Comparative Analysis of Unipolar and Bipolar Control of Modular Battery for Thermal and State-of-Charge Balancing

Thermal and state-of-charge imbalance is a well known issue to cause nonuniform ageing in batteries. The modular battery based on cascaded converters is a potential solution to this problem. This paper presents bipolar control (BPC) of a modular battery and compares it with previously proposed unipolar control (UPC) mode in terms of thermal/SOC balancing performance and energy efficiency. The BPC needs four-quadrant operation of full-bridge converter using bipolar pulse-width modulation (PWM) inside each module, whereas UPC only needs half-bridge converter with unipolar PWM. The BPC, unlike UPC, enables charging of some cells while discharging others. An averaged state-space electro-thermal battery model is derived for a convex formulation of the balancing control problem. The control problem is formulated on a constrained LQ form and solved in a model predictive control framework using one-step ahead prediction. The simulation results show that BPC, without even requiring load current variations, gives better balancing performance than UPC, but at the cost of reduced efficiency. The UPC requires at least current direction reversal for acceptable balancing performance. In short, the UPC is a more cost and energy efficient solution for EV and PHEV applications whereas the BPC can be beneficial in applications involving load cycles with high current pulses of long duration

On Thermal and State-of-Charge Balancing using Cascaded Multi-level Converters

In this study, the simultaneous use of a multi-level converter (MLC) as a DC-motor drive and as an active battery cell balancer is investigated. MLCs allow each battery cell in a battery pack to be independently switched on and off, thereby enabling the potential non-uniform use of battery cells. By exploiting this property and the brake regeneration phases in the drive cycle, MLCs can balance both the state of charge (SoC) and temperature differences between cells, which are two known causes of battery wear, even without reciprocating the coolant flow inside the pack. The optimal control policy (OP) that considers both battery pack temperature and SoC dynamics is studied in detail based on the assumption that information on the state of each cell, the schedule of reciprocating air flow and the future driving profile are perfectly known. Results show that OP provides significant reductions in temperature and in SoC deviations compared with the uniform use of all cells even with uni-directional coolant flow. Thus, reciprocating coolant flow is a redundant function for a MLC-based cell balancer. A specific contribution of this paper is the derivation of a state-space electro-thermal model of a battery submodule for both uni-directional and reciprocating coolant flows under the switching action of MLC, resulting in OP being derived by the solution of a convex optimization problem

Simultaneous Thermal and State-of-Charge Balancing of Batteries: A Review

The battery pack lifetime is severely affected by the State-of-Charge (SOC) and thermal imbalance among its cells, which is inevitable in large automotive batteries. In this review paper, the need of simultaneous thermal and SOC balancing is emphasized. Thermal and SOC balancing are two tightly coupled objectives. However, we argue here that it is possible to achieve these simultaneously by using a balancing device that enables the non-uniform use of cells, optimally using the brake regeneration phases and load variations in the drive cycle, and exploiting cell redundancy in the battery pack. The balancer must provide extra degree-of-freedom in control by distributing a large battery pack into smaller units to enable an independent cell/module-level control of a battery system

Feasibility Issues of using Three-Phase Multilevel Converter based Cell Balancer in Battery Management System for xEVs

The use of a three-phase multilevel converter (MLC) as an integrated cell balancer and motor driver is investigated for three-phase AC applications in EVs/HEVs/PHEVs. The paper analyzed an issue of additional battery losses caused by the flow of reactive and/or harmonic power from each power cell of the three-phase MLC battery system. The paper also investigates the size of shunt capacitor required for compensation of the losses to acceptable level. This study concludes that the size of the required capacitor is too big for the vehicle application unless some other active compensation is used as well. Another practical way to employ the MLC as a cell balancer is to use it in a cascaded connection with the conventional three-phase two-level voltage source inverter however it may not be a cost-effective solution either due to high component count

Performance Evaluation of Multilevel Converter based Cell Balancer with Reciprocating Air Flow

The modeling and design of an active battery cell balancing system using Multilevel Converter (MLC) for EV/HEV/PHEV is studied under unidirectional as well as reciprocating air flow. The MLC allows to independently switch ON/OFF each battery cell in a battery pack. The optimal policy (OP ) exploiting this extra degree-of-freedom can achieve both temperature and state-of-charge (SoC) balancing among the cells. The OP is calculated as the solution to a convex optimization problem based on the assumption of perfect state information and future driving. This study has shown that OP gives significant benefit in terms of reduction in temperature and SoC deviations, especially under parameter variations, compared to uniformly using all the cells. It is also shown that using reciprocating flow for OP gives no significant benefit. Thus, reciprocating flow is redundant for MLC-based active cell balancing system when operated using OP

