Solar Panel Optimizing Battery Charger

Posted on Feb 4, 2014

Figure shows the V-I curve for a typical solar panel (Sharp ND-224U1F), and output power under different lighting conditions. The solar panel behaves like a constant current source at high current, and like a voltage source at low current. The transition region is where the highest power can be obtained. The voltage and particularly the current at the optimum point vary considerably

Solar Panel Optimizing Battery Charger
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with the amount of light incident to the panel. The solar panel is typically used to charge batteries since solar energy is only available during the day, but typical electrical power usage extends during dark periods. On cloudy days, limited power may be available even at mid-day. Therefore, an optimized battery charger must be able to determine the "maximum power point" (MPP) of the panel under the current condition and adjust the load to match it, keeping in mind that the MPP varies during the day as a function of the amount of available light and the panel`s temperature. Fortunately, switching regulators of the type used in high efficiency battery chargers can vary the load while maintaining the proper output voltage for battery charging. It continuously monitors the panel`s output power while modulating the load by a small amount. When the power increases as the load is increased, the controller increases the average load. When the power does not change between the two load values, the panel is operating across the MPP. See figure 2 below. In the figure above, points P1 and P2 correspond to non-optimized operation. The current is high, but the delivered power is less than it would be if the load was a little lighter (less current and more voltage). When the charger operates between points P1 and P2, the unequal power levels drive the controller to a lower average load (lower current) in order to increase the delivered...

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