This would certainly be the case if we were in Spain where there is useful sunshine (insolation) falling on a correctly aimed Solar Panel.
However with our short winter days, low sun on the horizon, weedy panels on the market, the chances of actually finding any useful charge let alone overcharge are small.
With the NOCO BLSOLAR2 panel I have, when I last tested it at midday midwinter on a “bright sunny day” in London it could barely put out enough to overcome the NC dark current, let alone cook the battery. If I surrounded it with reflectors (opening the effective aperture) in bright midsummer then it almost rose to the rated output.
Here’s a bit of background using my NOCO (nominally 2.5Watt) panel as an example.
The Wiki article “Theory of Solar Cells” gives us an excellent in-depth explanation of how they work, and if you need more info it is well worth a look. Photons (light) enter the cell and electrons (current) leave it, and various factors affect the efficiency. The article gives us the equations and the diagrams and the theoretical details as well as an overview. I’ll try and apply it to this device below.
The limitations of the photovoltaic array are;
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The surface area (how much light it can catch, and therefore how much current.) This is crucial.
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The number of cells in the array (setting the maximum open-circuit voltage.)
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The `intrinsic’ (internal) series resistance (which limits the short-circuit current regardless of how much light is hitting it.)
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How hot it gets (hotter means more internal leakage and lower open-circuit voltage.)
This particular array has 31 cells (photo-diodes) arranged as stripes across the panel, and the open-circuit voltage (when it self-discharges through the 31 diodes) depends on how much light is coming in and varies from about 18V (bright indoors light, about 5mA short-circuit current) to about 27V (very bright sunlight, about 350mA short-circuit current, and that measurement in mid-summer at noon had the help of some mirrors to concentrate more light.)
Maximum output power available from an array is a trade-off between load voltage and current and intrinsic resistance. In the case of this Noco panel it is at its best with a normal 12V battery charging at about 13.5V with the nominal 180mA, because of the panel’s intrinsic series resistance of about 75 Ohms. However it will still charge lower voltage batteries (eg 6V) with similar amounts of current, just also with lower power because more of the power is being lost in the intrinsic resistance of the panel.
Under normal circumstances here in the UK expect it to charge at between 30mA to 100mA at midday. So over 24 hours in winter, expect maybe 0.15AH per sunny day! And in mid summer maybe 1.2AH could be harvested.