An Intelligent Distributed Embedded Fast Charge Equalization System For Series Connected Battery Strings

Series connected battery strings have been widely applied as power source in various applications. However, when charged, individual battery cells in the string can not be automatically balanced due to the series connected structure, which will have a great impact on performance and life cycle of the battery strings. Charge equalization systems can deliver charge energy equally to individual cells in the string. In this way, the available capacity will be enhanced and the life cycle will be extended. But, existing equalization systems have certain limitations in charging speed, extension, production and cost. It is very necessary to design a low cost non-dissipative equalization system, which can be easily extended and manufactured.This dissertation is written from the perspective of a real engineering project. Basic principles for designing charge equalization systems are given based on the analysis of existing system and the characteristics of charging series connected batteries. These principles and the defects in existing systems show that it is proper to introduce a distributed digital control mechanism into the equalization system. In order to facilitate this system architecture, the equalization circuit is realized with a new type of switch equalizer.The measurement and control subsystem is a distributed embedded computer system constructed with a main control module and several equalization modules. Multiprocessor UART protocol on isolated CAN physical bus is used for the communication between the modules. Switch equalizer are made of MOSFETs and driven with optically coupled isolators. In this way, power and analog circuitsof the system are isolated from each other, and any number of continuously connected cells can be selected for charging. Meanwhile, an average current mode controlled power source is used to provide constant charging current to these selected cells. Result of SPICE simulation shows that system with this control mechanism can response to step charge signal within 10ms.The algorithm of the system is based on the selective equalization model formed from the discretization of continuous charging process. Studies on this model put up the constant current block equalization algorithm and also the variable current equalization algorithm with key parameters. In addition, equalization algorithms designed to facilitate the transient response of battery cells to charging current can achieve more accurate voltage measurement.A verification experiment has been performed to show that this system can deliver a fast equalized charging process at 1C, and the equalization error is less than 4mV. It has been concluded that distributed digital equalization system can been made with limited cost to achieve a fast equalized charging. And the new type of switch equalizer and distributed embedded computers make the system smaller, also it's mV level of accuracy can meet the needs to equalize NiCd and NiMH batteries.