Detailed analysis of the initial charging of the battery

Detailed analysis of the initial charging of the battery

Usually, the first charge after the battery is unpacked is called the initial charge. For batteries that need to be injected with electrolyte, there is a process of “initial charging”. The initial charging mentioned here includes all the charging and discharging operations of the battery during the period from unpacking to the official use of the battery. The purpose of initial charging is to create PbO2 on the positive plate of the battery and spongy Pb on the negative plate. The quality of the initial charge directly affects the actual capacity and service life of the battery. For more information on affecting battery life, visit Tycorun Battery.

For different types of batteries, the initial charging requirements are different. For “dry charged” batteries,
For stationary batteries, during the storage period of the battery without adding electrolyte, there is lead sulfate (PbSO4) on the electrode plate, which is present when the battery is assembled. A portion of PbO is generated. During the battery storage period, due to the change of ambient temperature, the battery has a “breathing” process: when it is heated, part of the air in the battery is discharged from the battery case; when it is cold, the external air will be sucked into the battery. This kind of “breathing” brings moisture into the battery. If the storage period is too long, under the action of moisture, the self-discharge of the battery will increase, and it will be connected to each other to form dense and coarse grains.

Usually, the injection hole of the battery is thermally sealed with a metal film in order to prevent or slow down this damage. Usually, it is stipulated that the time from the date of manufacture to unpacking should not exceed one year, that is, only one rainy season. Therefore, when purchasing, storing and transporting, you should pay attention to the fact that if the original plate is in a good dry state and there is no moisture immersion during the storage period, it will not harm the quality of the battery after two years of storage. Compared with the cold and dry north, the south with high temperature and high temperature has a completely different degree of damage to the battery during the same storage period.

After unpacking the battery, first check whether the top cover of the case is cracked or not. If it is not checked at first, once the electrolyte is injected, it is found that there is cracking, and the loss is difficult to recover. The reasons are as follows: ①The cracked part of the battery shell is impregnated with electrolyte, which cannot be cleaned with water, and the adhesive repair surface cannot meet the requirements of the adhesive repair process. degree of cleanliness.
②As soon as the electrolyte is injected into the battery, the electrode plate reacts. During the time of sticking and repairing, the battery has been damaged by vulcanization, which is irreversible by ordinary charging. Put the battery in a well-ventilated workplace and inject the prepared electrolyte. The lower the temperature of the electrolyte, the better. Excessive electrolyte temperature can cause thermal damage to the battery.

a. The plastic separator and shell in the battery are easily deformed. The PVC plastic separator will aggravate its degradation at high temperature, release chloride ions, and damage the battery plate.

b. The electrode plate alloy of the battery is mostly lead alloy, and the high temperature will cause thermal dislocation of the alloy crystal, which will reduce the corrosion resistance. Therefore, the working temperature of the battery is usually specified below 45 °C. The lower the temperature of the injected electrolyte, the lower the temperature rise of the battery and the less possibility of thermal damage to the battery.

In the daily maintenance after the battery is put into use, generally only water is added, but no acid is added. Therefore, the acid added to the battery for the first time is the acid content required by the battery in subsequent use, and this value cannot be taken high. Many studies have shown that when the acid density is greater than 1.26g/cm³, due to the accelerated corrosion of the grid, the shedding of the active material on the plate increases sharply. In many cases, the damage of sulfuric acid electrolyte to the battery is more direct than the consequences caused by changes in production process conditions. In automobile batteries, the damage caused by the electrochemical corrosion of sulfuric acid is greater than the normal damage caused by the charge-discharge cycle. much more.

