Gabriele Fantechi

Batteries and battery management systems for electric vehicles

The battery is a fundamental component of electric vehicles, which represent a step forward towards sustainable mobility. Lithium chemistry is now acknowledged as the technology of choice for energy storage in electric vehicles. However, several research points are still open. They include the best choice of the cell materials and the development of electronic circuits and algorithms for a more effective battery utilization. This paper initially reviews the most interesting modeling approaches for predicting the battery performance and discusses the demanding requirements and standards that apply to ICs and systems for battery management. Then, a general and flexible architecture for battery management implementation and the main techniques for state-of-charge estimation and charge balancing are reported. Finally, we describe the design and implementation of an innovative BMS, which incorporates an almost fully-integrated active charge equalizer.

High-Efficiency Digitally Controlled Charge Equalizer for Series-Connected Cells Based on Switching Converter and Super-Capacitor

The charge stored in series-connected lithium batteries needs to be well equalized between the elements of the series. We present here an innovative lithium-battery cell-to-cell active equalizer capable of moving charge between series-connected cells using a super-capacitor as an energy tank. The system temporarily stores the charge drawn from a cell in the super-capacitor, then the charge is moved into another cell without wasting energy as it happens in passive equalization. The architecture of the system which employs a digitally-controlled switching converter is compared with the state of the art, then fully investigated, together with the methodology used in its design. The performance of the system is described by presenting and discussing the experimental results of laboratory tests. The most innovative and attractive aspect of the proposed system is its very high efficiency, which is over 90%.

Enhanced model for Lithium-Polymer cells including temperature effects

An accurate model of the elementary accumulation device is fundamental for sizing and controlling the battery pack to be used in electric and hybrid vehicles. Indeed, the implementation of such a model within the Battery Management System makes it possible to evaluate the status and the behavior of the battery pack in every condition and to apply a correct control strategy. This work shows the characterization and modeling of a commercial Lithium-Polymer cell, which properly considers thermal effects on cell behavior. The specific designed thermostatic chamber is described and the experimental results are presented and compared to those simulated with the developed model.

Wireless Sensor Node for Surface Seawater Density Measurements

An electronic meter to measure surface seawater density is presented. It is based on the measurement of the difference in displacements of a surface level probe and a weighted float, which according to Archimedes’ law depends on the density of the water. The displacements are simultaneously measured using a high-accuracy magnetostrictive sensor, to which a custom electronic board provides a wireless connection and power supply so that it can become part of a wireless sensor network. The electronics are designed so that different kinds of wireless networks can be used, by simply changing the wireless module and the relevant firmware of the microcontroller. Lastly, laboratory and at-sea tests are presented and discussed in order to highlight the functionality and the performance of a prototype of the wireless density meter node in a Bluetooth radio network. The experimental results show a good agreement of the values of the calculated density compared to reference hydrometer readings.

Design of a module switch for battery pack reconfiguration in high-power applications

This paper presents a novel approach for battery pack reconfiguration in high-power applications, where many high capacity elementary cells are series-connected to meet the power and energy requirements of the application. The novelty of the approach is that reconfiguration is applied at the module level rather than at the cell level, being a module a segment of the battery pack. A very good trade-off between reconfiguration flexibility and complexity of the implementation is achieved. A key point of the proposed architecture is the module switch, which must be able to connect or bypass a battery module by sustaining high currents without degrading the overall battery efficiency. The design of the module switch is presented, together with an implementation suitable for the battery pack of a hybrid fuel-cell small van. Experimental results show that the switch operates with continuous battery currents up to 160 A. To our knowledge, these experimental results are the first published on battery pack reconfiguration in high-power applications.

Hierarchical platform for monitoring, managing and charge balancing of LiPo batteries

One of the key aspects which determines the performance of a Lithium battery pack used for traction on electric vehicles is how the charge is equally distributed among the cells. Indeed, charge balancing is one of the main function that a Battery Management System (BMS) has to perform. Even though the study of balancing systems is currently considered a hot topic, there are no common procedures and metrics to evaluate their performance. A three-level hierarchy BMS with test benching features was designed for this purpose and it is discussed in this article. It is able to provide valuable comparisons and information about the efficiency and the balancing time of different circuitries, when used together with an ad-hoc defined test procedure. Some experiments carried out on different passive and active balancing circuits are presented and discussed.

Intelligent cell gauge for a hierarchical battery management system

Continuous monitoring of cell parameters (voltage and temperature) is fundamental for Lithium batteries, particularly when many cells are connected in series. It is also important to keep trace of particular events occurring during the life of the cell, preventing data losses if the cell is disconnected from the original battery management system for maintenance purpose or to be used in another battery in a second life market perspective. We present in this work a gauge that enables the cell monitoring and data storage in a non-volatile memory. The gauge is embedded in the cell and performs cell voltage and temperature measurements. It communicates with a battery management system through an isolated interface, so that multiple gauges, used on series-connected cells, can share a common bus without voltage conflicts.

Effective modeling of temperature effects on lithium polymer cells

The availability of an accurate model capable of simulating the dynamic behavior of a cell is important for sizing and controlling the battery for electric and hybrid vehicle. Moreover, including the thermal behavior of cell into the model is very useful when designing the battery pack layout and its cooling system. This work shows the characterization and modeling of a commercial Lithium-Polymer Cell. Particular attention has been paid to consider the thermal effects on the model parameters. The developed model has been used to predict the cell behavior in different operating conditions and the simulated data has been compared to equivalent experimental ones.

Design of the battery management system of LiFePO 4 batteries for electric off-road vehicles

This paper describes the design of a modular battery management system for electric off-road vehicles, where lithiumion batteries are expected to be widely used. A massive electrification of off-road vehicles can be enabled by the availability of a standard battery module, provided with an effective management unit. The design and some preliminary experimental results of the module management unit are discussed in this paper. The unit contains a high current active equalizer that enables the dynamic charge equalization among cells and maximizes the usable capacity of the battery.

Hardware building blocks of a hierarchical battery management system for a fuel cell HEV

The architecture of a hierarchical Battery Management System is presented focusing on the design of its hardware building blocks. The system is intended to be used in a fuel-cell powered van and the design is optimized for large lithium battery packs. The developed BMS provides monitoring and protection functions, together with a high efficiency cell equalization circuit that can selectively adjust the State of Charge of each cell of the pack. A preliminary experimental validation of some building blocks is presented and measurement results are discussed.