## Materials and Methods
### Charge and Discharge Control Strategy
Instead, the stability of the busbar is controlled by the post-stage phase-shifted full-bridge soft-switch circuit, and the post stage employs a bus voltage outer loop and a current inner loop to realize the control, thereby achieving MPPT constant-current charging. During the constant-current charging stage, the MPPT does not operate. Therefore, the bus voltage can be controlled by the interleaved parallel boost circuit in order to execute a constant voltage proportional-integral (PI) algorithm. The circuit uses a post-stage full-bridge voltage outer ring as the bus outer ring, combined with a current inner ring, to perform constant-current charging. During the constant-voltage charging stage, the post-stage full-bridge circuit uses the battery voltage loop as the outer loop. In this case, the bus voltage is clamped by the battery voltage, and the  
Figure 3: Flow chart of charging algorithm. Figure 2: Enhanced stage charging curve based on maximum power point tracking (MPPT). battery voltage loop alone can complete the constant voltage charging operation. The role of the current loop is to limit the current. The above procedures can not only complete the enhanced stage charging algorithm based on the MPPT described earlier, but can also maintain the stability of the busbar during charging. Figure 4 shows the detailed control block diagram. In Figure 4, when the system samples the photovoltaic output voltage/current, battery charging voltage/current, and bus voltage, the sampled values are inputted to the DSP to determine the charging mode, as follows:  
1. When the system is in the trickle pre-charging or the constant-current charging stage, the photovoltaic battery does not work in the MPPT state. Therefore, the interleaved parallel boost is controlled by the bus voltage loop, and the bus voltage is controlled by the boost. The charging current is controlled by the post-stage full-bridge circuit, which uses a double loop control comprising a current inner loop and a bus outer loop to achieve a constant current. 2. When the system is in the MPPT constant-current charging stage, the photovoltaic battery works in the MPPT state, which controls the output power of the photovoltaic battery to the maximum. Therefore, at this stage, the bus voltage cannot be controlled by the boost, instead, it should be controlled by the post-stage full-bridge circuit. Thus, the post-stage full-bridge circuit uses the bus voltage outer ring and the current inter ring to achieve a constant current and stable bus voltage, respectively. 3. When the system is in the constant-voltage charging or the trickle floating charging stage, the photovoltaic battery does not work in the MPPT state. Therefore, the busbar should be controlled by the post-stage full-bridge circuit. Moreover, the charging voltage of the battery needs to be controlled at this stage. Thus, at this stage, the battery voltage outer ring is used as the voltage outer ring, and the charging voltage is stabilized at a set value. The busbar can also be clamped to a certain stable value, and the role of the charging inner loop is to limit the current. After the specific control strategies for the pre-stage interleaved parallel boost circuit and post-stage full-bridge phase-shifted soft-switch circuit are determined, the corresponding PI or MPPT algorithm calculates the pre-stage boost duty cycle and the post-stage full-bridge phase shifting angle. The system then drives the pre-stage and post-stage switching tubes via the corresponding driving circuit so as to complete the charging process. Figure 5 shows the structure diagram of the specific loop control algorithm. Figure 4: MPPT-mode charging algorithm control block diagram. PI: proportional-integral.