The cost of solar cells is very high, accounting for more than 60% of the total cost of solar street lamps. Improving the utilization rate and charging efficiency of solar cells can make more effective use of solar energy and keep the batteries in good working condition. The PWM charging method can gradually reduce the current as the battery is fully charged, which meets the requirements of the battery for the charging process, can effectively eliminate polarization, and is conducive to fully restoring the battery’s power. Three-stage charging methods include: equalization charging, fast charging and floating charging. The battery has not been over-discharged. Floating charging is used during normal operation, which can effectively prevent over-charging and reduce the loss of water. When the depth of discharge of the battery exceeds 70%, a quick charge is implemented, which is beneficial to fully restore the capacity of the battery; once the depth of discharge (DOD) exceeds 40%, an equalization charge is performed, which is not only conducive to fully recovering the capacity of the battery, but also a slight deflation can play a role in stirring to prevent the stratification of the electrolyte in the battery.
According to the test results of the Florida Solar Energy Research Center, the controller using the pulse width modulation (PWM) three-stage charging method is 30% more efficient than the simple charge-disconnect controller (see Figure 1). The PWM three-stage charging method can maximize the use of expensive solar cells, greatly improve the charging efficiency, and ensure that the battery is always in good working condition.
The main circuit of the PWM three-stage charging controller is basically the same as that of the bypass and series controllers, except that the switching devices generally use power field effect transistors (MOSFETs) instead of relays.
The principle of the pulse width modulation circuit is shown in Figure 2. The modulation wave of the comparator is a triangular wave, which is input from the positive terminal, and the DC sampling voltage of the battery is input from the negative terminal of the comparator, and the triangular wave is cut by the DC voltage, a group of pulse width modulated waves is formed at the output end of the comparator, and this group of pulses is used to control the conduction time of the switching transistor to achieve the purpose of controlling the charging current.
As can be seen from Figure 2, for the series controller, when the voltage of the battery increases, the pulse width becomes narrower and the charging current becomes smaller; when the voltage of the battery decreases, the pulse width becomes wider and the charging current increases. For the bypass type controller, the DC sampling voltage of the battery and the modulating triangular wave should be adjusted at the input of the comparator, in order to achieve the purpose that the bypass current increases (the charging current decreases) as the battery voltage increases, and the bypass current decreases (the charging current increases) as the voltage drops.