The design of electrical car battery packs could fluctuate amongst producers however generally, the battery is hooked up to its cooling plate not solely with fasteners but additionally thermally conductive polymeric supplies known as hole fillers. Hole fillers can outperform thermal pads on this software to realize decrease thermal impedance, because the hole fillers conform to floor roughness earlier than curing. This permits hole fillers to stick properly to surfaces, present mechanical assist throughout regular operation, and move strict reliability requirements of OEMs. Nonetheless, the excessive adhesion values additionally pose a problem if the battery must be disassembled.
A common design of the battery pack is proven in Determine 1 the place battery modules are tightly packed collectively to maximise area and power density. Such tight meeting of batteries of their housing doesn’t enable quick access for elimination, reminiscent of peeling the battery from an edge, and vertically.
The reworkability of a battery pack entails eradicating and changing the faulty battery module with out destroying the opposite modules or cooling plate. The stress required to take away a battery module will depend on the adhesive power of the hole filler, which in flip will depend on the floor properties, mechanical power of the hole filler and battery elimination circumstances. One could ask the significance of reworkability within the EV market. Apart from a transparent environmental impression of a reworkable battery pack that permits reuse and recycle to cut back waste and encourage sustainability, there are actual financial impacts.
Adhesive Failure Modes
Within the field plots proven all through the subsequent few paragraphs, the arrows point out a rise (purple) or lower (inexperienced) within the imply pull-off stress when transferring from left to proper CoolTherm® hole filler pattern. A paired t-test for pattern means is carried out with alpha of 0.05 to judge if the distinction in pull-off stress means is statistically vital. The failure modes are proven via a consultant picture alongside every field plot. The failure modes are outlined as follows:
- Cohesive (coh): distinct failure within the bulk of the hole filler, each substrates have hole filler current. b. Adhesive (adh): failure on the interface between the substrate and hole filler, hole filler current solely on one floor.
- Blended (combine): a mixture of adhesive and cohesive failure the place some a part of substrate is clear of any hole filler.
- A cohesive (coh) materials failure is most well-liked by some OEMs from a thermal transport perspective. Nonetheless, an adhesive (adh) failure is straightforward to scrub. As we’ll see under, failure mode of a niche filler is just not solely a fabric property but additionally a results of the substrate floor.
For extra data on our testing methodology and an illustration of our t-bar take a look at samples, obtain the total white paper here.
Three Components that Affect Hole Filler Reworkability
1) Floor Results on Pull Off Stress
To guage the impact of tough floor on the pull-off stress, the T-bar floor was sand blasted. The floor roughness was measured utilizing a floor roughness tester (Mitutoyo, SurftestSJ-210) as per ISO1997. The typical roughness (Ra) for the clear aluminum floor was recorded as 0.35 ± 0.05 µm, whereas for the sand blasted floor the Ra was recorded as 4.66 ± 0.30 µm.
The floor roughness after sand blasting is roughly 10 occasions greater than the unique floor. The vertical pull-off stress for contemporary aluminum (contemporary Al) and sand blasted is proven in Determine 2, the place a 17% enhance within the imply pull-off stress is noticed for the sand blasted floor. The distinction in imply values is statistically vital.
The failure mode additionally switches from adhesive/combined (adh/combine) for contemporary Al floor to cohesive (coh) for sand blasted floor as seen from the pictures in Determine 2. The outcome highlights the significance of defining substrate floor roughness whereas testing the vertical pull-off for reworkable hole fillers.
To discover our different assessments on floor results reminiscent of first-use versus second-use aluminum and the impact of e-coat on pull-off stress, click here to learn the total white paper.
2) Impact of Pull Fee
With the viscoelastic conduct of polymers in thoughts, the impact of pull-rate (strain-rate) on the pull-off stress was measured and the result’s proven in Determine 3. The bond hole is stored fixed at 1 mm. Clearly, the pull-off stress will increase because the pull-rate will increase. Nonetheless, after 12 mm/min a much less vital enhance in pull-off stress is observed for the examined hole filler. A noticeable distinction within the failure mode is obvious. At 1 mm/min, failure mode was cohesive (coh). At and above 12 mm/min, the failure mode was principally adhesive (adh) with occasional combined (combine). See this demonstrated within the determine under.
3) Impact of Bond Line Thickness
The subsequent variable we investigated was the impact of bondline thickness on the pull-off stress (Determine 4). The pull-off stress decreases because the bond hole is elevated from 1 mm to three mm. The failure mode switched from adhesive/ combined (adh/combine) at 1 mm to fully adhesive (adh) for two and three mm bond hole.
If a battery could be pulled vertically from an edge, it might present greater than 50% discount within the pull-off stress as in comparison with drive being evenly utilized from the middle. The failure mode for our examined CoolTherm® hole filler was fully adhesive (adh) when pulling from an edge versus adhesive/combined (adh/combine) when pulling from the middle.
The tight meeting of battery modules could not enable for the peeling of the module from an edge. However, is an entire peeling from an edge required? The typical displacement of the sting earlier than joint failure was solely 0.6 mm when pulling from the sting That equates to the angle of 1.4 levels. If that angle of 1.4 levels is utilized to a battery module of common dimension (40 cm x 20 cm), it ends in the necessity for a 4 to five mm clearance between modules to permit for simpler elimination. This might scale back the required drive by greater than 50% as in comparison with lifting from the middle or making use of equal drive throughout the floor of the module. This could considerably scale back the possibility of damaging the cooling plate.
On the lookout for Extra Data?
Are you contemplating reworkability when designing your EV battery pack? Talk with a member of Parker LORD’s team for extra data on this subject, read the full study here or go to lord.com/CoolTherm for product data.