Simplifying solar-based battery charging: Page 3 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.
characteristics can occur when partial shading is present.  Figure 3 shows such a scenario where two local power maxima are present, one at 21 V and the other at 37 V.  If the above 32 V simple power point method were used, 79.4 W of power would be available compared to 90.1 W available at the true maximum power point of 21 V.  This represents a significant loss of 13.5% in this case.  Clearly, a system that can operate and track the true maximum power point would be a superior approach. 

Figure 3: With partial shading, more complex power curves exist for a solar panel

Design challenges of a solar powered rechargeable battery system

Typical solar panel efficiencies range from about 5% to 15% Combined with the fact that larger (i.e., more powerful) panels cost more, solar powered designs must maximize efficiency to minimize total system cost.  

To effectively harvest energy from the sun in a solar based product, the design must manage a widely varying input while also finding a way to operate the solar panel at or near its maximum power point.  Furthermore, the design must safely charge the battery chemistry of choice used in the product.

There are also other design problems encountered with solar charging systems. For any given solar-powered system, firmware development and debug can take a huge amount of time.  A more complex buck-boost topology is needed if the panel’s optimal power delivery point can be below, equal to, or above the battery voltage (this is a very common scenario). A buck-boost topology allows true isolation in both directions (when compared to a step-down or 'buck' topology only, if the panel is dark it might drain the battery through the body diode of the NMOS through the inductor). Proper voltage termination is needed to protect the battery.  Finally, since the panel is not a reliable source of power, in-situ charging of the battery

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