An IR-based Wireless Optical Communication System for New Generation Battery Pack

Abstract

International audienceThis study shows the design of a low-cost cell-module optical wireless communication system within an electric vehicle battery pack module.The increase in the energy density of the cells is accompanied by increasing constraints on the monitoring of their state. This implies an increase in the number and type of sensors used close to the cells and therefore an increase in the number of connections between the cells' surroundings and the cell monitoring/management system at module level.Wired connections are costly and prone to failure because they are often implemented manually. Monitoring/management systems therefore use distributed sensor systems connected by an isolated wired communication bus of the Daisy Chain type, often offering data rates of up to 1 Mbits.Studies have been carried out on wireless communication systems to replace these Daisy Chains [1]. Some use radio with a simple integration of Bluetooth gateway components [2]. Radio communications can have disadvantages in terms of confidentiality of the data exchanged and resistance to attacks from outside the battery pack.Powerline communication systems have also been proposed. These are sensitive to the noise generated on the power circuit. They are also sensitive to the variation in the impedance of the battery pack, which does not allow the case of advanced battery architectures such as Self Reconfigurable Batteries [3].An optical inter-module communication solution has also been proposed by LION Smart [4]. This solution is based on the same principle as the wired Daisy Chain, i.e. messages are sent back and forth between serial and pack stages. Thus, the data rate is divided as many times as there are communication nodes (one hundred to two hundred times for the case of a battery pack). The proposed solution is based directly on the integration of discrete infrared receiver circuits which do not solve the cost and power constraints.Most of the proposed solutions do not allow the management of events in real time, such as the synchronisation of measurements.This study proposes to investigate the implementation of an optical wireless communication bus between cells and modules that integrates the space, power and cost constraints imposed by this level of integration.The communication bus is based on the principle of open air communication between cells and module by exploiting the empty spaces within the module for light propagation. The environment of a module or even a battery pack offers interesting characteristics in terms of signal-to-noise ratio for light signals. This allows the use of very small and low cost solutions which is an important criterion for cell level integration.The characteristics of the communication bus are asymmetrical in order to take into account the specific constraints of the cell nodes and the module node. In contrast to the cell nodes, the module node is less constrained in terms of compactness and even cost because there are fewer of them in the system.The complexity of receiving an optical message, and therefore its cost and space requirements, increases with the bandwidth used, while the bandwidth of the transmission stage is only slightly affected. Thus, different bandwidths are used for uplink and downlink communications. Module-to-cell downlink communication is limited to a low bandwidth, while cell-to-module communication uses a higher bandwidth.This asymmetry is compensated by another asymmetry in the communication protocols used by the upstream and downstream channels. The slow communication between the module node and the cell nodes is based on periodic broadcast communication. The fast communication between the cell nodes and the module node is based on sequenced point-to-point communication.The broadcast communications from the module node are used as a time reference for the synchronisation of the sequential cell-to-module communications, thus reducing the clock accuracy constraints applied to the cell node sufficiently to avoid the need to use expensive time references such as crystal oscillators.A higher level protocol is then developed. This protocol is based on the request and answer principle. Each request and answer consists of a frame of several bytes including addressing and integrity information (in the form of an error checking code). The module node interrogates the cell nodes with a broadcast request addressing either a specific cell node or all the cell nodes (depending on the address contained in the question frame). The nodes concerned then respond in their respective slot times according to the nature of the request.A prototype of the communication bus that meets the integration constraints of a 12-cell pouch cell battery module has been realized. This prototype consists of 12 cell communication nodes and one module-level communication node. The open air propagation channel for the optical communications is made with a cross-section of 7x21mm over a length of 200mm. Each cell node is spaced at a distance of between 12 and 17mm.The cell communication nodes are implemented with a target component cost of less than $1 per cell (including encoding, transmitting, decoding, receiving, voltage and temperature measurement stages, an MCU monitoring the cell parameters and powering all these elements). The resulting bandwidths are 115.2kbits module to cell and 1.14Mbits cell to module with a minimum request-answers period of 10 ms.Tests of a low cost reflective coating were also carried out on one side of the propagation channel with improvements of around 40% on the amplitude of the optical signal.This work is part of the European Cobra project and will be used to produce 3 automotive modules of 12 cobalt-free cells equipped with advanced near-cell sensors.References[1] S. S. W. S. Samanta, «A Survey of Wireless Battery Management System: Topology, Emerging Trends, and Challenges,» Electronics, vol. 10, n° %118, p. 2193, 2021. [2] J. F. e. al., «Wireless Battery Management System for Safe High-Capacity Energy Storage,» Materials Research Society, San Francisco, CA, United States, 2014.[3] R. T. e. al., «A High Frequency Self-Reconfigurable Battery for Arbitrary Waveform Generation,» World Electric Vehicle Journal, vol. 12, n° %11, p. 8, 2021. [4] Lion SMART, «NEW BATTERY CONCEPT LIGHT BATTERY MODULAR, SAFE AND WIRELESS,» 05 06 2018. [En ligne]. Available: https://lionsmart.com/en/new-battery-concept-light-battery-modular-safe-and-wireless/