Researchers at Imperial College London have developed a battery cell cooling metric to compare the thermal performance of lithium ion batteries and are urging the industry to adopt it to improve the efficiency of electric vehicles.
The battery cell cooling metric was developed by a team at the Electrochemical Science and Engineering Group at Imperial to measure the temperature gradient across the layers in a lithium ion cell in operation. This would allow a comparison of different cooling techniques, particularly for battery packs in electric vehicles. The metric shows the difference in performance between the pouch cells used in the Nissan Leaf and the cylindrical cells used in the Tesla Model 3.
“Calculating the temperature gradient inside individual cells is key to designing thermal management systems for battery packs that contain large numbers of cells,” said Dr Greg Offer, reader in mechanical engineering at Imperial and principal investigator of the Faraday Institution Multi-Scale Modelling Project.
“Coolant fluid is pumped through a network of channels to carry heat away from the individual cells. But these cumbersome additions make the battery pack heavy and drain its energy. Developers are wasting time and money on these inefficient designs. Heat-removal strategies must be improved to make battery packs both light and powerful,” say the researchers in the journal Nature.
The problem is that there is no thermal performance metric for electrochemical cells that is easily reproducible anywhere in the world, and that does not reveal commercially sensitive information about how a battery cell is designed or manufactured. Heat-transfer specialists favour the Biot number, which describes a body’s capability to pass and dissipate heat. Mechanical engineers prefer definitions of thermal conductance and thermal conductivity; these define the rate of heat transfer that can be achieved through a material for a given temperature gradient.
None of these methods can calculate the temperature gradient across a cell when it is in operation, because electrochemical cells generate their own heat throughout their volume. If the temperature gradient across one cell is not known, it is impossible to design a thermal management system for a battery pack containing 1,000 cells.
The battery cell cooling can be used to describe the temperature gradient across a cell in operation in watts per kelvin (W/K). A cell will have a different value for surface cooling and for tab cooling, because each method results in a different temperature gradient. Such a coefficient would tell a designer how difficult it will be to manage heat in the selected cells in a pack.
A key point is that the cooling coefficient is straightforward to measure in the lab. Heat from a cell in operation can be measured using temperature sensors. Heat loss from the cell can be measured using heat-flux sensors. For surface cooling, where one side of the cell is cooled and the other remains hot, the cell cooling coefficient could be calculated by dividing the rate of heat loss by the temperature gradient from the hot side to the cold side. This would add just two hours to the characterisation process of a cell.
Next: Results of the battery cell cooling metric
A large battery cell cooling coefficient means more heat can be removed and there is a small temperature gradient inside the cell. Of the cells we have investigated, large pouch cells, such as the ones in the Nissan LEAF, seem to perform best and have a cell cooling coefficient close to 5 W/K. Small cylindrical cells, such as the ones in the Tesla Model 3, perform less well, with a cell cooling coefficient of less than 0.5 W/K.
Knowing the battery cell cooling coefficient will help designers to evaluate trade-offs between thermal management and energy density, improving the working performance of the whole pack. Computer simulations might be helpful for assessing the potential of cells.
The researchers are calling on manufacturers to publish the cell cooling coefficient of their products alongside other common metrics such as energy capacity and discharge rate. “Some manufacturers might be reluctant to adopt a metric like ours if their products fare poorly in comparison with the competition, but those that embrace it could gain a competitive advantage,” said Offer.
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