The distributed balanced charging structure adds balancing function on the basis of distributed charging. Therefore, the distributed balanced charging system of series energy storage power sources mainly includes three aspects: lower level unit balancing topology, distributed charging system structure, and balancing control strategy.
In response to the current situation of slow charging speed, low efficiency, and uneven charging in charging systems, this paper proposes a new distributed balanced charging system with balancing function based on the existing patents of the research group.
The system adopts a distributed parallel charging structure combined with a four cell direct balancing topology to form the entire system structure. According to the functions of the system, the distributed balanced charging system can be regarded as two layers of units. From a macro perspective, the charging balance and cross group discharge balance of the distributed charging system of the upper unit and the four cell balancer of the lower unit are implemented, and from a micro perspective, the dynamic self balancing of the battery cells inside the lower unit is implemented.
We use standard lithium iron phosphate batteries with a capacity of 20Ah, with charging and discharging cut-off voltages of 3.65V and 2.5V respectively, and a nominal voltage value of 3.2V. The lower unit of a distributed balancing charging system is composed of four sections of lithium iron phosphate batteries and a coupled inductor type multipath direct balancing topology, namely a four cell equalizer. Each lower unit has a nominal voltage of 12.8V, and the standard charging voltage of the charging system can be dynamically expanded and reduced according to the voltage multiples of the lower unit according to actual needs, which can adapt well to energy storage systems of different charging levels.
The main research content is divided into the following four parts:
(1) Principle Analysis and Simulation of Distributed Balanced Charging System with Series Energy Storage Power Sources
Introduce the basic circuit of the distributed balanced charging system for the series energy storage power supply, and analyze the working principle and operating mode of the bidirectional DC converter and four cell equalizer. Then, derive the model of the four cell equalizer, obtain relevant balance indicators, and conduct simulation verification.
(2) Design of Magnetic Components and Converter Control Loop for Balanced Charging System
Design the magnetic components of the balanced charging system according to the system indicators. Establish small signal models for bidirectional DC converters in different modes, design control loop parameters for bidirectional DC converters in different modes based on the system’s charge discharge balance requirements, analyze their stability through Bode plots, and finally build closed-loop simulation models for verification.
(3) Analysis of Balanced Strategy for Distributed Balanced Charging System with Series Energy Storage Power Sources
Based on the structural characteristics of a distributed balanced charging system with series energy storage power sources, a charging balance strategy is proposed. The relationship between intra group balance and cross group balance is analyzed. Based on the selected balance control variables, a time slice allocation parallel discharge balance strategy that considers efficiency and speed is proposed. The relationship between battery internal resistance loss and time slice size and position is analyzed, and the optimal balance scheme under different situations is obtained.
(4) Functional Experiment and Balance Strategy Verification of Distributed Balanced Charging System with Series Energy Storage Power Sources
Introduce the platform structure of a distributed balanced charging system for series energy storage power sources, and then conduct basic functional experimental verification and related balancing index testing for independent and combined operation of bidirectional DC converters and four cell balancers. Design a time slot allocation parallel discharge balancing strategy simulation program and conduct two different forms of complex unbalanced battery pack balancing strategy verification.