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Although there are test standards and legislative requirements for the vibration and shock testing of electric vehicle battery packs, some of these have been derived from data from consumer electronics, or transposed from data from more traditional internal combustion engine vehicles, research suggests.
So it is possible that EV battery packs are being either over-engineered to compensate for vibration inputs that they would otherwise never be subjected to, or under-engineered because the battery component is not subjected to the correct in-market vibration and shock loads.
To improve our understanding of vibration requirements in EV batteries, Millbrook has carried out tests on a range of such batteries, by subjecting them to “real world” road surfaces and rough road abuse environments. The tests revealed interesting results that differed considerably from what was previously thought to be the case.
We identified that the bulk of vibration energy in EV battery packs occurs from 0 to 100Hz, whereas it was previously thought to occur between 0 and 50Hz.
Another difference was the bottom end of the range. Current engineering specifications for battery vibration durability typically test from 7Hz upwards. However, the work highlighted the need to consider frequencies below 7Hz, as significant vibration energy occurred from 0 to 5Hz in all battery EVs tested.
Yet another difference arose with the batteries’ X and Y axes. An academic review and evaluation of current rechargeable energy storage system vibration test procedures highlighted that typically the X and Y axis of EV battery packs are evaluated using the same test profile. However, the X and Y vibration behaviour of the EV battery packs evaluated in our research differed significantly. So Millbrook would recommend that the X and Y axis of an EV pack be evaluated using specific vibration data, based on the vibration measurements it will receive in the in-vehicle conditions.
A mode typical of powertrain-induced vibration was noted in all EVs between 7 and 20Hz. High energy spikes were also noted in some EVs above 300Hz. They were likely caused by powertrain-induced vibration from the electric motor or transmission assemblies, and the effect of vibration induced by different cooling strategies or power electronics. Identifying the exact cause of these vibration energy spikes above 300Hz will require further research.
This study also highlighted that if it was determined vibration was being introduced to the pack through cooling strategies, the pack’s vibration life could be affected by different climates and driving styles.
There is evidence to suggest that because of the balanced weight distribution of EVs, lower vibration energy levels are typically witnessed in these vehicles than in their internal combustion vehicle counterparts.
Evidence was also found to suggest that vibration behaviour at 20 to 40Hz in EV battery packs is the result of a body torsion mode, because the pack forms a large structural member of the chassis. This idea could be validated by further investigation of the pack and chassis vibration response via a modal analysis in a laboratory.
However, some resonances between 20 and 40Hz were attributed to the strategies employed by manufacturers to isolate the pack from chassis vibration. So it is recommended that engineers consider the battery pack to chassis integration, as the pack’s installation can affect the frequencies and vibration energy it will see during its service life.
It is clear that engineers need to get to grips with vibration requirements for EV batteries, and to consider the battery pack to chassis integration.