Thermal Management 119 the cell is being tested, the thermal chamber is maintained at the exact same temperature as the cell throughout the test. This allows for the accurate definition of how much heat is generated by a cell during a specific operational cycle. The other two terms that I want to explain are exothermic reactions and endothermic reac- tions. An exothermic reaction is one that occurs when energy is discharged and heat is released into the environment thereby causing heating. An endothermic reaction occurs when energy is absorbed and heat is absorbed from the environment thereby causing cooling.
In lithium-ion cells, exothermic reactions occur when the cell is being discharged (releasing energy) thus generating heat that must be managed through the thermal management system of the battery. Why Cooling? As mentioned above, in order to maximize the potential of lithium-ion cells, they need to be maintained at about 23–25 °C (73–77 °F) throughout the majority of their use cycle. However, under operation, the cells experience an exothermic reaction—they begin generating heat due to the rate at which the chemical reaction occurs within the cell and the related increase in cell resistance.
This reaction in combination with high ambient (outside) temperatures means that your battery design must be able to cool the batteries down and maintain them within their optimal operating range in order to ensure the performance and life of the overall battery system. Additionally, high discharge rates generate the exothermic temperature increases within the cells. And when these discharges come frequently, it means that the cells do not have time to cool down between these pulses, which again drive higher temperatures. In addition to this, there is less time for the thermal management system to engage and reduce the temperature of the cells back down. Think of this as a stair-step effect:
when you hit your accelerator, a rapid discharge of the battery occurs; with frequent stopping and accelerating events the battery thermal management system will not have time to cool down the battery from the last dis- charge, causing the battery to gradually but steadily increase in temperature. An example of this is the traditional HEV, which will continually discharge and then regeneratively charge the battery regularly during a driving cycle. In this type of usage cycle, the battery may not have time to cool down the cells before another discharge–charge cycle is initiated. This will cause a slow and steady increase in the temperatures within the pack.
Allowing the pack to sit unused for a period of time will generally allow the system to reduce the temperature back down to its normal operating range. Figure 3 below offers an example of what this type of temperature increase may look like. While this is not actual performance data, it is consistent with the type of heat generation that could be expected to be seen in this type of application. As mentioned earlier, in high-temperature environments where the ambient air temperature is already in the 30–35 °C range or higher, it becomes even more important to be able to reduce