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Solar panels have an open circuit voltage, which is what you measure with a digital voltmeter and no load connected - its always quite a bit higher than the operating voltage - which depends on the number of cells in the panel. The operating voltage is the one which tells you for what battery voltage the solar panel is designed for.
There is also a short circuit current (what you would measure on an Amp meter that is shorting out the panel while the panel is aimed for maximum sun light)
Then there is an operating current (a little lower than the short circuit current) which is the maximum charging current you can get from the panel into a battery.
BTW, it does no harm to short circuit a solar panel with an Amp meter (unless you select the wrong meter range :wink: )
To further the discussion as the light on the solar panel varies, the open circuit voltage and short circuit current vary. Since this is the case, it takes some work to extract the maximum power from the panel. One method is to use a DC/DC converter and vary the duty cycle of the converter while measuring the change in output power. In this way you constantly dither the control signal and steer your output to maximum power.