Structure and working principle of VRLA battery

Structure and working principle of VRLA battery

Valve-regulated sealed lead-acid (VRILA) battery is the most common battery used in solar street lamps. Its structure and working principle are introduced as follows.

1. Structure of VRLA battery

The main components of VRLA battery are composed of positive plate, negative plate, separator, battery tank cover, sulfuric acid electrolyte, etc. (Figure 1).

Structure and working principle of VRLA battery
Figure 1 – Structure of VRLA battery

2. The role of the main components of VRLA battery

(1) Grid. Supports active materials and conducts current.

(2) Plates, positive and negative plates are the place of electrochemical reaction and the main restrictor of battery capacity. The negative plates are all paste-coated. Positive plates are generally painted (flat) and tubular, and tubular positive plates are generally used in traditional flooded batteries and gel batteries.

(3) Separator. Store electrolyte, gas channel, prevent active material from falling off, and prevent short circuit between positive and negative electrodes.

(4) Electrolyte. All lead-acid batteries use sulfuric acid electrolyte, which is a necessary condition for electrochemical reaction. For colloidal batteries, it is also necessary to add colloid to form a colloidal electrolyte with sulfuric acid gel, at this time, sulfuric acid is not only a reaction electrolyte, but also a gelling agent required by the colloid.

(5) Slot cover. Packing the pole group, the thickness and material of the groove directly affect whether the battery is bulged and deformed. Plastic slot covers are generally used, such as PVC or ABS slot covers.

(6) Pole, which conducts current.

3. Basic reaction principle of VRLA battery

(1) The principle of VRLA battery reaction

Positive electrode

Structure and working principle of VRLA battery

Negative electrode

Structure and working principle of VRLA battery

(2) VRLA battery sealing principle

In the later stage of lead-acid battery charging, the electrochemical reactions that occur on the electrodes are as follows:

Structure and working principle of VRLA battery

It can be seen that the generation of H2 and O2 during battery charging is unavoidable, and the recombination of the two gases can only be carried out in the presence of catalysts. From the 1950s to the 1960s, Pt-catalyzed explosion-proof plugs were studied, but they were eliminated due to their complex structure, high price and poor reliability.

The principle of gas recombination proposed by A. Dassler in 1938 played an important guiding role in the subsequent manufacture of sealed lead-acid batteries. In 1971, the Gates Company of the United States proposed the use of glass fiber separators, which provided the feasibility for the practical application of the oxygen composite principle and achieved a breakthrough in “sealing”. The composite principle is as follows.

Oxygen cycle is very difficult to seal lead-acid batteries. The balance voltage of lead-acid batteries is about 2V, and the decomposition voltage of water (oxygen is released on the positive electrode and hydrogen is generated on the negative electrode) is also about 2V. So thermodynamically lead-acid batteries do not work at all, and oxygen and hydrogen are already evolved during charging before lead dioxide and lead are formed from lead sulfate. Lead dioxide and lead electrodes have extremely high oxygen and oxygen overpotentials on the surface, so that the positive and negative electrodes can be recharged before a large amount of hydrogen and oxygen are precipitated. If the principle conditions of excess negative active material and limited electrolyte are met, oxygen cycling can be applied to lead-acid batteries. When the cathode adsorption sealed lead-acid battery is charged, oxygen is released on the positive electrode, which is compounded on the negative electrode, so as to maintain the negative electrode in a partially charged state, so that the precipitation of hydrogen can be suppressed.

During the oxygen cycle, the following reactions often occur: ① Oxygen is precipitated from the positive electrode (diffusion to the negative electrode); ② On the negative electrode, oxygen reacts with spongy lead (oxidation reaction) to form PbO; ③PbO reacts with sulfuric acid to generate lead sulfate and water, and the negative electrode is charged due to the conversion of lead sulfate into spongy lead in the middle. In summary, there are the following reactions.

Structure and working principle of VRLA battery

Combining the oxygen evolution process of the positive electrode and all the reactions on the negative electrode, it can be known that the negative electrode is consuming oxygen, and all the various reactions on the negative electrode are actually unlikely to occur, and it is more likely that the oxygen is directly compounded to generate water. The evolution of hydrogen on the negative electrode is not absolutely suppressed, so the necessary valve must be designed. When the internal pressure of the battery reaches a certain value, the valve is opened to release air (pressure relief), and then closed again after the pressure is released, preventing the gas (oxygen) in the atmosphere from entering the battery, and ensuring the safety of the battery when it is abused.

The composite battery can be widely used under different loads and has excellent performance. Its main advantage is that there is no need to add water, and it is mainly used in some application fields where it is difficult or expensive to obtain electric energy, such as power supply in remote areas. Since the hybrid battery is practically gas-free, there is no acid mist hazard caused by it, so it is very suitable for use in office equipment.

The porous glass wool separator (porosity>90%) provides a good channel for oxygen transfer between the positive and negative electrodes, and the oxygen evolved from the positive electrode is reduced at a very high speed at the negative electrode.

The above reaction achieves the recycle of oxygen, with the result that there is no accumulation of oxygen and no loss of water. The recombination of oxygen desorbed the negative electrode and slowed down the evolution of H2 (Figure. 2).

Structure and working principle of VRLA battery
Figure 2 – Oxygen recombination or cathode absorption principle of VRLA battery