Li-ion cells are a technologically important class of devices for electrochemical energy storage and conversion. Overheating of a Li-ion cell during operation is undesirable as it directly affects performance and safety. Although a number of methods have been used for temperature measurement in Li-ion cells, there is a lack of non-invasive techniques to determine the peak temperature at the core of the cell. Measuring only the outside surface temperature, while straightforward, is not sufficient since the core temperature is in most cases much higher. This paper presents non-invasive measurement of the core temperature of a Li-ion cell using a recently developed technique that utilizes space and time integrals of the measured temperature field at the outside surface. The surface temperature field of an operating Li-ion cell is measured using infrared thermography at multiple discharge rates up to 10C, using which, the core temperature is predicted as a function of time. In each case, there is excellent agreement throughout the discharge period between the predicted core temperature and measurements from a thermocouple embedded at the core of the cell. These measurements quantify the temperature gradient within the cell, which is particularly high at large discharge rates. This paper may result © 2016. This manuscript version is made available under the Elsevier user license http://www.elsevier.com/open-access/userlicense/1.0/ 2 in non-invasive core measurement methods that may contribute towards performance optimization and improved safety of Li-ion cells.

While temperature on the surface of a heat-generating solid body can be easily measured using a variety of methods, very few techniques exist for non-invasively measuring the temperature inside the solid body as a function of time. Measurement of internal temperature is very desirable since measurement of just the surface temperature gives no indication of temperature inside the body, and system performance and safety is governed primarily by the highest temperature, encountered usually at the core of the body. This paper presents a technique to non-invasively determine the internal temperature based on the theoretical relationship between the core temperature and surface temperature distribution on the outside of a heat-generating solid body as functions of time. Experiments using infrared thermography of the outside surface of a thermal test cell in a variety of heating and cooling conditions demonstrate good agreement of the predicted core temperature as a function of time with actual core temperature measurement using an embedded thermocouple. This paper demonstrates a capability to thermally probe inside solid bodies in a non-invasive fashion. This directly benefits the accurate performance prediction and control of a variety of engineering systems where the time-varying core temperature plays a key role.

Accurate measurement of temperature is critical for understanding thermal behavior and monitoring safety and performance of engineering systems involving heating and cooling. While a number of methods are available for measurement of temperature on the outside surface of solid bodies, there is a lack of contactless, non-invasive methods for determining temperature inside solid bodies. Development of such methods is likely to impact a wide range of engineering systems. This paper describes and validates a method for measurement of internal temperature of a solid body based on measurement of the temperature distribution on its outside surface. A theoretical model is developed for determining the core temperature of a cylinder based on surface temperature measurement. This method is validated by determining the core temperature of a thermal test cell using infrared temperature measurement on the surface, and comparing with measurements from an embedded thermocouple. The two measurements are found to agree well with each other in a variety of heat generation and cooling conditions. While this validation is presented for a cylindrical body, the method lends itself easily to bodies of other shapes. This work contributes towards fundamental thermal metrology, with possible applications in a wide variety of engineering systems.

Li-ion cells offer excellent energy storage and conversion characteristics, but also suffer from performance and safety problems related to overheating due to insufficient heat removal during operation. Traditional thermal management approaches cool the cell at its outer surface, whereas it is more critical to cool the core of the cell where heat accumulation occurs. This paper investigates thermal performance of a 26650 Li-ion cell with a heat pipe inserted into the core. Heat pipe integrated cells are assembled starting from unfilled, unsealed cells. Thermal benefit of heat pipe insertion is characterized at a number of discharge rates. Advantages of heat pipe cooling compared to traditional surface-based cooling approach are quantified. It is shown that active cooling of the protruding tip of the heat pipe results in maximum thermal benefit, which is shown to reduce the core temperature to as low as, or even lower than the surface temperature. The heat pipe is shown to reduce temperature rise in case of anomalous increase in heat generation. While heat pipe insertion involves significant manufacturing challenges to ensure long-term reliability, the thermal benefits in doing so may potentially outweigh these challenges, and offer an effective thermal management approach for future Li-ion cell designs.