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They also come in the form of straps, with solar cells on the outer surface and a rechargeable battery inside. Solar cell technology limits the effectiveness and practicality of phone solar chargers for everyday use. Phone charge times vary depending on the solar panel size and efficiency, or the battery capacity of models with batteries, further extending the charge times of solar chargers. The fold-out design provides a larger solar panel, hence higher charge current, and is compact when not in use.