Looking at the pouch cell design and edge cooling evolution allows us to appreciated the importance of battery cell electro-thermal behaviour.
Fundamentally pouch cells are made from multiple layers of active material all applied to copper and aluminium electrodes that are sandwiched and compressed together. The copper and aluminium electrodes have very good in-plane thermal conductivity. Hence out of the many pouch cell cooling options, cooling along the longest edge of the cell makes perfect sense.
The simple pugh matrix comparing the pouch cell cooling designs shows that Edge Cooling comes out as the best overall with a score of 29.
Let us look at how the edge cooling of pouch cells has evolved over time.
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Most of the original pouch cell designs had the two electrical connections on one edge.
However, as this cell is discharged the shortest path between the two tabs is the lowest resistance and hence discharges first. As the cell discharges this area heats up and the resistance decreases with temperature. Hence a positive feedback loop.
The original module edge cooling designs used a heat transfer plate between the cells to draw the heat to the cooling plate using a thick (~0.5 to 2mm) sheet of aluminium. This plate had a bent over edge to increase the interface area with the cooling plate.
This plate takes up space, adds weight and complexity.
Also, when we look at this plate it is running parallel to the copper and aluminium electrodes in the cell.
There is one other function that these heat transfer plates serve and that is they distribute the loads across the cell faces.
A better cell format is the long pouch with the tabs at each end.
A reduced cell and hence module height and a more even distribution in the cell discharge.
If we look at the cell design progress it has evolved around the electro-thermal performance.
Then looking at the module design we see this progress in performance and reduced complexity, cost and weight.
When we look at pouch cell design and cooling it is perhaps natural that automotive designs have evolved to an optimsed edge cooling.
References
- Anna Tomaszewska, Zhengyu Chu, Xuning Feng, Simon O’Kane, Xinhua Liu, Jingyi Chen, Chenzhen Ji, Elizabeth Endler, Ruihe Li, Lishuo Liu, Yalun Li, Siqi Zheng, Sebastian Vetterlein, Ming Gao, Jiuyu Du, Michael Parkes, Minggao Ouyang, Monica Marinescu, Gregory Offer, Billy Wu, Lithium-ion battery fast charging: A review, eTransportation, Volume 1, 2019
Porsche Taycan
This is an 800V pack designed for fast charge and fast discharge.
The modules fit into a robust waterproof frame and the cooling elements are glued on underneath the bulkhead plate using heat conductive adhesive.
This approach is simple and robust in terms of it’s approach to possible coolant leaks, but it does reduce the overall thermal performance of the pack.