In battery manufacturing, after the electrode plate is formed, when the electrode plate is taken out from the formation tank, the lead in the PbO2 on the positive plate is already in the Pb state, so it will no longer react with oxygen in the air. However, the spongy Pb on the negative electrode is in a highly activated state, which can spontaneously oxidize with oxygen in the air:

This reaction gives off heat, and the rate is much faster when the plate is wet than when it is dry. The PbO generated by the reaction continues to react with the acid in the capillary to generate PbSO4:

The reaction gave off heat again. In order to prevent the thermal intensity of the reaction from being too high and damaging the plate, it is necessary to discharge a part of the electricity in the formation tank first, that is, protective discharge, so that a layer of lead sulfate is formed on the surface of the plate; at the same time, when the negative plate is taken out from the formation tank , Immediately immersed in pure water, the acid in the capillary pores of the plate is leached, which can slow down the thermal intensity of the exothermic reaction.
In humid conditions, the positive plate can also react with CO2:

This reaction causes the positive plate to lose its activation capacity. In the process of plate assembly and battery storage and transportation, the above reactions are always going on. In order to minimize this harmful reaction, the plate should be kept as dry as possible, and the air outlet on the water cap should be sealed to prevent the intrusion of oxygen and moisture in the air.

Sulfuric acid is injected into the battery, and the sulfuric acid reacts with PbO on the negative plate to generate fine-grained PbSO4, which releases heat and increases the temperature of the battery. At the same time, the density of the electrolyte decreases, the more serious the oxidation of the negative plate, the higher the temperature rise of the battery after the electrolyte is injected, and the more the density of the electrolyte decreases. If charging is not performed within a short period of time after acid injection, the fine-grained lead sulfate attached to the surface of the electrode plate will gradually become larger, the number will gradually increase, and it will penetrate into the interior of the electrode plate, making the charging operation difficult.

It can be seen from the above analysis that the process of initial charging is mainly the process of restoring the activity of the negative plate. The conversion of electrical energy to chemical energy also occurs mainly on the negative electrode. Although the initial charging also has a deepening reaction of the positive plate, it is mainly the unipolar plate reaction of the negative plate, which is different from the daily supplementary charging. Therefore, the current conversion efficiency of the initial charging is very low, and the battery is also prone to heat. Since there is PbSO4 on the plate, in the later stage of initial charging, the measured electrolyte density will be 0.02~0.04 higher than the density of the injected electrolyte. It is formed by discharge, so the grains are coarse and the active area is small. At this time, the internal resistance of the battery is large, and the temperature rises quickly during charging, so the charging current cannot be large.

The first charge is usually performed with two current values. The first stage is carried out with 0.05C20~0.1C20. “C20” is the nominal capacity at 20h rate discharge. At this stage, charge the battery to 2.3~2.4V per single cell. The charging current in the second stage is 1/2 smaller than the charging current in the first stage. The purpose of reducing the charging current is to reduce the temperature rise of the battery and the precipitation of gas.

During the entire charging process, it must be carefully checked to ensure that the electrolyte temperature of each single cell does not exceed 45°C. At the beginning of charging, the electrolyte temperature of each single cell should be measured every 1~2h, and then select a single cell with the highest electrolyte temperature as the representative of the highest temperature rise of the battery pack. When the battery has begun to have obvious gas evolution, in the charging state, use a voltmeter to select the single cell with the highest terminal voltage as the pilot battery.

Generally speaking, this battery also has the highest temperature rise of the electrolyte. Using this method to determine the pilot battery is accurate and convenient. In the first stage of charging, the current receiving rate is very low, and the current of 0.05C20 is used to charge for a period of time. The principle is to control the electrolyte temperature below 40 °C. If the temperature rises to 40°C, the current should be reduced; if the temperature rises to 45°C, the charging should be stopped, and the charging should be carried out after the temperature of the electrolyte decreases. If the temperature of the electrolyte is too high, a large amount of active substances on the plate will fall off, resulting in permanent damage to the battery. In fact, the size of the charging current is mainly restricted by the temperature rise of the electrolyte and the amount of gas evolution. As long as the temperature does not exceed 40 °C, the gas output is not large. At this time, the proportion of current energy converted into chemical energy is very high. It doesn’t matter. Fast charging is designed based on this prerequisite.

