Digital Data Transmission System with Capacitive Coupling for In-situ Temperature Sensing in Lithium Ion Cells Nora Martinya,∗, Thomas M¨uhlbauera, Sebastian Steinhorsta, Martin Lukasiewycza, Andreas Jossenb aTUM CREATE, 1 CREATE Way, 10-02, CREATE Tower, Singapore 138602 bTechnische Universit¨atM¨unchen,Lehrstuhl f¨urElektrische Energiespeichertechnik, Arcisstr. 21, 80333 M¨unchen Abstract Precise temperature measurement within a narrow time frame is crucial for the safe operation of Li-Ion battery cells. This paper proposes a digital approach for in-situ cell temperature measurement, relying on capacitive coupled data transmission between the controller in a cell and an external controller. In contrast to purely analog solutions, the digital approach satisfies all requirements in terms of electrochemical, space and transmission quality. For this purpose, analog and digital conceptual hardware setups were implemented for comparison and extensive measurements were carried out. The experimental results for the digital implementation show the influence of capacitor sizes and baud rates, providing a guideline for transmission rates for different sizes of cells. The paper also discusses in detail integration aspects for the digital approach. From the digital concept setup, valuable information is deduced regarding the communication latency, achievable energy consumption and miniaturization potential: Depending on the employed sampling rate, the transmission duration for one message with five temperature values can be reduced to as little as 65 µs with a power consumption in the µW range. The proposed miniaturization enables an efficient, low cost, fully integrated logic circuit for data collection and transmission with a size not bigger than 2 mm 3 mm. × Keywords: Wireless data transmission, capacitive coupling, lithium ion pouch cell, digital data, temperature monitoring, in-situ sensing 1. Introduction considerable inhomogeneities within the temperature dis- tribution have to be faced. Arunachala presented in [2] Lithium Ion (Li-Ion) batteries are utilized increasingly in 25 simulation results of cells with different sizes and cooling everyday applications from mobile devices over power tools strategies and showed that temperature gradients can be up to electric cars. Compared with other chemistries, the above 10 K in large-scale automotive cells. To find optimal 5 success of Li-Ion batteries has its foundation in the com- operating conditions and to detect early failures, perma- bined high energy and power density as well as high volt- nent, space resolved and reliable temperature monitoring age, enhanced lifetime, and the absence of severe mem- 30 is therefore inevitable. ory effects. To ensure the operation within their prede- In most current applications, monitoring is performed fined bounds in terms of voltage and temperature, Li-Ion by attaching sensors to the surface of battery cells, see [3]. 10 batteries require sophisticated Battery Management Sys- However, this approach has severe drawbacks, particularly tem (BMS). With a well-designed and precise BMS, life- in large scale applications where active cooling is applied. time of batteries is optimized while safety risks are mini- 35 In this scenario, battery cells are actively cooled from their mized. As a result, the demand for safe and long-lasting surface either using cold air or liquid. The resulting tem- operation strategies is satisfied [1]. perature discrepancy between the cell surface and its in- 15 One crucial factor when dealing with Li-Ion batteries terior might become significant [2], especially in the event is the temperature. Higher overall temperatures in the cell of a thermal runaway [4]. Consequently, the delay in time mainly lead to an accelerated aging, resulting from a loss 40 response when detecting the actual cell temperature may of capacity and a rise in internal resistance. On the other result in an inappropriate battery management when only hand, an internal cell temperature exceeding the desig- the surface temperature is monitored. Therefore, an in- 20 nated range during cycling may be an early indicator for situ temperature monitoring as described in [5], [6] and [7] safety-relevant failures. With the emerging trend of in- is of great value. creasing cell sizes, particularly in automotive applications, 45 As mentioned above, space resolved temperature mon- itoring is necessary in order to observe the State-of-Safety (SoS) at any time, particularly for large cells. However, the ∗Corresponding author Email address: [email protected] (Nora Martiny) collection of a large amount of sensor data is challenging when applying in-situ sensing, see [5, 8, 9]. For wire-based Preprint submitted to Journal of Energy Storage, DOI:10.1016/j.est.2015.10.001 October 15, 2015 50 data transmission systems, leakage at the sealant becomes 2. Related Work a severe issue, resulting in long-term instability of cells. To eliminate these problems, the paper at hand proposes Due to the growing demand for Li-Ion batteries and data transfer without additional feed