In a photovoltaic system, a battery is a component used to store electrical energy. In the battery, electrical energy is converted into chemical energy, which is the process of battery charging; when the solar battery is fully charged to the battery and the load starts to use electricity, the chemical energy in the battery begins to be converted into electrical energy, which is the discharge process of the battery .
At present, the batteries commonly used in photovoltaic systems are basically sealed lead-acid batteries. Why? Because this battery is very cheap. However, because this kind of battery is sealed and maintenance-free, compared with the previous open-type lead-acid battery, it cannot be maintained by manually adding water, acid, and online testing the temperature, specific gravity, and voltage of the battery electrolyte. Sealed maintenance-free batteries have extremely precise and strict requirements for charging and discharging systems, and the extensive and simple charging and discharging methods of the past can no longer be used to manage batteries. Therefore, battery technology experts have now developed a more scientific and safe charging and discharging system. Now, for example, PWM (Pulse Width Modulation) charging technology is very good, and the charge and discharge controller is also embedded in a single-chip computer system to closely monitor the controlled battery pack. The knowledge about the controller will be introduced in the next chapter.
The battery is the most important component in the photovoltaic system. Why? Because the battery is too delicate and expensive. If it is not used properly, it will be finished. If the energy storage component is broken, can the photovoltaic system still work? Obviously not. Because the photovoltaic conversion efficiency of solar cells (currently) is too low, it is impossible to supply directly online like the generator set of a thermal power plant. When will the photoelectric conversion efficiency be increased to 85%, and when will the online direct supply of solar cells be envisaged. In this situation, we can only design, use, and maintain batteries well. Don’t let it overcharge, don’t let it overdischarge, don’t let it owe it every day. The hazard of overcharge is to cause the battery to lose water, because the overcharge state will cause the battery to outgas too much, and in severe cases, the extremely bad situation of water electrolysis will occur. After the loss of water, the electrolyte concentration becomes higher, the temperature rises, and the electrode voltage further rises, thereby accelerating the battery’s loss of water. After entering the vicious circle, the battery will soon be finished.
Similar to the overcharge hazard is the “thermal runaway” phenomenon of the battery, which is also a major hazard. The “thermal runaway” is also caused by too high charging voltage. It has been seen that charging is the deadly killer of lead-acid batteries. Undercharge or overdischarge will make the pure sulfate in the battery electrolyte, the negative plate will generate coarse and insoluble sulfate, the active material in the electrolyte will decrease, and the battery capacity will naturally decrease. During use, the battery will feel full when charging, but it will be emptied quickly when using electricity, and it cannot be used normally.