In photovoltaic power generation systems, especially photovoltaic lighting systems, the battery is in a deep discharge cycle every day. Usually, the depth of discharge is 25% to 35% every day and night. It is very important to detect and control the battery to avoid over-discharging. The discharge characteristic curve of lead-acid battery is shown in Figure 1. The battery discharge process has three stages:
In the beginning (OE) stage, the voltage drops rapidly; in the middle stage (EG), the voltage drops slowly and lasts for a long time; after the G point, the discharge voltage drops sharply. There are three main reasons why the voltage continues to decrease with the discharge process: firstly, with the discharge of the battery, the acid concentration decreases, causing the electromotive force to decrease; secondly, the active material is continuously consumed, the reaction area decreases, and the polarization continues to increase; thirdly It is because of the continuous generation of lead sulfate, the internal resistance of the battery continues to increase, and the internal resistance pressure drop increases. The voltage at point G in the figure indicates that the battery is close to the end of discharge, and the discharge should be stopped immediately, otherwise it will cause irreversible damage to the battery. The electrolyte specific gravity (concentration) method can be used to detect the battery state of charge. Since the load of the battery discharge may be changing, it is difficult to accurately detect the battery state online, and it is not easy to accurately control the cut-off voltage of the battery discharge. Generally speaking , the nominal 2V lead-acid battery has a cut-off discharge voltage of 1.7~1.9V, and the battery charge and discharge power statistics can also be used as a constraint condition for discharge control.
Figure 2 shows the discharge characteristic curve of a 100A·h single lead-acid battery, which shows that the time characteristic curve of the battery with different discharge currents has different discharge cut-off voltages. As can be seen from the figure, the larger the discharge current, the lower the discharge termination voltage. Taking a 2V single-cell 100A·h lead-acid battery as an example, when discharged at a current of 25A, it can be discharged for 3 hours, and the discharge termination voltage is 1.75V; when it is discharged at 1.5A, the dischargeable time is 100h, and the discharge termination voltage is 1.85V. In addition, the use of small current discharge can make the battery output more energy. In the actual photovoltaic system, the “threshold voltage” can be set according to the discharge and characteristics of the battery. Usually, the depth of discharge of the system every night is about 30%. Therefore, for the VRLA battery, the standard state (25℃, 0.1C The discharge end voltage under the discharge rate) is reasonable to be 1.8V.
In photovoltaic and wind power generation systems, the charging power of the battery comes from the solar cell array and the wind turbine, and its guarantee rate is much lower than that in the case of alternating current. The change of climate and the excessive use of electricity by the user will easily cause the battery to be overdischarged. . Figure 3 shows the effect of depth of discharge on battery life. If the lead-acid battery is frequently deeply discharged during use, the service life of the battery will be greatly shortened. Therefore, in order to prolong the service life of the battery, the remaining capacity (SOC) of the battery must be detected in real time, and the discharge process of the battery must be controlled according to the remaining capacity of the battery. Therefore, when designing a photovoltaic system, the battery should be in shallow discharge as much as possible, and the remaining capacity of the battery must be matched with the load to ensure that the battery is not overdischarged. (Tycorun Battery is a company specializing in the production of various types of batteries, you can visit their website if you are interested)
In the designed photovoltaic system, in order to improve the reliability of the system, the load working time can be controlled according to the remaining capacity of the battery, and the discharge of the battery can be controlled in the whole process. It is mainly used for unattended photovoltaic power generation systems that allow proper adjustment of working hours. , the most typical is solar street lights. In order to improve the reliability of the system, the solar street light system can adjust the working time of the street light under the condition of different SOC of the battery, and can also control the output power of the street light, especially the LED lighting system only needs to control the current.
Therefore, the controller can usually work regularly, and divide it into different levels according to the remaining capacity of the battery to control the working time of the load; it can also adjust the power work, set different levels of the load, and the controller adjusts the load according to the remaining capacity of the battery. power can also achieve the same purpose. Because the programming function of the intelligent controller is more convenient, the two control methods can also work at the same time to better optimize the system.