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