By zhanqun1 
Open-circuit Voltage, abbreviated as OCV, defines the voltage at both ends of a power supply in an opened state of a Circuit. It means there is no passage of current in it. It represents the essential parameter to indicate the possible capacity of power. Common uses are found within a field of power engineering and also include photovoltaic systems.
I. What is an open circuit voltage?
The open circuit voltage can simply be explained as the voltage value at both ends of a power supply when there is no connection of load to the power supply. For example, a photovoltaic cell; when the anode and cathode are unconnected to any kind of external load, which includes resistors, bulbs, and other electric appliances, the voltage across the anode and cathode of a photovoltaic cell is considered to be its open circuit voltage. In other words, the open circuit voltage refers to the potential difference within a photovoltaic cell during its open circuit state.
The photovoltaic cell can be regarded as a water pump for a more intuitive understanding; the water flow corresponds to the current. In this case, when the water outlet and water inlet of the pump are not connected with any water pipe-that is, in the open state-the pump will still have a certain pressure difference, which is similar to the open voltage. The open-circuit voltage is representative of the potential ability of the power supply itself, such as a photovoltaic cell, to drive current.
II. the importance of open circuit voltage
Open circuit voltage is an important parameter of the photovoltaic cell and other power supply equipment, with great significance for design, performance evaluation, and fault detection. What it does is as follows:
Thus, open-circuit voltage can reflect the possible performance of the photovoltaic cell, whose value depends on the material of the photovoltaic cell, process parameters, light intensity, and temperature. For example, with identical environmental light intensity and temperature, various materials of photovoltaic cells-monocrystal silicon, polysilicon, thin-film cadmium telluride solar cell, and so on-have different open-circuit voltages. It means that the higher the intensity of light, the higher the open circuit voltage of the photovoltaic cell, and as the temperature gets lower, the open circuit voltage is higher.
For the design with the photovoltaic systems, an open circuit voltage represents an important parameter in the design for series and parallel packs of a battery. For example, the total open circuit voltage is the sum of the open circuit voltage of each photovoltaic cell when many photovoltaic cells are connected in series. Reasonable series and parallel combination can be designed according to the open circuit voltage for the requirement of the specific voltage output.
Open-circuit voltage is considered a very significant index of photovoltaic cell performance evaluation. It also actually gauges an open circuit voltage to be measured with the theoretical or normal value, then judges by this whether photovoltaic cells have defects or the degradation of performances exists. At the same time, in a running photovoltaic system, such changes in the open-circuit voltage are taken as one basis of monitoring the system’s state of health.
III. Influencing factors of open circuit voltage
The magnitude of the voltage of the open circuit would depend on quite a few factors including but not limited to:
Materials: the photovoltaic cells of different materials have different width of bandgap, therefore, there is a difference in open circuit voltage. Example, the open-circuit voltage of mono crystalline silicon photovoltaic cell is usually higher than the polycrystalline silicon.
Light intensity: the open circuit voltage of the photovoltaic cell will be risen when the light intensity goes higher since strong light can provide more energy and push up a higher voltage.
Temperature: open circuit voltage is strongly depended on the temperature. Increased temperature can reduce the open circuit voltage of the photovoltaic cell since the electron moving increases due to the high temperature, which reduces the potential difference.
IV. Open circuit voltage practical application
Measurements and analysis that relate to open circuit voltage, have a wide array of practical applications, in particular, the following :
Inspection and maintenance of photovoltaic system.The open circuit voltage measure during the installation and operation represents the mean that measures and controls the performance of the battery module, the health condition of a system. For instance, in the case where the open circuit voltage of one component significantly remains below others that will equally imply faultiness of the particular component.
The open circuit voltage is used to set operating range and performance parameters in many power supply devices. For instance, an open-circuit voltage is used in the management system to regulate the charge/discharge strategy of electric vehicle batteries.
Research and development: Open circuit voltage is an important index in the study of new photovoltaic cells or energy storage equipment in research and development to characterize the performance of materials. For instance, by measuring the open-circuit voltage under different conditions, the potential efficiency of the material in photoelectric conversion can be studied.
The open-circuit voltage is the most important electrical parameter for power supply equipment, mainly the photovoltaic system, in wide use and of importance. The understanding of open-circuit voltage-definition, factors, and function in a practical application-not only gives optimization during the design but also serves as the scientific basis of performance evaluation and fault detection of a photovoltaic power generation system. Open circuit voltage will still be one of the core research and applications in the future energy field.