Research On Battery Management System Of Solar Electric Vehicle

The rapid development of auto industry brings not only economic benefits to thesociety but also many tough problems, for example， the energy crisis and theenvironment pollution. In order to solve these problems, researchers have beenlooking for a renewable and clean energy to replace traditional fossil fuels. At thesame time, new energy vehicles have attracted extensive attention of autocorporations. The solar electric vehicle which combines photovoltaic technology andpower drive technology, where solar power is used as auxiliary energy，is one of thebest solutions. Nowadays there exist many problems need to be solved in solarvehicles, such as how to improve the energy efficiency and how to prolong thebattery’s cycle life. These problems can be solved by developing a better batterymanagement system (BMS).This work was supported by the Jilin provincial scientific development projectof Microfluidics Solar Electric Vehicles (20080353). The main energy storage of thevehicle is a package of lead-acid batteries, with the solar cells used as the auxiliarypower source. The ultimate objective of this research is to develop a highperformance BMS for solar electric vehicles.First of all, the performances of lead-acid batteries and solar cells were presented.The discharge performance of lead-acid batteries involves the voltage change and the capacity change. While the charge performance just related to the Mas Law, based onwhich the optimal charge current and charge strategy can be obtained. In addition, thefailure mechanism of lead-acid batteries was also analyzed. The performancecharacteristics of solar cells include output power characteristics and I-Vcharacteristics.Then the overall design of the system was carried out. The hardware frame andthe control program flow were demonstrated to achieve the expected function of theBMS. And the key techniques of the system, such as lead-acid battery state-of-charge(SOC) estimating algorithm, the photovoltaic charging strategy and the maximumpower point tracking (MPPT) algorithm of solar cells, were discussed respectively.SOC estimating algorithm is the core of the BMS. Above all, we analyzed themerits and demerits of different algorithms those have been adopted, and then a newalgorithm based on the Kalman Filter was purposed. In order to fit the algorithm, webuilt a calculation model of lead-acid battery with experimental parameters to deduceperformance formulas. The applicability and accuracy of the algorithm were provedby computer simulations. The results demonstrated that the algorithm designed canprecisely estimate the SOC of the lead-acid battery. Moreover, in order to improve theaccuracy, we tried a new algorithm called UKF (Unscented Kalman Filter) whosetheory precision is higher than that of the Kalman Filter, which is also proved bycomputer simulations.In the aspect of Photovoltaic charging strategy, we analyzed the existing schemesto find that the3-phase charging is best, but it’s not suitable for the photovoltaicsystem. So we proposed a new improved strategy based on3-phase charging. By thisnew strategy, lead-acid batteries can avoid failing too soon, and thus the energyefficiency of the photovoltaic system can be improved at the same time. Moreover,the MPPT algorithm used in the system is a conductance increment method which hasa good stability and less calculated complexity. Then the development process of the hardware system was presented, includingthe principle and circuit of all blocks. We chose the AVR ATmega16microcontrolleras the core chip of the system. The system hardware involved the peripheral circuit,the measurement block and the photovoltaic charging block.Finally the refitting process of the solar electric vehicle was illustrated. Theprototype vehicle was tested to confirm its performance including the max speed, thebreaking performance and the acceleration performance, etc. The photovoltaiccharging performance and the BMS performance were tested respectively. The resultindicated that the BMS can meet the operating requirement of the solar electricvehicle and the photovoltaic system well.