Lead-acid battery – the capacity of the battery is related to the polarization of the plate

Lead-acid battery - the capacity of the battery is related to the polarization of the plate
  1. Capacity of lead-acid battery

Under certain discharge conditions, the amount of electricity that can be obtained from the battery. It is expressed in w.h or A.h. The w.h capacity represents the energy of the battery’s working ability, and the A.h capacity refers to the power output by the battery. Obviously, the larger the capacity of the lead-acid battery, the more electricity the electric throw can output, and the stronger the ability to do work. In addition to the lead-acid battery, I have also learned about a lithium battery, which has a large capacity and is durable and has excellent performance. If you are interested in learning about it, please click here to open.

The capacity of the battery can be divided into three types: the first is the theoretical capacity, which is calculated according to Faraday’s law based on the amount of active material. In fact, it is impossible; the second capacity is the actual capacity. Since it is impossible for all active substances to participate in the reaction, the actual amount of electricity released under certain conditions is always lower than the theoretical capacity; the third capacity is designed The minimum amount of power that should be released under the specified discharge conditions when a battery is used. The ratio of the actual capacity to the theoretical capacity is called the utilization rate of the active material. The method of increasing the actual capacity of lead-acid battery is as follows.

(1) Increase the number of active substances The amount of active substances involved in the reaction is related to the thickness of the plate. When a small current is discharged for a long time, the electrolyte can penetrate into the pores of the active material in the deep layer of the electrode plate, the utilization rate of the active material is high, and the discharge capacity is large; on the contrary, if the discharge current is too large in a short time, the sulfuric acid generated on the surface of the electrode plate Lead is easy to block the pores of the active material, which leads to the fact that the active material in the deep layer of the plate cannot be replenished in time by the electrolyte, and the reaction is interrupted. Therefore, when a large current is used for short-term discharge, the discharge capacity only depends on the size of the plate area.

(2) Influence of the porosity of the active material The ratio of the total volume (volume) occupied by the pores in the active material to the total volume (volume) of the active material is called the porosity (also known as porosity) of the active material. By definition, the larger the porosity of the active material, the more actual pores and the less active material. Although the porosity is large, the contact area between the electrolyte and the active material is large, and the discharge of the lead-acid battery is large, but due to too many pores and too little active material, the discharge of the lead-acid battery decreases, so there must be an optimal porosity. Generally, the porosity of the positive plate is about 55%, and the porosity of the negative plate is about 60%. In addition, when there is a large amount of β-PbOz in lead dioxide in the active material composition, the discharge capacity is large.

(3) The temperature, density and purity of the electrolyte have an impact on the capacity of the lead-acid battery. The temperature is low, the viscosity and resistance of the sulfuric acid electrolyte increase, the diffusion is difficult, the concentration difference increases sharply, and the resistance also increases, which makes the active material inside chemical reactions are difficult to carry out. The density of the electrolyte decreases, the amount of sulfuric acid participating in the reaction is not enough, and the density is too high, because the viscosity and resistance of the electrolyte will also increase. The lead-acid batteries used at first generally used electrolytes of 1.270 to 1.290 g/cm3. In addition, the discharge current also affects the capacity of the lead-acid battery. When discharging with a small current, the current density is small, and the number of lead ions is small near the electrode, that is, the supersaturation degree of lead ions is small, and it is easy to form a loose, coarse-grained lead sulfate salt layer, which is conducive to the diffusion of sulfuric acid electrolyte through pores. The discharge capacity is also improved when the active material is brought into contact with the deep electrode plate.

(4) Lead-acid batteries can not be idle for too long. If the lead-acid battery is idle for too long, the capacity will become lower and lower, as shown in the figure.

The relationship between idle time and residual capacity ratio
The relationship between idle time and residual capacity ratio
  1. Plate polarization into

Plate polarization is the process of converting plate electrodes into positive and negative electrodes with electrochemical characteristics using chemical and electrochemical reactions. When the polar plate is formed, it is necessary to apply a voltage between the positive and negative plates with a DC power supply, so that the current passes through the electrode to realize the oxidation-reduction reaction of the electrode material. This process is called the electrochemical reaction process. Oxygen and hydrogen will be discharged in the middle and late stages of plate polarization, which is due to the decomposition of the electrolyte into water, which is further decomposed into oxygen and hydrogen. Therefore, the plate is polarized so that there must be a small amount of gas discharged. If there is a large amount of gas discharged, it indicates that the battery is overcharged. At this time, sparks are often generated, which may cause the battery to explode.

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