M. Muneeb Ur Rehman

Active balancing system for electric vehicles with incorporated low voltage bus

Electric-drive vehicles, including hybrid, plug-in hybrid, and electric vehicles, require a high-voltage (HV) battery pack for propulsion and a low-voltage (LV) dc bus for auxiliary loads. This paper presents an architecture that uses modular dc-dc bypass converters to perform active battery cell balancing and to supply current to auxiliary loads, eliminating the need for a separate HV-to-LV high step-down dc-dc converter. The modular architecture, which achieves continuous balancing of all cells, can be used with an arbitrary number of cells in series, requires no control communication between converters, and naturally shares the auxiliary load current according to the relative state-of-charge (SOC) and capacities of the battery cells. Design and control details are provided for LV low-power dual active bridge (DAB) power converters serving as the bypass converter modules. Furthermore, current sharing is examined and worst-case SOC and current deviations are derived for mismatches in cell capacities, SOCs, and parasitic resistances. Experimental results are presented for a system consisting of 21 series 25 Ah Panasonic lithium-ion NMC battery cells and 21 DAB bypass converters, with combined outputs rated to supply a 650-W auxiliary load.

Modular approach for continuous cell-level balancing to improve performance of large battery packs

Energy storage systems require battery cell balancing circuits to avoid divergence of cell state of charge (SOC). A modular approach based on distributed continuous cell-level control is presented that extends the balancing function to higher level pack performance objectives such as improving power capability and increasing pack lifetime. This is achieved by adding DC-DC converters in parallel with cells and using state estimation and control to autonomously bias individual cell SOC and SOC range, forcing healthier cells to be cycled deeper than weaker cells. The result is a pack with improved degradation characteristics and extended lifetime. The modular architecture and control concepts are developed and hardware results are demonstrated for a 91.2 Wh battery pack consisting of four series li-ion battery cells and four dual active bridge (DAB) bypass DC-DC converters.

Active Balancing System for Electric Vehicles With Incorporated Low-Voltage Bus

Electric-drive vehicles, including hybrid (HEV), plug-in hybrid (PHEV) and electric vehicles (EV), require a high-voltage (HV) battery pack for propulsion, and a low-voltage (LV) dc bus for auxiliary loads. This paper presents an architecture that uses modular dc-dc bypass converters to perform active battery cell balancing and to supply current to auxiliary loads, eliminating the need for a separate HV-to-LV high step-down dc-dc converter. The modular architecture, which achieves continuous balancing of all cells, can be used with an arbitrary number of cells in series, requires no control communications between converters, and naturally shares the auxiliary load current according to the relative state-of-charge (SOC) and capacities of the battery cells. Design and control details are provided for low-voltage, low-power dual active bridge (DAB) power converters serving as bypass converter modules. Experimental results are presented for a system consisting of two series 3.6 Ah NMC battery cells and two DAB bypass converters, with combined outputs rated to supply a 12 V, 35 W auxiliary load.

Control of a series-input, parallel-output cell balancing system for electric vehicle battery packs

Cell balancing circuits are required in battery packs to equalize the state-of-charge (SOC) of series-connected cells. This paper presents a control strategy for a modular cell balancing architecture based on low-voltage bypass dc-dc converters that perform real-time active cell balancing using a shared low-voltage (LV) bus. Each bypass converter includes an autonomous controller, which employs a faster loop with a standard PI compensator to regulate the LV bus voltage and a slower loop with droop control to regulate SOC of each cell. The control strategy results in balanced LV load current sharing among bypass converters. Design and stability analysis details are provided for the cell balancing system and bypass converters. The control approach is verified by experimental results for a three series-connected Li-ion NMC battery cell system with three digitally controlled dual-active bridge bypass converters.

Assessment of harmonic pollution by LED lamps in power systems

Modern energy-efficient LED lamps draw non-sinusoidal currents from the grid. The harmonic distortion level varies with the quality of lamps non-linear driver circuit. This paper studies the problem of system-wide harmonic impact with consideration to both the variation in lighting load and difference in quality of LED lamps available in the market. An electrical classification model is proposed for LED lamps based on their harmonic spectrum and an analysis is carried out to quantify the distortion and losses possible in power system if such lamps are to be installed. Extending prior work in this area, we use advanced harmonic analysis techniques and conduct evaluation with voltage-dependent harmonic current model for LED driver. We built a harmonic analysis tool using Gauss-Seidel Method to carry out our evaluations on IEEE-30 test bus system. We report our results using IEEE standard 1459 which provides definitions of power quantities under non-sinusoidal conditions. Evaluations show that poor quality LED lamps can cause significant distortion in a power system and good quality LED lamps introduce significantly lower distortion while providing high economic and environmental benefits.

