Prokura Innovations

Daedalus, Icarus, and Li-ion battery

When Daedalus and Icarus were prepared for flight, Daedalus warned Icarus not to fly too high, because the heat of the sun would melt the wax, nor too low, because the sea foam would soak the feathers.

Ignoring the warning of his father, Icarus rose higher and higher…… the blazing sun softened the wax that held the feathers together and they came off. Icarus quickly fell in the sea and drowned.

Flying the UAV with a too low and too high temperature of the battery pack are undesirable. This time we want to heed the warnings and prepare before our UAV even takes its first flight!

 The performance of the Li-ion battery will degrade below 0-degree C. While such low temperature may not be attainable under normal flying conditions, the high temperature can easily be reached due to the fact that heat is generated inside the batteries. Failure to dissipate heat will lead to its accumulation. Also, if the heat dissipates unevenly, it will cause a large temperature difference inside the battery pack. The unevenness of the battery pack temperature field will cause an unbalance of the battery modules and each cell performance, and finally, affect the performance of the entire battery pack and security. The desired operating temp of the battery is less than 40 degrees C. 

Getting ready

When you are new to the work you are doing, one way to start is to skim the related works on research papers and videos. This gives you an idea if similar works have been done already. It was found that several works were done on thermal management of Li-ion battery pack although all of them were tailored for use in hybrid and electric vehicles. 

Comparison of work of others to your own

It is unlikely that the exact work you are doing is already done. But these sources can be of great help in identifying the tools and techniques to get your work done. When you cannot conduct an experiment on your own but if somebody has validated a numerical model with the experiment, then you can use that numerical model to carry out your own numerical experiment. The development of the thermal management of the Li-ion battery pack under discussion was done on a similar basis. 

Simulation of the temperature based on computational fluid dynamics

ANSYS fluent has a module for the simulation of the electrochemical reaction of Li-ion battery-MSMD (multi-scale multi-dimensional) Dual-Potential Model. One important advantage of such modeling is that you need not know how the heat is being generated inside the battery (although you can know-how by visiting the documentation). This is great for simulating the prismatic Li-ion cells on which it is based. But you may need to carry your own experiment to have proper results for your model. 

Another easy but less accurate way of carrying this simulation using fluent is to give each cell a heat generation rate and measure the temperature under various conditions- air flow rate, airflow configuration, gap spacing between cells, cell alignment, etc. 

The simulation of the battery pack requires you to have knowledge of the heat generation rate inside the battery. It was difficult to know the rates because 1) no experiment was found to have been carried on the Samsung 30Q (INR18650-30Q), candidate cell for the pack and 2) the results on other batteries showed that the rate is not constant but varies with discharge rate and temperature. 

During this work, the value of heat generation was first chosen based on the value of the heat generation rate of INR18650-25R and modified such that the temperature distribution matches the one given in the datasheet. 

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Description of Author

  • Bikash Kunwar is mechanical design engineer for drones at Prokura Innovations. He graduated from Pulchowk Campus with a degree in Mechanical Engineering. He was a member of the team that developed a search and rescue fixed wing UAV as their final year project. He is interested in interdisciplinary research concerning flying objects and using the technology to solve problems of the society. When he is not in the design room or slaving over a computer, Bikash can be found reading, travelling, or having fun with family and friends.

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