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Embedded Temperature Instrumentation for Battery Thermal Management
Rechargeable lithium-ion batteries (LIB) are extensively employed to underpin the design of energy storage systems (ESS). They are typically used within the automotive and wider electrical generation sector, due to their relatively high gravimetric energy density, power density and low financial cost. It is widely accepted within both academic literature and the industrial community that the degradation rate of an ESS and its performance are affected by its operating temperature. Equally, the operating temperature can provide significant insight into ESS health and the propagation of defects. Therefore, a Battery Thermal Management System (BTMS) with Real-time temperature monitoring is fundamental for the reliable and safe operation of battery systems.
There has been a recent surge in EV uptake with consumers anticipating and expecting longer ranges, higher performance and exponentially faster charging speeds all of which subject the ESS to additional thermal stresses. The Media’s portrayal of EV accidents often results in apprehension among new EV purchasers regarding ESS safety, and although EV’s are inherently safe, it is important ESS functional safety is continuously enhanced. Within a conventional BTMS, considerable effort, resource and development time are required to ensure that the battery system meets the safety requirements mandated by the target application. “Over engineering” of the system design is often undertaken to address areas of uncertainty in LIB performance and safety.
WMG centre High Value Manufacturing Catapult are investing considerably into the development of smart cell technology capable of accurately measuring thermally induced stress though real-time internal temperature monitoring. Current ESS employ traditional methods of temperature measurement and often rely on fewer than 20 surface data points, even for packs containing many hundreds of cells. Blanket measurements of temperature reduce the BTMS ability to optimise the operation of the battery system. A reliance on sophisticated software functions, coupled with “safety margins” and thermal materials reduces the energy density of the ESS while increasing its cost. The ability to monitor abnormal events, through internal cell instrumentation opens-up new possibilities in LIB diagnostics and prognostics, further improving system safety and increasing life.
Research and the availability of data on ESS operation and capability within the aerospace industry, where cells are subjected to harsh environmental conditions, is limited. As safety and reliability in aerospace application is of paramount importance, it is critical the internal core temperatures of a battery module are known to predict and mitigate excessive degradation or possible component failure, from conceptualisation to deployment.
The Cell Instrumentation team at WMG specialise in embedded battery instrumentation and employ multiple temperature sensing methods to support the advanced thermal characterisation of LIBs for enhanced module performance and safety, including traditional thermocouples, thermistors, and fibre optics. Dr Tim Vincent (CI Team) has developed a novel cost-effective (<$1 per flexible PCB assembly) and scalable solution for internal battery temperature monitoring. By integrating thermistor sensing arrays within 21700 lithium cells and combining with miniaturised interface circuitry, Dr Vincent has developed an on-cell thermal management solution capable of detecting and monitoring the real time formation of “hots spots” within the cell whilst in operation. The ‘Smart Cell’ assembly exploits Powerline-communication (to be covered in a follow-up article) for data transmission, removing the need for additional wiring, reducing system ancillary demands, therefore increasing energy density.
Interface and PLC circuitry incorporated on smart cell PCBInterface and PLC circuitry incorporated on smart cell PCB
Figure 1: Interface and PLC circuitry incorporated on smart cell PCB
Dr Yifei Yu (also CI Team) leads fibre optics sensing at WMG and recently evaluated the use of DFOS (Distributed Fibre Optic Sensing) to assess the external thermal behaviour of LIBs over a wide range of ambient temperatures and electrical loads. Optical fibres are small in format, provide high spatial resolution sensing, and are resilient to electromagnetic and chemical interference providing an ideal solution for in-situ ESS thermal management. The DFOS system in Dr Yu’s study reported cell surface temperatures up to 307% higher than that measured using traditional thermocouples emphasising the requirement for enhanced thermal sensing in ESS. More recently Dr Yu embedded a distributed fibre optics sensor within an A5 pouch cell for real-time measurement of the solid electrolyte interface formation and the structural deformation within the anode.
The CI team recognise the importance of robust and repeatable instrumentation and have developed and rigorously tested and validated a series of gas-tight fittings to support internal sensing activities. The fitting is secured to the negative terminal of the cell forming a strong seal which facilitates internal temperature measurement throughout severe thermal events where significant pressures form within the cell. The team are currently exploring combined internal sensing including electrode potential for advanced cell characterisation.
Further information on these innovations can be found in relevant publications, for example:
The CI team are available for consultation and can support the implementation of embedded thermal sensing for ESS development and optimisation. For further information please contact: James.Marco@warwick.ac.uk
Figure 2: Instrumented smart cell: interior