This single-buck converter system is unable to implement MPPT and charge control simultaneously. When this single-stage design is applied to MPPT, the lithium battery load has not controlled the voltage and current and the battery life may be reduced.This selleck chemicals llc study designed a portable solar energy lithium battery charger, on the principle of two-stage system. The first stage uses a DC/DC boost converter, which adopts the variable step size incremental conductance (VSINC) method to control the solar power system working at the maximum power point [7�C9]. The second stage uses a DC/DC buck converter [10], which controls the charging system by battery voltage and current feedback for the lithium battery, so as to make the system attain constant voltage, current, and voltage charge.
The PI controller based on optimal algorithm is designed to achieve the charge control of second stage [11, 12]. This two-stage system can achieve simultaneous MPPT of solar energy and lithium charge control. The two-stage design not only controls the two-stage system but also there is a control system when the battery is being charged, thus prolonging the charge cycle of lithium battery. This study used Simulink and Simpower of MATLAB simulation software for simulation analysis of system. In terms of portable hardware, the embedded PIC18F8720 microcontrol chip developed by Microchip was used to compose the MPPT algorithm and battery charge controller. The experimental results were compared with simulation result to prove the effect of the proposed control theory.2.
Solar Cell CharacteristicsSolar cell is mainly made of PV wafers, converts the light energy of solar irradiation into voltage and current directly for load, and conducts electricity without electrolytic effect. The electric energy is obtained from the PN interface of semiconductor directly; therefore, the solar cell is also known as PV cell [1, 2].The mathematical model of solar cells in series or parallel connection can be simply expressed asIPV=Iph?Isat?(exp?qBkTVPV?1),(1)where IPV is output current of solar cell (A); VPV is output voltage of solar cell (V); Isat is reverse saturation current of solar cell; Iph is current output of solar cell; q is quantity of electronic charge; k is boltzmann constant; B is ideal factor of solar cell; and T is solar cell surface temperature.
The power-voltage and current-voltage curves of mathematical model corresponding to solar cell in different illuminations are shown in Figures Figures11 and and22.Figure 1Current-voltage characteristic curve diagram of solar cell under varying irradiance.Figure 2Power-voltage curve diagram of solar cell under varying irradiance.3. Maximum Power Point System DesignIn application, AV-951 the MPPT algorithm is between the solar cell and DC/DC boost converter.