|MadSci Network: Engineering|
When you say you keep charging it will the cells die I presume you are referring to the photovoltaic cells (the solar cells) not any rechargeable battery cells the device may have.
The solar cells will become less effective as time goes by, but quite slowly. There will be changes in both the internal structure of the device, the electrical connections and even to the protective coatings (plastic covers and the like).
Most materials, especially when exposed to heat or UV light, tend to slowly break down, become brittle or otherwise lose some integrity. The actual silicon semiconductors (the component that makes electrical energy from light energy) would most likely go on producing a voltage for a very very long time, but the structure holding them in place, the wiring, the covers etc would all tend to slowly break down.
However the solar cells should be able to power your calculator for at least ten years and probably much longer. It is more likely other parts of the calculator would fail before the solar cells.
Commercial solar cells in outdoor settings would be expected to last 20 years or more.
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