Basically, the charge curve of a nicad cell isn't particularly linear. Your length of time is more dependant on the amperage rating of your cell. There's a difference between a 1.4 Amp and a 2.2 Amp capacity. As a nicad charges the bulk of the charge occurs typically over a 3-4 hour period (all of this is also relative to the strength (wattage) of your solar cell. During the final phase of charging, negative delta V, or sudden voltage drop, will occur and heat will become generated. This is indication that the cell is fully charged and since no more energy can be held in the cell, it dissipates as heat (thank you Mr. Newton). Feel free to contact me if I can help any further. I used to design solar cell arrays
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