Answer by Yrjo Koyen for Solar panel charging circuit

If your 18V and 280mA are respectively Voc (open circuit voltage the solar cell can produce) and Isc (short circuit current), then that means the cell can produce 280mA of current at an output voltage of 0V and when no current is being drawn from the cell there would be 18V at the output. In between these points, the Current-Voltage characteristic (general example of a solar cell) looks like this:
You cannot use the solar cell on its own in an operating point 'outside' of the curve.