Study On The Batteries Energy Management In Stand-alone Photovoltaic System

Storage batteries are a complex and weak link in a stand-alone PV system, so efficient energy management on batteries is a great significance for improving energy efficiency and reducing the cost of whole PV system.The output characteristic of photovoltaic arrays and which is effected by temperature and light intensity is analyzed, the changes of voltage and current of lead-acid battery during charging and discharging and why there is a polarization voltage are analyzed and the major factors impacting on the life of storage batteries are also discussed in the paper for the sake of knowing more about the characteristic of stand-alone PV system.The storage battery working status in the stand-alone photovoltaic system is as follows:working in a small charge and discharge rate; partial cycling in low state of charge; batteries are easily over charged in large light intensity.SOC is an important parameter to reflect the real capacity of batteries. The proposed battery model improved from the ideal circuit is present for the real-time calculation of SOC, in which terminal voltage, current, temperature are as its input and battery state of charge as its state variable. The relationship between battery state of charge and the battery external characteristics is obtained in the model.A battery energy management system is designed, in which the batteries are grouped. The strategy includes how the batteries are grouped, how to select the charge team and the discharge team and how to design the control methods.The grouping principle:The maximum charge current of the grouping batteries can not be smaller than the PV array output current.The selection of charge team and discharge team:the status of the batteries is divided into four types:FULL, GREEN, YELLOW, RED by SOC. We can obtain the batteries charge and discharge priorities based on the battery status. The numbers of discharge teams id determined by the relationship of the total current and the nominal current.The battery charge and discharge regulation:A fast-charging method by the direct controlling of the battery charge current is proposed according to the fast-charging principle the fast-charging principle is the actual battery charging current is equal or close to the maximum acceptable charge current. The battery discharge processing is setup by controlling the depth of discharge and the discharge current.Finally, the circuit and the software are designed based on TMS2812DSP.The results have shown that the calculation of the battery SOC is accurate and universal; the batteries can obtain more energy by the fast-charging method proposed; batteries can avoid partial cycling in low SOC in grouped system.