Nickel-hydrogen Battery Equalization Charging System Design And Pilot Study

Electric Vehicle (EV) and Hybrid Electric Vehicle (HEV) are considered as the direction for future automobile development The Ni/MH battery became the first choice in EV and HEV due to its excellent properties such as high energy density, long life-span, pollution-free and the ability to discharge at a large current, etc.As an innovative traction technology, The Ni/MH batteries must be arranged in series to reach the required voltage. Unfortunately, for the battery string in series, the un-uniformity among the string would result in a decrease of the life-span of the battery and have an impact not only on the energy utilization of EV and HEV, but also the normal operation of the automobile. So it's very urgent to tackle the technology problem of the application of traction batteries.The equalizing charge system for Ni/MH traction batteries, aimed at the special requirement of its application in EV and HEV, performs its equalizing function by adopting the equalizing charge based on inverted and equational voltage. The system consists of mainly two modules, namely the equalizing charge module which is the core part of the system and the data acquisition module. The former module was used to regulate the output characteristics of this module. Meanwhile, the latter samples the battery voltage, current and temperature at a given intervals and then transmits the data to the BUS by its serial port to achieve the information exchange among the whole system.Based on the analysis of the principle and charge/discharge characteristics of Ni/MH battery, Orthogonal experimental design is applied in,which the PWM duty ratio, frequency, the primary and secondary turns of the coil with three levels respectively are chosed .the battery decentralized degree was adopted to specify the un-uniformity degree among the cells in the string and used as the main index while the charging time as the secondary index in the quantitative analysis to evaluate the equalizing effect of the system under different parameters, to study on the effect of exterior characteristics on the equalizing process and acquire the optimal controls parameters.It could be seen from the data that there was nearly a linearity relationship between the equalizing charge current and the extent that the voltage of the substring deviates from the average level of the whole string,namely,the system could automatically distribute the energy according to the un-uniformity extent of the battery string without the participation of MCU allthrough the equalizing process, which would simplify the control logic and hardware structure while achieve the effect of equalizing charge.