Storage battery for solar cell and its working principle

The battery is a component used to store the electric energy (DC) generated by the solar cell module for the subsequent load. In an independent photovoltaic system, a controller is generally required to control its state of charge and depth of discharge to protect the storage battery and extend its service life. The deep-cycle battery is made with a larger electrode plate, which can withstand the specified number of charge and discharge. The so-called deep cycle refers to a depth of discharge of 60% to 70%, or even higher. The number of cycles depends on the depth of discharge, rate of discharge, rate of charge, and so on. The main feature is the use of thicker plates and higher density of active materials. The thicker plate can store more capacity, and the release rate of capacity is slower when discharging. The high density of active materials can ensure that they are attached to the electrode plate/grid of the battery for a longer time, thereby reducing the degree of attenuation. It has a longer service life under deep cycle; the recovery ability after deep cycle is good. Shallow cycle batteries use lighter electrode plates. Shallow cycle batteries cannot be recycled as many times as deep cycle batteries. The voltage of the solar battery must exceed the working voltage of the battery by 20%~30% to ensure the normal power supply to the battery. The battery capacity should be more than 6 times higher than the daily consumption of the load.

At present, batteries mainly include lead-acid batteries, nickel-metal hydride batteries, lithium-ion batteries, and fuel cells. Among them, the price of lead-acid battery is low, its price is 1/4~1/6 of the price of other types of batteries, one-time investment is relatively low, and most users can afford it; the technology and manufacturing process are mature. The disadvantages are large mass, large volume, low energy-to-mass ratio, and strict requirements for charging and discharging. Nickel-cadmium batteries are used in some countries, they are usually more expensive than lead-acid batteries, but nickel-cadmium batteries have a long life, low maintenance rate, durability, can withstand extremely hot and cold temperatures, and can be completely discharged. Because it can be completely discharged, the controller can be saved and used in some systems. The controller is not universal. Generally, the controller provided is designed for lead-acid batteries.

The battery capacity determines the number of days the load can be maintained. It usually refers to the number of days that the load can be maintained by the power stored in the battery when there is no external power supply. The battery capacity can be determined by referring to the local annual average number of consecutive cloudy and rainy days and the needs of customers. The design of the battery includes the design calculation of the battery capacity and the series-parallel design of the battery pack.

The principle of commonly used batteries

The battery is a reversible direct current power source, an electrochemical device that provides and stores electrical energy. The so-called reversible means that it can be restored and used after being charged. The electrical energy of a battery is produced by a chemical reaction between two different plates immersed in the electrolyte. Battery discharge (outgoing current) is the process of converting chemical energy into electrical energy; battery charging (inflowing current) is the process of converting electrical energy into chemical energy. For example, a lead-acid battery is composed of positive and negative plates, electrolyte and electrolyzer. The active material of the positive plate is lead dioxide (PbO2), the active material of the negative plate is gray spongy metallic lead (Pb), and the electrolyte is an aqueous sulfuric acid solution. The total chemical equation of battery charge and discharge is:


During the charging process, under the action of an external electric field, the positive and negative ions migrate to the two poles each, and a chemical reaction occurs at the interface of the electrode solution. When charging, the PbsO4 of the positive plate is restored to PbO2, and the PbSO4 of the negative plate is restored to Pb, the H2SO4 in the electrolyte increases, and the density rises. The charging continues until the active material on the electrode plate is completely restored to the state before the discharge. If you continue to charge, it will cause water electrolysis and release a lot of bubbles. After the positive and negative plates of the battery are immersed in the electrolyte, a small amount of active material is dissolved in the electrolyte solution to generate electrode potential, and the electromotive force of the battery is formed due to the difference in the electrode potential of the positive and negative plates. When the positive plate is immersed in the electrolyte, a small amount of PbO2 dissolves into the electrolyte, forms Pb(OH)4 with water, and then decomposes into tetravalent lead ions and hydroxide ions. When the two reach a dynamic balance, the potential of the positive plate is about +2V. The metal Pb at the negative electrode plate interacts with the electrolyte to become Pb2+, and the electrode plate is negatively charged. Because the positive and negative charges attract each other, Pb2+ tends to settle on the surface of the electrode plate; when the two reach a dynamic balance, the electrode potential of the plate is about -0.1V. The static electromotive force E0 of a fully charged battery (single cell) is about 2.1V, and the actual measurement result is 2.044V.

When the battery is discharged, the electrolyte is electrolyzed inside the battery, the PbO2 of the positive plate and the Pb of the negative plate become PbSO4, the H2SO4 in the electrolyte decreases, and the density decreases. Outside the battery, the negative charge on the negative electrode continuously flows to the positive electrode under the action of the electromotive force of the battery. The whole system forms a loop: an oxidation reaction occurs at the negative electrode of the battery, and a reduction reaction occurs at the positive electrode of the battery. Due to the reduction reaction on the positive electrode, the electrode potential of the positive electrode plate gradually decreases, and the oxidation reaction on the negative electrode plate promotes the increase of the electrode potential. The whole process will cause the electromotive force of the battery to drop. The battery discharge process (Figure 1) is the reversal of its charging process.

Storage battery for solar cell and its working principle
Figure 1 – Schematic diagram of battery discharge process

After the battery is discharged, there are still 70% to 80% of the active material on the electrode plate that has no effect. A good battery should fully increase the utilization rate of the active material on the electrode plate.

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