During the charging process, when measuring the terminal voltage of the battery with a voltmeter, the charging current should not be disconnected. It is best to use a 3-0-3 voltmeter or a digital voltmeter. Since the single cell may have a large internal resistance difference, the average value of the total charging voltage on a single cell cannot be simply used as the terminal voltage of the single cell. At the beginning of charging, the interval time for measuring the voltage can be limited to 3~4h, and it should be measured once every hour after 12h. In the initial charging operation, all the active materials on the original plate should participate in the electrochemical reaction (Figure 1):

Figure 1 - The activation reaction process of the positive and negative plates at the initial charge
Figure 1 – The activation reaction process of the positive and negative plates at the initial charge

If the lead sulfate on the electrode plate is not completely converted into PbO2 and Pb during the initial charging, it is impossible to activate that part in the ordinary supplementary electric operation during the application process. Moreover, due to the existence of coarse PbSO4 seeds, the vulcanization of the electrode plate is accelerated. Therefore, it must be fully charged. The charged power should be 1.5 to 3 times the capacity of C20. The duration of initial charging is sometimes as long as 70h. If the battery load voltage value no longer increases as measured by the battery capacity meter, the charging can be stopped.

When the battery is fully charged, it has the following characteristics.
① The terminal voltage of the battery and the density of the electrolyte remain stable for 3 hours.
②Stop charging for 15 minutes. When charging again, air bubbles immediately emerge from the battery. In the initial charging operation, the electrolyte has a process of changing from “clear” to “mixed” and then to “clear”. When the electrolyte is first added, due to the violent reaction of the battery, some substances on the plate are dissolved into the electrolyte, and some additives in the plate, such as carbon black, will also enter the electrolyte in a suspended state, so the electrolyte changes. “Mixed”. With the progress of the charging process, some ions enter the plate under the action of the electric field force, and the oxygen generated during charging oxidizes part of the suspended matter. Both processes play a role in purifying the electrolyte. As a result, the electrolyte gradually changes from “mixed” to “clear”. This phenomenon can also be used as a basis for judging the degree of charging.

The above are all qualitative judgments. The quantitative basis for judging the degree of charge is to directly measure the retained capacity of the battery with the battery capacity meter. If the holding capacity of the battery increases gradually with the charging time, charging is beneficial; if the holding capacity of the battery does not increase with the charging process, the charging at this time is harmful and unhelpful to the battery. If the electrode plate has broken ribs in the battery and a large amount of active material has fallen off, the retained capacity of the battery can only reach a level corresponding to the degree of damage. Therefore, when the electricity meter determines whether the battery is fully charged, it has nothing to do with the magnitude of the value, but only depends on whether the measured value is still increasing. After the first charge, if the measured battery capacity has reached or even exceeded the nominal capacity, the battery can be put into use.

The batteries produced in the 1950s and 1960s had a cycle requirement of 6 times of charging and 5 times of discharge. At that time, only by doing this could the battery reach its rated value. With the advancement of technology, in the 1990s, many manufacturers have been able to reach the level of “a single charge to reach the nominal capacity”, which brings a lot of convenience to users. The next discharge cycle should only be performed if the nominal capacity is not reached on the first charge.

The first discharge operation, in order to improve the efficiency, can be carried out with a 5h rate current, the current should be stable, and it should be carried out continuously and uninterruptedly. During discharge, the terminal voltage of each single cell must be carefully checked. Measure every 1h when starting to discharge. When the terminal voltage of a single battery reaches 1.8V, it should be changed to be measured every 15min. When a single cell drops to 1.70V, the discharge stops.

If the depth of discharge is too large for the first discharge, the effect of the second charge will be reduced. If a single cell shows a significant increase in temperature and low capacity, the single cell should be removed from the battery pack.

The second charge should be carried out immediately after the first discharge, and the time between discharge and charge cannot be extended arbitrarily. The selection of the second charging current depends on the temperature of the electro-hydraulic fluid, and the maximum temperature limit is the same as the first charging.

The basis for judging the end of the second charging may refer to the first charging. If there is no abnormal damage inside the battery, there is no need to adjust the electrolyte density. When density adjustment is required, it should be carried out in the charged state. If after the second charge, the battery capacity still does not reach the nominal capacity, another discharge and charge cycle should be performed. If the nominal capacity is still not reached after the third charge, the battery should be used at a reduced level.

Now the initial charging process program is drawn as Figure 2. At points 1 to 3 in the figure, use a CB meter to measure the remaining capacity of the battery. If it has reached the standard of use, it can be used. It is not necessary to complete 3 charging and 2 discharging.

Figure 2 - The process of 3 charging and 2 discharging of the battery
Figure 2 – The process of 3 charging and 2 discharging of the battery

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