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

A Powertrain LQR Torque Compensator with Backlash Handling

This paper derives an LQR anti-jerk controller for an automotive driveline. The time derivative of the drive shaft torque, which is closely related to the vehicle jerk, is used as a virtual system output and regulated to zero. Thereby, the controller does not need a reference model for generation of reference trajectories for the control law evaluation. The controller acts as a torque compensator for the driver’s torque demand which the controller output asymptotically follows. The properties of the controller are discussed and the behavior is illustrated by simulation examples and verified with experiments on a heavy duty truck

Proactive control of wind turbine with blade load constraints

This paper describes an easy-to-implement, proactive control strategy for wind turbines, incorporating constraints on blade loads. The control strategy is aimed for rejecting wind gusts and is based on upwind speed measurements and a new statistical wind gust detection mechanism. The control action comprises simultaneous driveline and collective pitch control with constraints on flapwise bending moment. The controller is evaluated by simulation on a transient between two steady-state operational modes of the wind turbine. A new driveline backstepping-based controller with integral action for compensation of steady-state errors is also proposed and verified by simulations

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

Influence of State of Charge Estimation Uncertainty on Energy Management Strategies for Hybrid Electric Vehicles

This paper studies how the optimal energy management of a hybrid electric vehicle and a plug-in hybrid electric vehicle is affected by uncertain estimates of the battery state of charge. A simple model for the battery dynamics and the state of charge estimation is postulated, inspired by the known characteristics of previously proposed estimation schemes. Based on the assumption that the drive cycle is perfectly known, the effects of state of charge estimation uncertainty is studied by including the estimation uncertainty in the optimization of the energy management strategy. The simulations indicate lower battery usage and higher fuel consumption as the estimation uncertainty increases

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

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

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