State-of-charge estimation based on microcontroller-implemented sigma-point Kalman filter in a modular cell balancing system for Lithium-Ion battery packs

Cell balancing in large battery packs requires accurate state of charge (SOC) estimation for individual cells. This paper presents a low complexity sigma-point Kalman filter to estimate the state-of-charge (SOC) of Lithium-Ion battery cells. The proposed sigma-point Kalman filter is of 1st order, and can be easily implemented on a simple microcontroller around a dc-dc converter in a modular cell balancing system. The approach is verified experimentally on a battery pack containing twenty-one balancing converters and twenty-one 25 Ah Lithium-Ion cells under high-current (up to 100A) cycling.

Design and control of an integrated BMS/DC-DC system for electric vehicles

Electric-drive vehicles require a battery management system (BMS) for normal, safe operation of the battery pack, and a DC/DC converter to supply auxiliary loads on the low-voltage (LV) DC bus. This paper presents a shared, central control approach for an integrated BMS/DC-DC system that uses modular DC/DC bypass converters to achieve battery management and supply auxiliary loads in electric vehicles. The motivation to integrate the BMS and DC/DC is to provide differential control of battery cells using the LV bus load and provide an opportunity for advanced battery management to achieve longer battery life and higher power limits. The proposed control approach, system modeling, loop gain analysis, and design of the feedback compensators are provided. Experimental results are presented for a prototype system consisting of eighteen series connected 25 Ah lithium-ion NMC battery cells and three isolated dual-active bridge bypass converters, with combined outputs rated to supply a 1.44 kW auxiliary LV load.

SIMULINK based hardware-in-the-loop rapid prototyping of an electric vehicle battery balancing controller

Large strings of battery cells require a conditioning system to avoid parameter divergence, especially for Lithium based battery cells. The development procedure of a real time, central-control system with a high control loop rate, sophisticated battery internal state estimation, low cost and short developing time is presented. The method is based on a real plant and controller emulating system. The emulating system is implemented using a standalone PC, running a Real-Time SIMULINK environment. The approach does not require a computationally powerful DSP, or ECU, no additional programming skills are needed, and it could be implemented using software and hardware readily available in most laboratories. The computationally intensive algorithm used in this study was a sigma-point Kalman filter SOC estimator. Validation of the method was carried out on a battery pack consisting of 21 series 25 Ah Li-ion NMC cells, and a total conditioning power of 650 W.

Improved steady-state model of the dual-active-bridge converter

An improved steady-state model is developed in this paper for the dual-active-bridge (DAB) dc-dc converter. It is first shown how the well-known ideal model fails to predict DAB dc characteristics, and that differences in model predictions compared to experimental results can be very large, especially in cases where the DAB series tank inductance is relatively small. A new analysis method is then developed and applied to derive an improved steady-state model that correctly predicts DAB dc characteristics. The improved model is validated by simulations and by experimental results.

Hybrid balancing in a modular battery management system for electric-drive vehicles

A hybrid balancing approach that combines active and passive balancing is presented for a modular battery management system (BMS) in electric-drive vehicles. The hybrid system combines advantages of module-level active balancing with cost-effectiveness of cell-level passive balancing. A module consists of a number of cells connected in series, with cell-level passive balancing performed within a module, together with a module-level dc-dc converter that performs active balancing among the modules. Furthermore, the module-level dc-dc converters also supply auxiliary vehicle loads on the vehicle low-voltage bus, thus removing the need for a separate high-voltage to low-voltage dc-dc converter. System control and communication are shared among active and passive balancing functions, reducing the system complexity. Design details and experimental results are provided for a hybrid balancing system implemented on a lithium-ion battery pack, which consists of eighteen 25 Ah cells grouped in three 6-cell modules, each including a 480 W dc-dc converter supplying power to the vehicle low-voltage bus.