Lead-acid sealed batteries are composed of positive and negative plates, separators and electrolyte, battery tanks and connecting strips (or lead parts), wiring terminals and exhaust valves. The electrode plate is the core component of the battery. It is a lead grid plate with a grid structure, which is divided into two types: positive plate and negative plate. The active material on the positive plate is lead dioxide, which is brown-red; the active material on the negative plate is spongy pure lead, which is blue-grey. Batteries used in stand-alone photovoltaic systems should be of deep-cycle heavy-load type. Because the lead plate material is relatively soft, some elements such as antimony or calcium should be added to strengthen the hardness of the lead plate, which can improve the performance of the battery. Valve-regulated sealed lead-acid batteries have the advantages of no need to add acid water, no acid mist precipitation, can be placed arbitrarily, convenient to handle, and clean in use. In recent years, they have been widely used in photovoltaic power generation systems. However, the battery pack is relatively expensive and has a short lifespan. Generally, the maintenance-free working life is 5 years, while the stable working life of photovoltaic panels is 25 to 30 years. The existence of batteries will inevitably affect the life of the photovoltaic system. Therefore, by using appropriate charging and discharging methods to extend the life of the battery as much as possible, the maintenance cost of the photovoltaic system can be greatly reduced. The structure of the lead-acid sealed battery is shown in Figure 1.
The plate is the core of the battery. During the charging and discharging process of the battery, the conversion of electrical energy and chemical energy is realized by the electrochemical reaction between the active material on the positive and negative plates and the sulfuric acid in the electrolyte.
The battery plate (Figure 2) is divided into positive and negative plates, which are composed of a grid frame and an active material. The active material on the positive plate is lead dioxide (PbO2), and the active material on the negative plate is lead (Pb). The amount of active material on the single plate is small, and the stored electricity is small. In order to increase the battery capacity, the positive and negative plates are connected in parallel, and a separator is inserted in the middle to form a positive and negative plate group. The lead dioxide and dilute sulfuric acid are adjusted into a paste and coated on the grid to form the positive plate (Figure 3). Mix pure lead powder and dilute sulfuric acid into a paste and apply it on the active material of the grid to form a negative plate (Figure 4). In a single-cell battery pack, there is one more negative plate than the positive electrode. The reason is that during charging and discharging, the two electrode plates and the electrolyte react chemically to heat the electrode plates to expand, but the heating degree of the two electrode plates is different. The positive plate generates a large amount of heat and the expansion is serious, while the negative plate is very slight. In order to make the same chemical changes occur on both sides of the positive plate, the expansion degree is balanced, and to prevent the plate from bending and breaking, so one more negative plate is needed. Although only one side of the outer negative plate undergoes chemical changes, it does not cause deformation or fracture due to its small heat generation.
The function of the separator (Figure 5) is to separate the positive and negative plates to prevent short-circuiting of the plates. Commonly used materials for partitions are usually wood, glass, plastic, hard rubber, etc. The partitions are made with grooves on one side and smooth on the other.