through wiring from their safe and reliable operation [11, 12], the estimation of multiple sensors in the cell. 105 the internal temperature of Li-Ion cells is a focus interest of 55 In [5, 8, 9], thin-film thermocouples are implemented ongoing research. Several papers use internal cell parame- as simple and reliable solution. Since these sensors require ters like the impedance spectrum to estimate the tempera- a reference temperature at the cold junction, they are not ture, see [13, 14]. Other research considers the integration suitable for fully integrated solutions. Therefore, resis- of sensors physically into the cells [5, 6, 7, 8, 9, 15]. Both tance based sensors have been applied in [10] to measure110 methods, however, have their specific drawbacks as out- 60 the cell temperature at different points and read the data lined in the following. out via capacitive coupling. However, an efficient imple- While using internal parameters has the advantage of mentation of the proposed data transfer scheme is prohib- being non-invasive and in result non-influential on the cell ited due to the high practical complexity. As a remedy, behavior, so far no possibility has been found to estimate the paper at hand proposes a fully digitized solution. 115 inhomogeneities and temperature hotspots with these meth- 65 Contributions of the paper. This paper proposes a ods. This means, that only an average temperature of the fully integrated digital solution for in-situ temperature cell can be detected and safety hazards are hardly recog- sensing in Li-Ion cells. As a result, it overcomes all draw- nizable. backs of existing data-transmission solutions for in-situ Placing physical sensors into the cells solves these prob- measurements and allows for an efficient integration on120 lems and provides the possibility of monitoring the tem- 70 a large scale. perature distribution in a cell more accurately [8, 9]. Most In Section 2, related work in the domain of Li-Ion and of the present projects, however, use thermocouples for in-situ temperature sensing is discussed. Particularly, lim- temperature measurements [5, 6, 8, 9] that require a ref- itations of previous work as well as resulting challenges erence temperature at the cold junction of the sensor in are pointed out. A more detailed discussion on in-situ125 order to estimate absolute temperature values. A feed 75 temperature sensing is given in Section 3, deducing the through the cell casing is mandatory which can lead to requirements for the proposed solution. leakage [8, 9, 10]. The proposed approach for a fully integrated digital In [7, 10, 15], resistance based sensors are introduced solution and the setup of a proof of concept is described to overcome the drawbacks of requiring a reference tem- in Section 4. This solution has several benefits: 130 perature. They provide in principle the possibility of en- capsulated temperature measurements in the cell if data 80 The delay in time response when detecting the actual transmission can be performed wirelessly. • cell temperature is reduced to a minimum. [16] proposes to move the monitoring and control hard- ware of the lowest layer of a hierarchical battery manage- Space resolved temperature monitoring becomes pos- • 135 ment system closer to the cells. It is suggested that sensing sible with the used data transmission protocol. elements for temperature, current, voltage, impedance and A fully integrated solution of the described hardware some other cell parameters along with basic cell balanc- • 85 might be produced at very low cost. ing systems, data processing and the data communication system are integrated into one macro-cell. The macro-cell It furthermore integrates with the emerging technology140 consists of four pouch cells, connected in series. of smart cells [1] where the battery management is dis- [17] suggests, similiar to the paper at hand, to use a ca- tributed onto controllers which are directly attached to pacitive data communication for battery cell monitoring. the batteries. The proposed monitoring system provides one tempera- 90 Section 5 discusses solutions for the integration as well ture sensor, that, along with other monitoring electronics as the potential towards miniaturization of specific compo-145 is attached to the surface of a cell. The capacitive data nents. Performed experiments and their results are elab- transmission is used to send data from each cell in a mod- orated and the robustness of the proposed system is in- ule to the common BMS. By contrast, the novelty in the vestigated. Furthermore, the energy consumption of the paper at hand is that the described communication system 95 solution is determined and strategies to keep it at mini- is designed such that it can monitor multiple in-situ tem- mum are discussed. 150 perature sensors and communicate data wirelessly through Experiments that show the influence of capacitor size the cell casing. and baud rate on the signal quality and the reliability of In [10], different methods for data transmission are in- the data transmission are presented in Section 6.