130 Chapter 10 be designed to operate within. However, the thermal management system must still be designed to keep the lithium-ion cells operating within their optimal range. So for the higher temperature climates, chilled air or refrigerant-based cooling system may be needed in order to reduce the cells down to the 23 °C range. And in colder climates, a heater may be needed in order to provide warmed air or liquid to the battery in order to bring it up to its operating range. Two of the main types of temperature-measuring thermocouples that are used in battery systems today are thermistors and negative thermal coefficients (NTC).
A thermistor is a device that measures temperature through the use of two dissimilar metals that contact each other in one or more locations. When a temperature difference is experienced in one of the two conductive metals, a voltage is produced. This actually operates using the same principal as that of the Peltier device, which was discussed earlier. Thermistors operate over a very wide range depending on the type and selected from −180 °C and all the over 1800 °C, but typical measurement range is from 0 °C to over 1100 °C.
The negative thermal coefficient (NTC) thermistor is a temperature measurement device that operates by measuring a decrease in electrical resistance as the temperature increases. NTC thermistors operate in the range of −50 °C to 150 °C but can operate up to about 300 °C for some of the glass-encapsulated NTC units. Finally, thermocouple selection will depend on the desired temperature range, tolerance, cost, stability, and output. Design Guidelines and Best Practices Lithium-ion cells like to be maintained at about the same temperature range that people are comfortable with, which is about 23 °C (73 °F).
Thermal management of a battery is 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. Thermal management system will be dependent on three factors: the duty cycle that the pack will operate under, the region where it will be used in, and the cell itself. Three different types of heat transfer need to be considered: conduction, convection, and radiation. Active thermal management involves using some medium such as air, liquid, or refrigerant that is forced through the pack and over the cells to reduce temperatures.
Passive thermal management is the process of managing the temperature of the cells and the pack without forcing air, liquid, or other cooling medium into the pack.