At (+) electrode:
PbO2 + 3H++ HSO4– + 2e –→PbSO4 + 2H2O
At (-) electrode:
Pb + HSO4–→PbSO4 + H+ + 2e–
Overall reaction formula:
PbO2 + Pb + 2H2SO4 → 2PbSO4 + 2H2O
It can be seen from the reaction equation that PbSO4 is formed on the positive and negative plates after discharge, and the reaction can proceed to the left only under the condition of charging. Reducing the acid concentration in the electrolyte favors this reaction, and the more it is reduced, the easier it is to charge. Commonly referred to as plate vulcanization, it means that the positive and negative plates have become PbSO4, which has a large resistance value and is difficult to carry out a charging reaction. The way to deal with this kind of battery is to pour out all the electro-liquid in the battery, inject distilled water, and charge it, which is most conducive to the charging reaction. During the charging process, we will measure that the density of the electrolyte in the battery gradually increases. When the rising speed slows down, pour out all the acid solution, refill it with distilled water, and charge it. This is repeated several times, and finally the measured density does not increase with the extension of the charging time. At this time, the plate is activated. This process is obtained by using the principle of the artificial catalytic equation reacting to the left. This is the traditional method.
After the lead (Pb) on the electrode plate is tightly wrapped by the lead sulfate (PbSO4), the discharge product, it cannot come into contact with the sulfuric acid electrolyte. Charging cannot be carried out, which is called plate vulcanization. The schematic diagram is shown in Figure 1. Capacity recovery is to open some gaps on the surface of dense PbSO4, so that Pb can contact the electrolyte, the charging reaction can be carried out, and the battery capacity is activated.
There are two kinds of processes for removing vulcanization, namely chemical method and physical method. The chemical method is to add some activators. The activators mainly contain alkali metal ions such as K+ and Na+. Under the action of current, these ions will be enriched on the surface of the negative electrode, so that the surface of the electrode plate will locally form a condition with a low pH value. PbSO4 is very soluble in alkaline environment, so some PbSO4 on the surface is dissolved to expose Pb, which is the activation point shown in Figure 1. Lithium ions are used in some activators. In the periodic table of elements, lithium ions have the smallest atomic radius and the strongest permeability, so the activation effect is the best. Since no new alkali metal elements are available, it is technically impossible to exceed this limit.
Although this method works quickly and has a high capacity increase, it has side effects. Once the activator is added, the dissolution of PbSO4 on the surface of the electrode plate is uncontrollable. Dissolution inevitably occurs at the junction of the Pb particles in Figure 1, so the plate is softened, which accelerates the capacity decay of the battery. The “chemical formation” process of the battery factory in the production of batteries is to turn the deeply vulcanized green plate into an active Pb cooked plate. The method of alkali metal additives is not used, but the pulse formation technology is generally used. The process is to use pulse current to eliminate vulcanization and physically remove vulcanization of the vulcanized battery. The capacity increase is small, but there is no side effect of softening the plate, and the two indicators of capacity and life are taken into account.
After the battery is restored to its capacity, it goes online, and after a few months of use, there will be a single battery behind. This is normal and this capacity fade has nothing to do with the company undertaking the recovery. If the company undertaking the restoration does not have the online retention capacity detection technology, it is difficult to find a few failed single sections, and it is difficult to eliminate the hidden quality problems. This is because when the battery is damaged by vulcanization, the electrode plate is deeply expanded, resulting in more shedding. After activation, the non-conductive lead sulfate (PbSO4) that falls off becomes conductive lead (Pb). This discrete lead is connected between the positive and negative plates, and the self-discharge of the battery increases significantly. Many restoration companies publicize There is no explanation for controlling self-discharge in the data. When the self-discharge increases to a certain extent, under the condition of low-current floating charge of the base station, the amount will be lost in 2-3 months. Retrieve the failed single cell in the middle of the base station, and the discharge capacity can still reach 80% after charging, which meets the use standard. Disassemble one of the batteries and see that the separator between the plates has been contaminated with lead powder that has fallen off, see Figure 2.
In the current capacity recovery process, the battery is checked for capacity with a current of several tens of amps. After reaching the base station, the floating charge current is usually only about 0.3A. Therefore, a battery that has passed the capacity test after activation cannot be guaranteed to be used safely in the base station. The self-discharge index is an important index to ensure the safe operation of the battery. The new battery standard is no more than 0.14% per day. Due to the limitations of the process and contract terms, the cost of testing this indicator is high, and neither the recovery company nor the communication department can detect the technical parameter of self-discharge. After the battery is restored to its capacity, the self-discharge will increase tenfold, and many technical directors of communication companies do not know the impact of this indicator on the quality of operation. It is not feasible to rely on emergency process measures such as capacity recovery to improve battery operation quality.
It is also understandable that the valve-regulated battery uses a high-density electrolyte, and the battery is easily vulcanized.
Now there is electronic desulfurization technology, which can easily remove sulfur. By incorporating this technology into the charger, the charging process and the devulcanization process can be combined. Long-term use of this charger to charge, the battery will not be damaged by vulcanization. The power module used in the communication base station has this function.