When the electrolyte is initially injected into the cell, the density is uniform throughout the solution. However, in the process of charging and discharging, due to the progress of the electrochemical reaction, the density of the electrolyte is different everywhere, and the final result is that the density of the lower part is greater than that of the upper part. It can be seen from the reaction formula that sulfuric acid is consumed during discharge and water is generated. During the discharge process, the sulfuric acid in contact with the surface of the active material first participates in the reaction to generate water. At this time, the external sulfuric acid should continuously diffuse into the electrode plate to supplement the sulfuric acid consumed by the discharge and maintain the discharge current uninterrupted. There must be two movements: one is the water generated by the reaction to diffuse outward; the other is the diffusion of sulfuric acid into the plate. The driving force for this diffusion movement is the difference in electrolyte concentration during discharge. If the discharge is terminated, the diffusion motion ceases very quickly. Since the density of the newly generated water is smaller than that of the electrolyte, in the diffusion motion, there must be an upward floating motion component, as shown in Figure 1.
During charging, new ecological sulfuric acid is continuously produced on the plate. Because the density of the new ecological sulfuric acid is larger than that of the electrolyte in the middle of the two plates, there is also a corresponding sinking component in the homogenization diffusion. As a result of these two movements, after a period of use, the density of the battery’s electro-hydraulic fluid increases.
The stratification of the electrolyte is extremely harmful, and the damage to the car battery starts from the electro-hydraulic stratification, which is analyzed below.
①The density of the upper and lower parts of the electrolyte is different, and the active material on the plate shows a difference in potential, which causes the short-circuit discharge of the plate itself, that is, the concentration discharge.
②The higher the temperature, the more serious the corrosion, and the delamination causes the chemical corrosion of the lower part of the plate to be larger than that of the upper part.
③Due to the high density of the lower part, the resistance value of electrolysis between the plates has changed. In Figure 2, when charging, the current flowing between the two points of C and D is intense, and the electrochemical corrosion of the grid is aggravated. The charging and discharging depth of the lower active material is larger than that of the upper part, which accelerates the damage of the lower part of the battery.
This kind of damage to the battery, in all the batteries, there are almost no exceptions. It is generally believed that in car batteries, this stratification of electrolyte does not exist in car batteries. The actual investigation shows that the electro-hydraulic stratification cannot be eliminated by the driving vibration of the vehicle alone. At present, it can only be solved by appropriately increasing the charged power. In the case of frequent charge and discharge cycles, the battery works on a battery tractor with deep charge and discharge, and its damage often starts from the upper part of the plate group. In a 600A*h large stationary battery, if the charging voltage is lower than the decomposition voltage of water, basically no bubbles are generated during the charging process. After 6 charge-discharge cycles, the concentration at the bottom of the battery is actually twice that of the top, and the density at the bottom is 1.30 g/cm³, the density of the upper part is 1.15g/cm³, which is why the corrosion damage of the lower part of the battery is always more serious than that of the upper part.
When there is no charge-discharge reaction, the electrolyte will not be layered under the action of gravity, and the sulfuric acid in the electrolyte will combine with water to form hydrated ions, which have a stable structure and are evenly dispersed in the solution. In a static state, after several months, the difference in density of the upper and lower electrolytes could not be measured with a specific gravity balance.
In order to eliminate the stratification of the electrolyte, the gas generated by decomposing water by increasing the charging voltage is often used, and the electro-liquid is stirred when the bubbles float to make it uniform, which is called balanced charging in the charging process. This is a necessary measure, not an ideal method. Agitating the electrolyte with compressed air is a good idea, but requires pre-set gas channels in the cell structure.