In-situ instrumentation of cells and power line communication data acquisition towards smart cell development

The internal core temperature of cells is required to create accurate cell models and understand cell performance within a module. Pack cooling concepts often trade off temperature uniformity, vs cost/weight and complexity. Poor thermal management systems can lead to accelerated cell degradation, and unbalanced ageing. To provide core temperature an internal array of 7 thermistors was constructed; these in conjunction with cell current, via bus bar mounted sensors, and voltage sensor measurements, we have developed instrumented cells. These cells are also equipped with power line communication (PLC) circuitry, forming smart cells. We report upon data from these miniature sensors during cell cycling, demonstrating successful operation of the PLC system (zero errors compared to a reference wired connection) during typical cell cycling (C/2 discharge, C/3 charge) and the application of automotive drive cycle, providing a transient current test profile. Temperature variation within the cell of approximately 1.2 °C gradients, and variation of >2.8 °C during just 30 min of 2C discharging demonstrate the need for internal sensing and monitoring throughout the lifetime of a cell. Our cycling experimental data, along with thorough cell performance tracking, where typically <0.5% degradation was found following instrumentation process, demonstrate the success of our novel prototype smart cells

Development of an in-vehicle power line communication network with in-situ instrumented smart cells

Instrumented cells, equipped with miniature sensors, are proposed to aid the next stage of electrification in the automotive and aerospace industries. To optimize the energy density available within a lithium ion (li-ion) pack we demonstrate how a power line communication (PLC) network can be formed at an individual cell level. This reduces the need for complex communication cables within a vehicle wiring loom. Here we show a unique prototype smart cell (instrumented cell equipped with interface circuitry and processing capability) can be connected via a PLC network, to enable monitoring of vital parameters (temperature, voltage, current), regardless of cell state of charge (2.5 V to 4.2 V DC operating voltage). In this proof-of-concept study, we show the reliable system (0 errors detected over ∼24 hr experiment, acquired data (logged at 10 Hz) from cells (in a parallel configuration), and comparative data for cell internal and external temperature was recorded. During a prolonged discharge (1C, 5A discharge) a peak core temperature >3 °C hotter than surface temperature was observed, highlighting the need to understand cell operation in cooling system design

Battery cell temperature sensing towards smart sodium-ion cells for energy storage applications

Battery cell instrumentation (e.g., temperature, voltage and current sensing) is vital to understand performance and to develop/contrast different cell designs and chemistries. Sodiumion batteries (NIBs) are emerging as an alternative solution to lithium-ion (LIB) technology, particularly in the field of grid energy storage. The relative abundancy of sodium (Na) and superior charge/discharge capability, fuel the development effort to match the desirable energy density properties of LIBs. Internal temperature sensing is of particular value during cell development, offering insights into hot spots and manufacturing defects, in-advance of detection via voltage or surface temperature measurement. We developed novel thermistor arrays (7x miniature sensors) inserted into the core of a 21700 format LIB via flexible PCBs. These arrays were protected using a covering tube, and successfully provided temperature measurements throughout an ageing experiment consisting of 100 cycles (1C charge, 0.3C discharge). For the first time, we report on our performance tests prior to this ageing study (capacity, internal resistance) to highlight the instrumented cells show comparable degradation (∼5 %) to an unmodified cell. We extend this study by verifying that our scalable low-cost solution to sensor protection can be migrated to NIBs. The resilience of the protected PCBs to electrolyte was tested via a longer-term test (preliminary results from a 90-day study are reported here) submerged within the solution. The findings offer a promising outlook to lower-cost cell instrumentation and will provide a tool to optimize these novel cell chemistries

Global thermal image of cylindrical 21700 Li-ion batteries with distributed optical fibre sensor

The ability to monitor the thermal behaviour of lithium-ion batteries (LIB) is an essential pre-requisite to optimise performance and ensure safe operation. However, traditional point measurement (thermocouples) faces challenges in accurately characterising LIB behaviour and notably in defining the hotspot and the magnitude and direction of the thermal gradient. To address these issues, an optical-frequency-domain-reflectometer (OFDR) based distributed-optical-fibre-sensor has been employed to quantify the heat generation within a cylindrical 21700 LIB. A 3 mm spatial resolution within the optical sensor is realised. The optical fibre has been wound around the cell surface for over 1300 unique measurement locations; distributed around the circumference and axially along the LIB. Distributed measurements show the maximum thermal difference can reach 8.37 °C during a 1.5C discharge, while the point-like sensors have 4.31 °C thermal difference. While a temperature gradient along the cell axial length is well understood, for the first time, this research quantifies the temperature variations along the circumference of the cell. The global thermal image highlights heat generation is accumulated around the positive current tab, implying that a fundamental knowledge of internal LIB structure is required when defining sensor placement within the traditional characterisation experiments and deployment within the battery management system (BMS)