Evaluating the Potential for Cell Balancing Using a Cascaded Multi-Level Converter Using Convex Optimization

The modeling and design of an active battery cell balancing system using Multi- Level Converter (MLC) for EV/HEV/PHEV is studied. The MLC allows to independently switch ON/OFF each battery cell in a battery pack . This extra degree-of-freedom (DoF) can be exploited to optimally use each cell in order to balance among them the temperature and state-of- charge (SoC). This study has shown that the constrained convex optimization based control policy, exploiting the extra DoF of MLC, gives significant benefit in terms of reduction in temperature and SoC deviations, especially under parameter variations, compared to uniformly using all the cells. Thus, the MLC has promising potential to offer extra benefit of achieving cell balancing while being simultaneously used as a motor driver

Electro-thermal Control of Modular Battery using Model Predictive Control with Control Projections

This paper proposes a novel model predictive control algorithm to achieve voltage regulation and simultaneous thermal and SOC balancing of a modular battery using limited future load information. The modular battery is based on multilevel converter (MLC), which provides a large redundancy in voltage synthesis and extra degree-of-freedom in control. The proposed algorithm is based on orthogonal decomposition of controller into two components, one for voltage control and the other for balancing control. The voltage control decisions are made using a simple minimum norm problem whereas the balancing control decisions are made in two stages. The first stage computes a balancing control policy based on an unconstrained LQ problem and the second stage enforces constraint on control actions via projection on a time-varying control constraint polytope. The control algorithm shows promising performance in a simulation study of a four cell modular battery. The performance and the simplicity of the control algorithm make it attractive for real-time implementation in large battery packs

State of Power Prediction for Battery Systems with Parallel-Connected Units

To meet the ever-increasing demand for energy storage and power supply, battery systems are being vastly applied to, e.g., grid-level energy storage and automotive traction electrification. In pursuit of safe, efficient, and cost-effective operation, it is critical to predict the maximum acceptable battery power on the fly, commonly referred to as the battery system’s state of power (SoP). As compared to the SoP prediction at the battery cell level, predicting the SoP of a multi-battery system, especially including parallel-connected cells/modules/packs, is much more complicated and far less investigated. To solve this problem, a system-model-based SoP prediction method is first proposed in this paper. Specifically, based on the formulated system model and generic state-space representation, the challenge of non-monotonic system state evolution, arising from the dynamic parallel current distribution, is identified and systematically addressed by the proposed method. As demonstrated by tests on a battery system set up with experimentally verified parameter values, the proposed method outperforms the commonly applied cell-SoP based methods for providing a more accurate and reliable prediction of the battery system SoP. Moreover, the proposed prediction framework presented in generic forms can be readily applied to other system structures

Role of Surgery in Spinal Metastases

Spinal metastases constitute two-third of all cases of bone metastases. Surgical intervention in these tumours has been recommended in highly selective cases, for confirming diagnosis, stabilizing the spine, and decompressing nerves or spinal cord for alleviation of pain and preservation of function.In this article,the authors have reviewed available evidence on role of surgery in spinal metastases and have presented results of selected papers on this vast topic

Screening of Arabidopsis mutants for functional genomic studies

Eight photosynthetic Arabidopsis mutants were screened for co-segregation of a photosynthetic phenotype with the T-DNA insertion. These mutants were selected from 80 photosynthetic mutants with genetic background of Columbia-0. Two different screening approaches were used to study the T-DNA insertion in the genome of mutant Arabidopsis lines. The sulphonamide sulfadiazine was found to be an effective selective agent and a single copy of sulfonamide resistant gene was found to be completely resistant to the optimal concentration i.e., 5mg mL-1. The maximum number of Arabidopsis mutant plants had confirmed insertions. Some of the plants did not show any amplification with gene specific primer combination, and it was assumed that either they were wild type plants or they had random T-DNA insertion and the insertion was not found in the gene under study but it could be found in any where in the genome. Some mutant plants were morphologically different from the wild type plants e.g., ALP105. These plants grew as small in size and dark green in color. After PCR screening with gene specific and T-DNA border primers all such mutant plants were confirmed as heterozygous T-DNA insertion plants