Squeeze the Lemon: Balancing as a Way to Use Every Drop of Energy in a Lithium-Ion Battery

This work discusses recent research results obtained in tackling one of the most limiting factor for an effective use of a Lithium-ion battery: the charge unbalance between the cells constituting the battery. First, it is recalled how unbalancing affects the performance of a battery consisting of series-connected cells, then some possible techniques to balance the battery are described and compared to each other. The comparison is made by modeling the balancing circuit topologies and by performing statistical simulations. Finally, we describe two balancing circuits that efficiently address the problem and we report on the experimental results that validate the circuits

Performance comparison of active balancing techniques for lithium-ion batteries

A simple but effective analysis to calculate the performances achievable by a balancing circuit for series-connected lithium-ion batteries (i.e., the time required to equalise the battery and the energy lost during this process) is described in this paper. Starting from the simple passive technique, in which extra energy is dissipated on a shunt resistor, active techniques, aiming at an efficient energy transfer between battery cells, are investigated. The basic idea is to consider the balancing circuit as a DC/DC converter capable of transferring energy between its input and output with a certain efficiency and speed. As the input and output of the converter can be either a single cell or the entire battery pack, four main active topologies are identified: cell to cell, cell to pack, pack to cell and cell to/from pack (i.e., the combination of the cell to pack and pack to cell topologies when the converter is bidirectional). The different topologies are compared by means of statistical simulations. They clearly show that the cell to cell topology is the quickest and most efficient one. Moreover, the pack to cell topology is the least effective one and surprisingly dissipates more energy than the passive technique, if the converter efficiency is below 50%

Investigation of series-parallel connections of multi-module batteries for electrified vehicles

Large-format Lithium-ion battery packs consist of the series and parallel connection of elemental cells, usually assembled into modules. The required voltage and capacity of the battery pack can be reached by various configurations of the elemental cells or modules. It is thus worth investigating if different configurations lead to different performance of the battery pack in presence of a mismatch in the cell characteristics. A simulation tool is developed in this work and applied to a battery pack consisting of standard 12 V modules connected with various serial/parallel topologies. The results show that battery configurations with modules directly connected in parallel and then assembled in series are more robust against variation of the cell capacity through the battery. Moreover, given the cells and the battery configuration, we show that changing the position of the cells has a significant impact on the usable capacity of the battery

On the Sizing of the DC-Link Capacitor to Increase the Power Transfer in a Series-Series Inductive Resonant Wireless Charging Station

Wireless inductive-coupled power transfer is a very appealing technique for the battery recharge of autonomous devices like surveillance drones. The charger design mainly focuses on lightness and fast-charging to improve the drone mission times and reduce the no-flight gaps. The charger secondary circuit mounted on the drone generally consists of a full-bridge rectifier and a second-order filter. The filter cut-off frequency is usually chosen to make the rectifier output voltage constant and so that the battery is charged with continuous quantities. Previous works showed that an increase in power transfer is achieved, if compared to the traditional case, when the second-order filter resonant frequency is close to the double of the wireless charger excitation and the filter works in resonance. This work demonstrates that the condition of resonance is necessary but not sufficient to achieve the power increment. The bridge rectifier diodes must work in discontinuous-mode to improve the power transfer. The paper also investigates the dependence of the power transfer increase on the wireless excitation frequency. It is found the minimum frequency value below which the power transfer gain is not possible. This frequency transition point is calculated, and it is shown that the gain in power transfer is obtained for any battery when its equivalent circuit parameters are known. LTSpice simulations demonstrate that the transferred power can be incremented of around 30%, if compared to the case in which the rectifier works in continuous mode. This achievement is obtained by following the design recommendations proposed at the end of the paper, which trade off the gain in power transfer and the amplitude of the oscillating components of the wireless charger output

Hardware-in-the-Loop Platform for Assessing Battery State Estimators in Electric Vehicles

The development of new algorithms for the management and state estimation of lithiumion batteries requires their verification and performance assessment using different approaches and tools. This paper aims at presenting an advanced hardware in the loop platform which uses an accurate model of the battery to test the functionalities of battery management systems (BMSs) in electric vehicles. The developed platform sends the simulated battery data directly to the BMS under test via a communication link, ensuring the safety of the tests. As a case study, the platform has been used to test two promising battery state estimators, the Adaptive Mix Algorithm and the Dual Extended Kalman Filter, implemented on a field-programmable gate array based BMS. Results show the importance of the assessment of these algorithms under different load profiles and conditions of the battery, thus highlighting the capabilities of the proposed platform to simulate many different situations in which the estimators will work in the target application

System on chip battery state estimator: E-bike case study

This paper discusses the hardware implementation and experimental validation of a model-based battery state estimator. The model parameters are identified online using the moving window least squares method. The estimator is implemented in a field programmable gate array device as a hardware block, which interacts with the embedded processor to form a system on a chip battery management system (BMS). As a case study, the BMS is applied to the battery pack of an e-bike. Road tests show that the implemented estimator may provide very good performance in terms of maximum and rms estimation errors. This work also proposes a new methodology to assess the performance of a battery state estimator

Performance measurements of energy storage systems and control strategies in real-world e-bikes

The paper presents a measurement campaign (electrical, thermal and user comfort) for the performance characterization of energy storage systems in real-world electric bicycles. Specific sensors were added to characterize three vehicles which differ for electric motor, energy storage system size and control strategies. The controller can implement energy recovery strategies when braking and change the level of electric assistance depending on the desired trade-off between the comfort of the driver and the battery duration. Experimental results show that a control strategy aiming at preserving the SOC (State-Of-Charge), together with regenerative braking, can ensure very long battery duration with no need of recharge. The SOC is kept at about 50% for a long period. Instead, control strategies optimizing the full comfort of the driver by maximizing the level of assistance can ensure real-world e-bicycle missions of about 2 h and 40 km, when the SOC of the battery drops down from 95% to 5%

Experimental Analysis of Open-Circuit Voltage Hysteresis in Lithium-Iron-Phosphate Batteries

This paper aims at investigating and modelling the hysteresis in the relationship between state-of-charge and open-circuit voltage of lithium-iron-phosphate batteries. A first-order charge relaxation equation was used to describe the hysteresis dynamics. This equation was translated into a voltage-controlled voltage source and included within an equivalent electric circuit of the battery used in online state-of-charge estimators. The effectiveness of the obtained battery model was verified comparing simulated and experimental data

Smart LiFePO4 battery modules in a fast charge application for local public transportation

This paper describes the research effort jointly carried out by the University of Pisa and ENEA on electrochemical energy storage systems based on Lithium-ion batteries, particularly the Lithium-Iron-Phosphate cells. In more detail, the paper first illustrates the design and experimental characterization of a family of 12 V modules, each of them provided with an electronic management system, to be used for electric traction. Then, the sizing of the energy storage system for an electric bus providing a service with 'fast and frequent' charge phases is described

Design of the battery management system of LiFePO4 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