# Adaptive Thermal Control of Cell Groups to Extend Cycle Life of Lithium-Ion Battery Packs
## 4 Discussion  
It is important to note some of the practical challenges while implementing thermal control for individual cell groups for future studies and the potential commercialization of this method. First, it is important to electrically isolate each series group along with its cooling plate to avoid shorting along the termination points. A series cooling loop is possible, but it must be ensured that the coolant is not electrically or ionically conductive, and a non-conducting pipe is used between each series group. To our knowledge, no  
Figure 4: Capacity degradation of cell groups with thermal control applied (solid lines represent active cooling, and dashed lines denote no active cooling). Discharge rate of 5C, charge rate of 3C throughout cycling. previous studies have performed such thermal-based capacity balancing, so it is not possible to draw direct comparisons between our results and other prior work. Furthermore, thermal control of individual cell groups could be accomplished with a system of valves and a bypass pipe as shown in Figure 5[16]. instead of individual coolant pipes going to each cell group. Essentially, an optimization strategy such as the one used in [18] could be used to control the amount of cooling applied to each parallel stack of cells. This arrangement would enable an autonomous system that would use capacity feedback to adaptively control the temperature of specific cell groups. One challenge is that a capacity measurement of each individual parallel cell group would need to be recorded periodically (every 10-20 cycles) at a low C-rate for accuracy. Cooling for each cell group would then be adjusted for the next interval of cycling before repeating the capacity check. This would involve a more complex electronic BMS system that has accurate capacity measurement, such as the method used in our study with a slow 1C discharge rate. An alternative method to maintaining cell-to-cell capacity balance throughout the lifetime of a battery pack is to consider installing \"disconnecting\" fuses on a certain number of parallel cells in a pack. For example, consider the Tesla Model S 5.3 kWh battery module, which consists of 444 18650 cylindrical cells. These are arranged in 6 series groups of 74 parallel cells (6874P) with friction-welded fuses inserted between each cell and the main busbar. Some of these fuses could then be replaced with a controllable fuse/relay which could be activated by a microcontroller. Let us now consider a situation where one of the groups of 74 parallel cells has lost a significant amount of capacity in the mid- to end-of-life of the pack. At that point, several of these controllable relays from cells in the other 5 groups which have higher capacity could be opened, equalizing the capacity between all 6 groups, and restoring functionality to the module again which does not take an inordinate amount of time to balance during charge or after discharge. However, this method has a major drawback in removing functional cells from the pack and having them ride along in the EV as dead weight. Therefore, the extra upfront cost and space allocated to an adaptive thermal control implementation could have high utility. This type of adaptive cooling configuration would be particularly well suited to grid storage applications where space is not as much of a constraint as in EVs. One drawback of this approach is that it would take up more space than existing series cooling methods  
Figure 5: Potential implementation method for adaptive thermal control. Indicated valve position (open) directs coolant toward the selected cell group. Closing the valve would cause the coolant to bypass the cell group. and incur a higher initial capital cost. However, this could be offset by the increase in the lifespan of battery packs, especially for grid storage. As an example of a basic preliminary techno-economic analysis, we consider this adaptive thermal control method applied to the Tesla module of 444 cells mentioned above.