Researchers from Sakarya University and Duzce University together with the specialists from Scientific and Technological Research Council of Turkey studied in detail the behavior of lithium-ion battery in the conditions of low earth orbit. This is of crucial importance for satellites: the battery needs to stay within safe temperature range both in the bright sun and in the cold Earth’s shadow. The heat fades away very slowly in the vacuum, and external temperature differentials can easily result in over-cooling of cells and facilitate the degradation of electronics. That is why, as early as at the design stage it is necessary to create a digital model capable of calculating the thermal regime of the battery under any scenario of flight.
To create this model the scientists performed a heat-vacuum balancing test – an experiment close to the outer space conditions to the maximum extent. A full-size mockup battery was placed into thermal-vacuum chamber, where it is possible to establish pressure close to space vacuum and the temperature from –10 to +40 °C. The mockup battery was made of the same materials and according to the same design as the real battery to be used during the flight. The heaters imitated the work of electronics, and 27 thermal sensors fixed the temperature of all the key elements – from control card to battery cells.
The collected data were used as the basis for the node thermal model: this method stipulates for dividing a complex structure into several zones, inside which the temperature is believed to be evenly distributed. For example, the control model framework and its lid were heated practically simultaneously, hence, they were combined in a single zone. The first version of the model took into account only thermal conductivity, i.e., thermal transmission via materials. For moderate temperatures that was enough, and the calculations matched the experiments.
However, in the “hot” modes this simple model failed. It turned out that under high temperatures thermal radiation starts playing an important role – the same physical effect due to which our hand gets hot when we hold it close to the fire. The radiation intensity grows drastically along with the temperature, and accurate forecast becomes impossible without taking it into account. That is why the model was enhanced by adding the radiation calculation and the impact of multi-layer thermal insulation, which covers the satellite equipment.
After all these updates the hybrid model practically completely coincided with the results of the experiments across the entire range of operation – from 0 to +40 °C. It is especially important that it accurately reflects temperature behavior of the most sensitive elements: battery cells and electronic circuit card, and the reliability of the entire system depends on their stable performance.
Eventually, the researchers received a compact, but very precise mathematical model consisting of seven nodes capable of describing the thermal performance of the battery both under standard and under extreme conditions. The difference between the estimated and the measured temperatures dropped down to several degrees, which is in line with international standards for thermal modeling of space vehicles.
