Battery Charging by Novel MPPT Method
Perturb and observe (P&O) MPPT method is adopted for its important advantages as simplicity and
applicability to almost any PV system configuration. Conventional P&O technique requires to sense PV panel output voltage and output current for MPPT implementation. In off-grid solar system with battery as energy storage, the charging current and battery need to be monitored to realize charging control. By taking the PV panel and the MPPT converter as an integrated
energy source, the MPPT algorithm could be implemented from the battery side to obtain the maximum charging power without the information of PV panel. Less sensing circuitry contributes to decrease the complexity and cost of the MPPT controller in off-grid solar applications.
The MPPT control method based on load parameters have proved the feasibility of the P&O MPPT
implementation via load parameters. However, the optimization of transient MPPT behaviour has not been covered by these literatures.In practical off-grid solar applications, the control objective is to maximize the power/energy flow delivered to the load, e.g. energy storage. From this point of view, it is reasonable to choose load power instead of PV power as control variable and to estimate output power of PV panel by battery charging power. Analysis shows that the mathematical expression of typical single switch DC/DC converter suitable for solar charge controller is monotonous regarding duty cycle. In this way, it is possible to judge the operating point location without the information of PV panel. Improved P&O MPPT algorithm based
on load parameter is implemented with the control flow chart in. Both the continuous current mode (CCM) and the discontinuous current mode (DCM) of Sepic converter are studied, in which the definition of DCM is the sum of both inductors current has the duration of being zero. Solar charge controller based on Sepic topology is shown in Fig. 3, where the port connected with inductor La and power switch Qb are defined as port a and port b respectively, Va and Vb
represent output voltage of PV panel and battery voltage, and Cc is coupling capacitor. There are two operating modes in CCM and three modes in DCM, respectively. In CCM, by applying voltage-second balance principle for La and Lb,