Analysis of internal temperature variations of lithium-ion batteries during fast charging

One of the major challenges that limits the fast charging of Lithium-ion batteries is Lithium (Li) plating at low temperatures. To reduce Li-plating an increased environmental temperature is commonly used. However, the uncertainties in the measurement of key battery internal states such as temperature, is a limiting factor to find the best fast charging profile that considers battery performance, degradation, and safety of the electric vehicles (EVs). We have used our state-of-the-art instrumented cells equipped with internal data acquisition and microcontroller, forming smart cells, that enable sensor data to be transmitted via a USB to a data logger. We demonstrate here that commercially available 21700 format cells were successfully instrumented and gave direct information on internal temperature for continuous fast charging rates from C/2 to 2.5C. The internal temperature was found to be considerably higher than that of the surface of the cell (between 10 and 14°C at 2.5C charge rate). A gradient of up to 2°C was found between the positive and negative end of each cell that became more prominent for higher charge rates. Li-plating was detected for all C-rates below 25°C even though, the internal temperature rose above 30°C when the cells were charged at 2.5C with an ambient temperature of 0°C. At a higher ambient temperature of 40°C, the cell’s internal temperature rose (to ~62°C) beyond the safe limits defined by the manufacturer’s datasheet whilst the external temperature recorded (~52°C) was within the manufacturer’s defined safe operating limits

A compatibility study of protective coatings for temperature sensor integration into sodium-ion battery cells

Instrumented battery cells (i.e. those containing sensors) and smart cells (with integrated control and communication circuitry) are essential for the development of the next-generation battery technologies, such as Sodium-ion Batteries (SIBs). The mapping and monitoring of parameters, for example the quantification of temperature gradients, helps improve cell designs and optimise management systems. Integrated sensors must be protected against the harsh cell electrolytic environment. State-of-the-art coatings include the use of Parylene polymer (our reference case). We applied three new types of coatings (acrylic, polyurethane and epoxy based) to thermistor arrays mounted on flexible printed circuit board (PCBs). We systematically analyse the coatings: (i) PCB submersion within electrolyte vials (8 weeks); (ii) analysis of sample inserted into coin cell; (iii) analysis of sensor and cell performance data for 1Ah pouch SIBs. Sodium-based liquid electrolyte was selected, consisting of a 1 M solution of sodium hexafluorophosphate (NaPF6) dissolved in a mixture of ethylene carbonate and diethylene carbonate in a ratio of 3:7 (v/v%). Our novel experiments revealed that the epoxy based coated sensors offered reliable temperature measurements; superior performance observed compared to the Parylene sensors (erroneous results from one sample were reported, under 5 d submersed in electrolyte). Nuclear magnetic resonance (NMR) spectroscopy revealed in the case of most coatings tested, formation of additional species occurred during exposure to the different coatings applied to the PCBs. The epoxy-based coating demonstrated resilience to the electrolytic-environment, as well as minimal effect on cell performance (capacity degradation compared to unmodified-reference, within 2% for the coin cell, and within 3.4% for pouch cell). The unique methodology detailed in this work allows sensor coatings to be trialled in a realistic and repeatable cell environment. This study demonstrated for the first time that this epoxy-based coating enables scalable, affordable, and resilient sensors to be integrated towards next-generation Smart SIBs

A smart cell monitoring system based on power line communication—optimization of instrumentation and acquisition for smart battery management

Energy density of current generation battery packs is insufficient for next generation electric vehicles nor the electrification of the aerospace industry. Currently, approximately a third of energy density is lost due to ancillary demands (e.g., cooling and instrumentation) within a pack, relative to cell energy density. Smart cells, instrumented cells with sensors and circuitry, offer a means to monitor cell performance (e.g. temperature, voltage, current data). Uniquely here we demonstrate our 21700 cells instrumented with internal thermistor sensing arrays with custom miniature interface circuitry including data acquisition and communication components. This circuitry including a power line communication (PLC) system, enables sensor data to be collected and transmitted to a master controller without requiring additional wiring, and can achieve an excellent <0.005 % message error rate. The control and communication system includes the use of adaptive sampling algorithms (during identified periods of low demand, through temperature and current measurements) the cells transmit data at 0.2 Hz, increasing to 5 Hz (normal operation) or 10 Hz (beyond operating limits). This method was demonstrated via drive cycling and external heating to alert the master controller to abnormal operating conditions (rapidly, to avoid missing key features) while saving 65% volume of data during a 90 minute experiment