How VRLA batteries work?

The working principle of VRLA battery basically follows the traditional lead-acid battery. Its positive active material is lead dioxide (PbO2), the negative active material is spongy metal lead (Pb), and the electrolyte is dilute sulfuric acid (H2SO4).

The design principle of VRLA battery is to inject the required amount of electrolyte into the electrode plates and separators. There is no free electrolyte. The ability to absorb oxygen is improved by the humidity of the negative plate. In order to prevent the reduction of electrolyte, the battery is sealed. VRLA battery is also called “lean liquid lead-acid battery“.

VRLA battery has made important improvements in structure and material. As shown in Figure 3-2, the positive plate is made of lead-calcium alloy or lead-cadmium alloy, low antimony alloy, the negative plate is made of lead-calcium alloy, the separator is made of ultra-fine glass fiber separator, and the tight assembly and lean liquid design technology are used. , The chemical reaction of the entire VRLA battery is carried out in a sealed plastic battery shell, and a one-way safety valve is provided on the air outlet. When the battery with this structure is charged at a specified charging voltage, the oxygen (O2) precipitated by the positive electrode can be transferred to the surface of the negative plate through the separator channel and reduced to water (H2O). % capacity, the potential increases when hydrogen is evolved, and the difference in reaction area and reaction speed makes the positive electrode appear oxygen before the negative electrode appears hydrogen.

Oxygen passes through the separator channel or top to reach the negative electrode for a chemical reaction.

The negative electrode is oxidized to lead sulfate, which is converted into spongy lead after charging.

This is the unique internal oxygen cycle reaction mechanism of VRLA batteries. In this charging process, the water in the electrolyte is almost not lost, so that VRLA batteries hardly need to add water during use.

Manufacturers take various technical measures to reduce the precipitation of H2 and O2 gases, so that they can be digested inside the VRLA battery as much as possible. For example, let the excess negative active material absorb part of the precipitation O2, so as to effectively control the electrolysis of water and reduce the consumption of electrolyte.

However, it is impossible to absolutely control the precipitation of H2 and O2. In fact, a small amount of electrolyte is still consumed, and a small amount of hydrogen and oxygen is still released. In this respect, VRLA batteries are not “maintenance-free” but less maintenance. With the development of science and technology and technological level and the accumulation of experience, the ability to control electrolyte consumption is getting stronger and stronger, thus effectively reducing the maintenance of VRLA batteries.

The pole shed of VRLA battery is mainly made of lead-calcium alloy to increase the overpotential of its positive and negative gas evolution (H2 and O2) and reduce the gas evolution during the charging process. When the positive plate reaches 70% charge, oxygen begins to generate, and the negative plate starts to generate hydrogen when it reaches 90%. In the production process, the thickness ratio of the positive and negative plates is generally 6:4. According to the change in the mass ratio of the positive and negative active materials, when the velvety Pb ​​on the negative electrode reaches 90%, the PbO2 on the positive electrode is close to 90%. After a little charging, the active materials on the positive and negative electrodes are oxidized and reduced by 95% respectively, which is close to full charging, which can reduce the evolution of H2 and O2 gases. Ultra-fine glass fibers (or silica gel) are used to absorb and store the electrolyte, and at the same time provide a channel for the oxygen evolved on the positive electrode to diffuse to the negative electrode. In this way, once oxygen diffuses to the negative electrode, it is immediately absorbed by the negative electrode, thereby suppressing the generation of hydrogen gas on the negative electrode.

In the open circuit state of VRLA battery, the reaction of positive and negative active materials PbO2 and spongy metal lead with dilute sulfuric acid of electrolyte tends to be stable, that is, the oxidation rate and reduction rate of the electrode are equal, and the electrode electromotive force at this time is the balance electrode electromotive force. When the charge and discharge reactions are carried out, the positive and negative active materials PbO2 and spongy metal lead are respectively converted back and forth through the electrolyte and the discharged material lead sulfate.