Advances in battery technology and device performance have made it possible to produce complex electronics that run for long periods between charges. Even so, for some devices, recharging the batteries by plugging into the grid is not possible. Emergency roadside telephones, navigation buoys, and remote weather monitoring stations are just a few applications that have no access to the power grid, so they must harvest energy from their environment.
Solar panels have great potential as energy harvesting power sources—they just need batteries to store the harvested power and to provide carry-though during dark periods. Solar panels are relatively expensive, so extracting maximum power from the panels is paramount to minimizing the panel size. The tricky part is a balancing of solar panel size with required power. The characteristics of solar panels require careful management of the panel’s output power versus load to effectively optimize the panel’s output power for various lighting conditions.
For a given illumination level, a solar panel has a specific operating point that produces the maximum amount of power (see Figure 1
). Maintaining this peak-power point during operation as lighting conditions change is called maximum peak power tracking (MPPT). Complex algorithms are often used to perform this function, such as varying the panel’s load periodically while directly measuring panel output voltage and output current, calculating panel output power, then forcing the point of operation that provides the peak output power as illumination and/or temperature conditions change. This type of algorithm generally requires complex circuitry and microprocessor control.
Figure 1: Current vs voltage and power vs voltage for a solar panel at a number of different illumination levels. The panel output voltage at the maximum power point (VMP) remains relatively constant regardless of illumination level.
There exists, however, an interesting relationship between the output voltage of a solar panel and the power that the panel produces. A solar panel output voltage at the maximum power point remains relatively constant regardless of illumination level. It follows that forcing operation of the panel such that the output voltage is maintained at this peak power voltage (VMP
) yields peak output power from the panel. A battery-charger can therefore maintain peak power transfer by exploiting this VMP
characteristic instead of implementing complex MPPT circuitry and algorithms.
A few features of the LT3652 battery charger
The Linear Technology's LT3652 is a complete monolithic step-down multi-chemistry battery charger that operates with input voltages as high as 32V (40V abs max) and charges battery stacks with float voltages up to 14.4V. The LT3652 incorporates an innovative input regulation circuit, which implements a simple and automatic method for controlling the charger’s input supply voltage when using poorly regulated sources, such as solar panels. The LT3652HV, a high voltage version of the charger, is available to charge battery stacks with float voltages up to 18V.