Mobile devices are small and cost is a key driver, so only small solar cells can be used. A typical mobile phone can accommodate a 24cm2 solar cell, while in the current state of technology, 15% efficient solar cells can be considered a commodity (low cost). More efficient solar cells are commercially available, but still prohibitively expensive for mass-market applications.
With solar power irradiation levels ranging from 50W/m2 (shade/clouded) to 1000W/m2 (full sun), using a 24cm2, 15% efficient solar cell, maximum electric charge current of the solar cell then falls within the 18mW - 360mW range.
In everyday use the amount of solar radiation incident on mobile devices is significantly lower than one would intuitively expect. Measurements have shown that relatively low light conditions (under 25% of full solar irradiation) occur most of the time due to diffuse lighting, sub-optimal angles of the cell towards the sun, clouds or other obstacles. The Intivation technology has been optimized to give the best possible performance under these conditions so that resulting efficiency curves yield an unprecedented performance under real world conditions.
The most commonly adopted approach to solar-charging small Lithium batteries is to use a number, typically eleven or more, solar cells connected in series. Individual solar cells behave like P-N junctions with a forward voltage of about 0.3V to 0.5V, based on irradiation and temperature. The series connection raises the output voltage of the string of cells to a value suitable for charging a Lithium-Polymer battery, which requires a voltage of 3.0V up to 4.2V.
However, series-connecting solar cells introduces several technical challenges.
Soldering the cells requires manual labour and is therefore prone to errors and expensive. Additionally, the resulting assembly is fragile and can be damaged easily.
Series connecting solar cells also impairs the efficiency due to the introduction of additional edge defects (the series connected cell has more edge length than a single solar cell with identical surface area). Also, the electrical interconnections and the spacing between cells reduce the effective area of the solar cell.
Aside from these disadvantages, the efficiency of the serial array is also seriously impaired whenever the array is partially shaded. Shading just a single cell will render the entire array useless.
The Intivation SunBoost allows us to charge a Lithium Polymer battery directly from a single solar cell, circumventing all the problems associated with the use of serially connected solar cells.
The Intivation technology is based on a boost converter, which is controlled with a Maximum Power Point Tracking (MPPT) algorithm. While the electrical efficiency is certainly important, the integral efficiency (from sunlight to electrical power into the battery) is what really counts. Where most other solutions are promoted based on their electrical performance only, the use of MPPT will make sure that all available power is extracted from the solar cell, thus increasing the integral system efficiency significantly. Using MPPT has a huge positive impact on system performance.
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