Simplifying solar-based battery charging: Page 2 of 7

May 06, 2014 //By Steve Knoth and Albert Wu
Simplifying solar-based battery charging
Steve Knoth, Senior Product Marketing Engineer, Power Products Group and Albert Wu, Design Manager, Power Products Group, Linear Technology Corporation explains how a buck-boost solar powered battery charging controller can offer an efficient solar-powered solution.
simple load box to accomplish the sweep.  For this particular case of conditions, the maximum power point is at 32 V and the panel can deliver 140 W. Once the panel temperature is allowed to vary, which it certainly will in a real world setting, the maximum power point can vary between 28 V on a hot day to 44 V on a cold winter’s day. 

Figure 1: With no partial shading, a simpler power curve can exist for a given solar panel

Many simpler solar charging systems set the panel voltage operating point to a fixed level.  In the case of this particular panel, these simpler systems would set the operating point of the panel to be 32 V in order to extract the most power at a given temperature, 60°C in this case.  However, when the panel temperature changes, significant power is wasted because the panel is no longer operated at its true maximum power point.  Upwards of 20% to 30% of the available power can be wasted in these cases.

To make matters worse, most panels are required, by safety standards set in place, to have bypass diodes built into the solar cell array.  The reason for this has to do with what occurs when only portions of the panel are shaded from sunlight, while other areas get full sun. When this occurs, the solar cells that are shaded become reverse biased but still have high currents flowing through them because the other illuminated cells are providing the current.  High temperatures in the shaded cells can occur and this can pose a fire hazard.  To help lower the risk of fire, manufacturers place bypass diodes throughout the panel.  Figure 2 shows how bypass diodes can be placed in the 72 cell panel.

Figure 2: 3 bypass diodes placed in a 72 cell solar panel for safety considerations

With bypass diodes in the panel, complex power versus voltage

Design category: 

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