displacement.
• Add a foam or composite filler and study energy per kilogram.

Viva and review questions

Learn it at DIYguru

These DIYguru courses and programmes teach what this project needs.