Meaning
Ratio of the vibrational response of a specific component to the input excitation from the vehicle chassis determines the durability of internal battery connections. Structural transmissibility identifies how much of the external vibration is amplified or dampened as it travels through the battery pack frame to the individual cells. This value is a function of the frequency of the input and the natural resonances of the internal components.
If the transmissibility is high at a frequency that matches a natural resonance, the resulting movement can lead to the fatigue failure of busbars or signal wires. The analysis of this ratio is the primary tool for designing the isolation systems that protect the battery from the harsh environment of the road.
Dynamic Analysis
Frequency response functions are used to plot the transmissibility across the entire range of vibration seen during the operation of a vehicle. Structural transmissibility is usually highest at the resonant frequencies where the damping in the system is lowest. Engineers use shaker tables to measure these values and verify that the design meets the safety requirements.
If a component shows a high peak in the plot, it indicates a need for additional stiffening or better damping materials.
Isolation Strategy
Placement of rubber mounts and foam pads is optimized to keep the transmissibility below a value of one in the critical frequency ranges. Structural transmissibility can be reduced by shifting the natural frequencies of the battery pack away from the common excitation frequencies of the motor and tires. This is achieved by changing the mass or the stiffness of the support structures.
A well designed system will isolate the sensitive electronics and cells from the most damaging vibrations.
Failure Mitigation
Monitoring the changes in transmissibility over time can reveal the loosening of fasteners or the degradation of isolation materials. Structural transmissibility provides a metric for the health of the entire assembly during long term durability testing. If the response of the pack changes significantly, it indicates that the structural integrity has been compromised.
This data is used to improve the design of future generations of battery modules to ensure they remain reliable for the life of the vehicle. The final validation ensures that the electrical connections remain secure even under extreme mechanical stress.
