Thermal Management 129 In some systems, the battery enclosure itself may act in this manner and no additional shield- ing may be necessary. Another method that is frequently used for keeping the heat either in or out of the battery enclosure is the use of insulation. In these systems, the insulation is used for two reasons. In the summer time and in hot ambient environments, it may be used to protect the battery from the high ambient temperatures. And in cold regions or in winter usage, it may be used to avoid heat loss to the outside environment. But even in these systems, it may be necessary to shield the electronics from the batteries.
Thermocouples and Measurement Temperature measurement is one of the most challenging issues in thermal system design and management. The most accurate location to measure the temperature of a lithium-ion cell is actually inside the cell. However, it is also the most difficult location to install a temperature sensor unless it is designed into the cell itself. Therefore, typical cell measurements are taken from the surface of the cell. When outfitting a cell for testing, many thermal engineers will install temperature sensors in three locations on the cell: near to the terminals, on the middle of the surface of the cell, and on the surface farthest away from the terminals.
This is usually only done during characterization of the cell however, as the cost of installing these many thermistors in a large pack would be very cost prohibitive. For these reasons, production designs will usually include two to three temperature sensors per module. Installation of the temperature sensors in this case is not dissimilar to how the sensors are located during cell characterization. One temperature sensor may be mounted on one of the first cells in the module, one in the middle, and one near the end.
Of course, the proper method to determine exactly how many sensors are required and where to locate them should be based on your module characterization testing and CFD analysis, both of which will inform you where the “hot” spots in the module design are. Remember that the purpose of the temperature sensors is to identify and manage the overall temperature of the pack, not necessarily to monitor the temperature of each cell. The tempera- ture sensors in the thermal management system (TMS) are intended to monitor the tempera- ture of the pack and inform the BMS when to send more heating or cooling to the battery.
In the thermal system design, for both liquid- and air-cooled designs, it is also good practice to install temperature sensors to monitor the temperature of the incoming fluid (liquid or air) as well as the temperature of the outgoing fluid (liquid or air). Note that this is in addition to the temperature sensors that are located inside the battery modules. Another important concept to remember is the difference between outside ambient tempera- tures and the internal energy storage system temperatures. Many specifications will describe a temperature range for a system as ranging from −40 °C to 70 °C, but this is not referring to the temperature that the battery will achieve.
Only the external temperatures that the battery must