3.1. Thermal Characterization
In this experiment, a 60 Ah high energy density lithium mangan oxid (LMO) cell with physical dimensions of 175 mm × 125 mm × 45 mm is tested. For the characterization measurement, all THFS units are operated in mode 1, which means that the whole surface of the prismatic cell is held constant at 25
. Meanwhile, the cell is cycled at a rate of 2C (120 A) between the lower and upper voltage levels of 3.5 V and 4 V, respectively. Comparing these values to the open circuit voltage of the cell, the cycle corresponds to 10% to 80% state of charge (SoC). However, since the 2C rate leads to a considerable voltage drop across the internal resistance, the effective charge during one cycle measured by coulomb counting is 21 Ah, or 36% of the nominal capacity. In order to minimize the effect of variations of the generated heat, the constant voltage phase at the upper boundary of 4 V is kept to a minimum. To do this, the unloading cycle is triggered as soon as the loading current drops below 119 A. The applied load profile is depicted in Figure 3
At the beginning of the experiment, internal temperature gradients are building up, which leads to the underlying exponential behavior of the heat flux measurements shown in Figure 4
. In addition, a superimposed heat flux variation is visible, creating a periodic pattern, which is synchronized with the loading and unloading cycles. It is well known that the heat generation within a Li-ion battery cell is not constant for a given current. According to Liu et al. [16
], the generated heat consists of joule heat and reaction heat, which are both dependent, amongst others, on state of charge. However, it can be stated that the system reached a quasi-steady state after about two cycles or after 50
The post-processing and visualization of the measurement data, as presented below, was conducted in Matlab (Version R2016a, The MathWorks, Inc., Natick, MA, USA). Figure 5
shows the measured heat flux distribution after one hour at steady state with all 87 THFS units operated in temperature controlled mode. Therefore, the total area of the active cooling elements is equal to 19,575 mm
, or 27% of the total battery cell surface. Based on these results, the following observations can be formulated:
These results may be partly explained by taking the internal structure of the battery cell into account. In order to do this, the tested prismatic cell is disassembled for further examination. Thereby, the aluminum enclosure is found to contain four stacked pouch cells. The current collectors for anode and cathode are placed on opposite sides of the pouch cells and are bound together inside the left and right parts of the prismatic cell in Figure 6
. Furthermore, they extend to the top side of the enclosure, in order to make a connection to the electrical terminal. Plastic mounting brackets are used for the proper positioning of the pouch cell stack within the enclosure. In addition, the plastic parts act as electrical insulation between the current collectors and the enclosure.
In general, electrical conductors also exhibit a low thermal resistance. Therefore, a fairly good heat dissipation within the yz
-plane (see Figure 7
) as well as towards the electrical terminals can be expected. Thus, the low heat flux density on the side faces is a rather surprising result. A possible reason for this observation are the plastic spacers on each side of the pouch cell stack, which introduce an additional thermal resistance to this path (y
-direction), resulting in a reduced heat transfer. On the other hand, the high heat flux density measured in close proximity to the electrical terminals can be explained by the low thermal resistance of electrical conductors. Another interesting result is the observed asymmetry between the two electrical terminals. The fact that the positive terminal is in contact with the enclosure is one explanation for the high heat flux density in this region.
The above listed observations regarding the heat flux distribution can be qualitatively explained, or even predicted, by examining the internal structure of the cell. However, the extent to which the heat flux density varies between the different sides of the cell is surprisingly large. As shown in Table 2
, the average heat flux density on the front and rear sides is higher by a factor of 8, compared to the bottom side. Taking the integral over all THFS units, the average amount of dissipated power is found to be 12.7 W during a discharge and 13.6 W during a charge cycle, respectively. Since the measured values for charge and discharge differ, the generated heat is not only due to joule effect, but indicates the presence of reversible entropy heat. However, the values are slightly lower than the average joule heat dissipated in the internal resistance derived from voltage and current measurements, which is 15.2 W. This difference is due to the fact that some portion of the generated heat is not captured by the THFS units, which will be discussed in more detail in Section 4
. Therefore, the presented test rig is not suitable for calorimetric experiments such as ARC, but is able to reveal the heat release capabilities of different areas of the cell.
The described thermal characterization of a battery cell provides useful information for the design of an efficient thermal management system. The results presented in this paper clearly show that the amount of heat which needs to be dissipated is not the only important design criterion. In fact, the proper selection of the location for cooling a battery cell is a crucial point in order to minimize the internal temperature gradients.
3.2. Cooling Strategies
In order to emphasize the point made in the previous section, three tests were performed with different cooling strategies. For such a verification, some THFS units are operated in mode 1 (cooling), while the remaining THFS units are operated in mode 2 (insulation). Therefore, the main path for heat dissipation is the actively cooled face. In the first case, only the bottom side is held at a constant temperature of 25
, which corresponds to a situation with the cells packed side by side in such a way that the large sides are in contact with each other. From the design point of view, this arrangement is very appealing because it does not require a lot of space and the bottom side of the module is accessible for cooling purposes. Moreover, it is rather simple to accomplish the series connection of the cells by means of short metal straps if the cells have alternating opposing orientations. For these reasons, this configuration is nowadays used in many applications. However, in terms of temperature gradients within the cell, this arrangement is not favorable. The surface temperature distribution after four loading/unloading cycles at a rate of 2C is shown in Figure 7
and Table 3
. As can be seen, at a steady state, a temperature gradient of 6.5
developed over the height of the battery cell.
In order to reach a more homogeneous temperature distribution, the most promising locations for cooling the battery cell seem to be in proximity to the electrical terminals or in the center of the front/rear face. This assumption is made because, in those areas, the dissipated heat flux density is the highest (see Figure 5
). In order to confirm this, two more experiments are performed.
During the first experiment, seven THFS units with a total active area of 1575
(i.e., 2.7% of the total battery cell surface area, which is 70,750 mm
) are used to keep the bottom side of the battery cell at a constant temperature of 25
. For the sake of comparability, a similar active area is chosen for all subsequent sets of experiments. In order to implement the terminal cooling strategy, a total of eight THFS units is operated in temperature controlled mode: two of them at each terminal on the front and rear side, respectively. Figure 8
shows the temperature field after approximately 120
of 2C loading/unloading cycles. The actively cooled areas next to the electrical terminals are clearly visible. Furthermore, a temperature gradient has built up towards the bottom of the cell. The maximum temperature measured during this experiment is 30.1
, which is 1.5
less than in the first experiment. In other words, when choosing a more appropriate location for the active cooling of the battery cell, the resulting surface temperature gradient can be reduced by about 23%.
This effect becomes even more apparent in the case of the third scenario. Here, the active cooling is placed right in the center of the front and rear face, respectively. Thereby, four THFS units are used on each side. In this case, the maximum temperature gradient can be reduced further to a level as low as 3.6
, as shown in Figure 9
, which corresponds to a reduction of 40% compared to the first scenario. Obviously, this could be further improved by increasing the actively cooled area on the front and rear faces, which would make sense also from a design point of view. This means, in a real world application, iso-thermal heat sink plates could be located between each pair of battery cells. For the battery cell under investigation, such a configuration would lead to differences in the surface temperature that are literally not measurable.