Whatever the cell format and however we cool it there will be a temperature gradient. This is described by Fourier’s Law of Conduction. An empirical law stating that the rate of heat flow is directly proportional to the temperature gradient and the area of heat transfer, and inversely proportional to the thickness of the material.
Whichever surface the heat sink is applied to, or even to more than one surface, a temperature gradient will be formed if we are to remove heat or if we add heat.
Thermal Conductivity of the Active Layers
Thermal Conductivity versus SoC – the through plane thermal conductivity of a pouch cell was measured [Ref 3] and showed a negative parabolic dependence for fresh cells, where the highest thermal conductivity values were shown for the highest state of charge. This increase in conductivity at high SoC was shown for new and aged cells.
Thermal Conductivity versus SoH – for cells where a pressure was applied to the cell the thermal conductivity in plane decreased with age by around 4%.
Importance of Pressure – the through-plane thermal conductivity degraded by 4% when a pressure was applied to the cells [Ref 3]. When the cells were not under pressure the thermal conductivity degraded by 23% when they were aged.
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Cooling on more both sides will reduce the path length and for a given heat rejection rate will reduce the temperature gradient.
Battery Cooling Options
There are many options and all have their place depending on the application and cell requirements.
A temperature gradient within the cell will be there if we are removing or adding heat. Even assuming the heat generation is uniform through the active cell a temperature gradient with exist.
Non-uniformity of heat generation is another factor and dependent on:
- electrical pathways
- ageing of the active materials
- uniformity of the active cell stack
- uniformity of the materials
- uniformity of the thickness
- compression
Anna Tomaszewska et al [1] show simulated (a and c) versus measured (b and d) temperature gradients in a pouch cell during a 5C discharge.
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
See also: Thermal Management System, Cell Temperature Sensing