CHAPTER 10 Thermal Management In this chapter, we will discuss the different manners in which a battery pack can be cooled and, if necessary, heated to maintain the cells at optimum operating temperature. When it comes to lithium-ion cell temperatures, the most important thing to remember is that lithium- ion cells like to be maintained at about the same temperature range that people are comfort- able, which is about 23 °C (73 °F). Thermal management of a battery is, in its most simple form, the integration of some form of heat exchange device into the battery system in order to maintain the battery cells at a near constant temperature.
There are many potential methods that can be used to achieve this, but it typically involves moving heat through some medium away from the cells and out of the pack. The complexity of the thermal management system will be highly dependent on three factors. The first is the duty cycle under which the pack will operate. If it is intended to be used as a high-power application, the battery cells will generate more heat than if it is used as an energy application. The second factor is the region where it will be used in.
If the pack is intended to be used in a region of the world that frequently has very high ambient tempera- ture, it means that the batteries will begin their cycle at an already elevated temperature level. Finally, the third factor is the cell itself. Different chemistries perform differently under high load and temperature operation. Therefore, it is important to consider all of these factors when beginning to design your battery’s thermal management system. The thermal management system should be able to maintain a temperature difference of about 2–3 °C from the coolest cell to the warmest cell. At the worst-case condition, usually for larger packs the difference can be as much as 6–8 °C.
The reason why this is important is that a large temperature gradient between the cells will cause the cells to age at different rates. So the hotter cells will age faster than the cooler cells, and if there is a large gradient, this could mean that the battery’s calendar life will be reduced prematurely. There are three different types of heat transfer that need to be considered in battery design: conduction, convection, and radiation. Conduction refers to a direct transfer of heat energy from two objects that are in direct contact. Convection occurs when heat is conducted through a liquid medium to a heat-sinking device.
Radiative heat transfer refers to heat energy that generated through electromagnetically thermally charged particles of matter that radiate from one source to another, generally through the air. All three methods of heat transfer must be considered in the battery system design, but conduction and convection will have the greatest impact on the thermal system design. The Handbook of Lithium-Ion Battery Pack Design. http://dx.doi.org/10.1016/B978-0-12-801456-1.00010-5 Copyright © 2015 Elsevier Inc. All rights reserved. 115