In order to understand the image, keep in mind that solar cells in an open-circuit state are about 2-3 K warmer than cells operating based on maximum power point (MPP).
A solar panel is roughly a current source over most of its characteristic, and the impedance of the load is setting the operating point's voltage, which is much lower than the panel's voltage at its MPP. At its MPP, it would be delivering more power than is needed.
Solar cell I-V characteristic curves that summarise the relationship between the current and voltage are generally provided by the panels manufacturer and are given as: = open-circuit voltage – This is the maximum voltage that the array provides when the terminals are not connected to any load (an open circuit condition).
The open-circuit voltage, V OC, is the maximum voltage available from a solar cell, and this occurs at zero current. The open-circuit voltage corresponds to the amount of forward bias on the solar cell due to the bias of the solar cell junction with the light-generated current. The open-circuit voltage is shown on the IV curve below.
What happens if a solar cell is not connected to a load?
With the solar cell open-circuited, that is not connected to any load, the current will be at its minimum (zero) and the voltage across the cell is at its maximum, known as the solar cells open circuit voltage, or Voc.
Why is my solar panel string's open-circuit voltage too low?
There is also another situation where the affected panel string's open-circuit voltage is the typical 11 or 13 volts too low but none of the bypass diodes are defective; instead, there's an interruption between the junction box and the solar cells.
What are the electrical characteristics of a photovoltaic array?
The electrical characteristics of a photovoltaic array are summarised in the relationship between the output current and voltage. The amount and intensity of solar insolation (solar irradiance) controls the amount of output current ( ), and the operating temperature of the solar cells affects the output voltage ( ) of the PV array.