The failure of lead-acid batteries may be caused by a variety of reasons, such as vulcanization, water loss, thermal runaway, active material shedding, plate softening, etc. Next, we will introduce and analyze them one by one.

1. Vulcanization

The process of charging and discharging lead-acid batteries is a process of electrochemical reaction. When discharging, lead sulfate is generated, and when charging, lead sulfate is reduced to lead oxide. This electrochemical reaction process is cyclically reversible under normal circumstances, but lead sulfate is a salt that is easy to crystallize. When the concentration of lead sulfate in the electrolytic solution in the battery is too high or the static idle time is too long, it will "hold" These small crystals then attract the surrounding lead sulfate to form large inert crystals like snowballs, which destroys the originally reversible cycle and makes the lead sulfate partially irreversible. When the crystallized lead sulfate is charged, it can not be reduced to lead oxide, but also adsorbed on the grid, resulting in a decrease in the working area of the grid, the lead-acid battery loses water due to heat, and the capacity of the lead-acid battery decreases. This phenomenon is called vulcanization. Also known as aging. Vulcanization can also lead to "complications" such as short circuit, active material relaxation and shedding, and grid deformation and fracture.

As long as it is a lead-acid battery, it will be vulcanized during use, but the lead-acid battery in other fields has a longer life than the lead-acid battery used in electric bicycles. This is because the lead-acid battery of electric vehicles has a Easier vulcanization working environment. Different from the starting battery for automobiles, after the automobile battery is ignited and discharged, the battery is always in a floating state, and the lead sulfate formed by discharge is quickly converted into lead oxide. When the electric vehicle is discharged, it is impossible to charge at the same time, which causes sulfuric acid. A large amount of lead accumulates. If it is deeply discharged, the concentration of lead sulfate will be higher at this time, and it is difficult to charge the electric vehicle in time after riding. The lead sulfate formed by the discharge cannot be charged and converted into lead oxide in time, and crystals will be formed. Therefore, the cycle life varies greatly according to the depth of discharge. The deeper the depth of discharge, the fewer the number of cycles, the shallower the depth of discharge, the more the number of cycles. According to the test results, the depth of discharge and the number of cycles are related to the following table:

Some lead-acid batteries do 70% 1C charging and 60% 2C discharge, because the continuous high current cycle is used, which destroys the conditions for the battery to generate large lead sulfate crystals, so it may not be seen that lead-acid battery vulcanization damages the battery. . If the test is stopped in the middle, the problem of lead-acid battery vulcanization will appear. Due to the heavy weight of the battery, some users often recharge the battery after multiple uses and discharge, so that the lead-acid battery is not charged in time after being discharged, and the vulcanization of the lead-acid battery is more serious. In addition, the proportion of sulfuric acid in lead-acid batteries is relatively high, which is also an important factor in the vulcanization of lead-acid batteries. The vulcanization of lead-acid batteries destroys the ability of the negative plate to circulate oxygen, resulting in accelerated water loss. In this way, the proportion of sulfuric acid in the lead-acid battery is higher, which makes it easier to cause the lead-acid battery to vulcanize. Therefore, the degree of vulcanization of lead-acid batteries may be different, but the impact on the life of lead-acid batteries is common.

2. Loss of water

One of the most basic principles of sealed lead-acid batteries is that after the oxygen evolution of the positive plate, the hydrogen evolution of oxygen directly to the negative plate and the negative plate is reduced to water. The parameter for evaluating the technical index of lead-acid batteries is called "sealing reaction efficiency". This phenomenon It's called the "oxygen cycle". In this way, the water loss of the lead-acid battery is very small, and the "maintenance-free" is realized, that is, no water is added. But this oxygen cycle of sealed lead-acid batteries is disrupted on electric bicycles, causing a lot of water loss in the battery.

In order to ensure that the battery is fully charged within 8 hours, in the three-stage constant voltage current-limiting charging, for example, the constant voltage of a 36-volt charger is 44.4 volts, and there are 18 cells in three single cells, and the voltage per cell is 2.466 V. In this way, it greatly exceeds the 2.35V of the oxygen evolution voltage of the positive plate of the battery and the 2.42V of the hydrogen evolution voltage of the negative plate. In order to reduce the charging time indication, the products of some charger manufacturers increase the current from constant voltage to float charging, so that after the charging indication is fully charged, it is not fully charged, and the float voltage is increased to make up. In this way, the float voltage of many chargers exceeds the single-cell voltage of 2.35V, so that a large amount of oxygen is still evolved during the float charge stage. The oxygen cycle of lead-acid batteries is not good, so it is also constantly exhausted during the float charging stage.

A set of 36-volt lead-acid batteries has 3 single cells, each single cell has 6 cells, and each cell has more than 15 positive and negative grid plates, and a set of batteries has at least 270 solder joints. One-thousandth of the virtual welding will inevitably lead to a group of 4 batteries that are unqualified, and the lead-calcium board is very easy to cause virtual welding due to calcium precipitation, so battery manufacturers generally use low-antimony alloy plates, and low-antimony alloys The gas evolution voltage of the battery is lower, the gas output of the battery is larger, and the water loss is more serious.

The standard proportion of sulfuric acid for floating lead-acid batteries should be between 1.21 and 1.28. However, in order to meet the requirements of large-capacity and high-current discharge of electric bicycles, the proportion of sulfuric acid in batteries is generally around 1.36 to 1.38. Since the proportion of sulfuric acid in the battery is relatively high, the vulcanization of the battery is relatively serious. The vulcanization of batteries with high specific gravity of sulfuric acid is obvious from the discharge of the battery to the charge of the next day. In this way, the ability of the negative plate to circulate oxygen is further reduced. After the battery loses water, the main loss is water, leaving the composition of sulfuric acid, which is equivalent to further increasing the proportion of sulfuric acid, which makes the lead-acid battery easier to vulcanize. Therefore, lead-acid battery vulcanization aggravates water loss, and water loss aggravates vulcanization. For users, "sealing" is necessary, otherwise the consequences of acid overflow will be disastrous, but it is inappropriate to promote the concept of "maintenance-free" excessively in the field of electric vehicles.

3. Thermal runaway

After the lead-acid battery is charged to 70%, the polarization voltage of the lead-acid battery is relatively high, the side reaction of charging begins to increase gradually, and the electrolysis of water begins. After the charged cell voltage reaches 2.35V, the positive plate begins to evolve oxygen, and after reaching 2.42V, the negative plate begins to evolve hydrogen. At this time, the charged electrical energy is converted into chemical energy, and the energy converted into electrolyzed water is increased. Whether or not gassing during the charging process depends on the charging voltage, and the amount of gassing depends on the charging current after reaching the gassing voltage. Therefore, during the charging process, after the charging voltage enters the constant voltage, the voltage begins to approach the highest, and the charging current also maintains the current limit value. At this time, the gas evolution is the largest. After entering the constant voltage, the charging current should gradually decrease, and the gas evolution should also decrease gradually. Charging itself is an exothermic reaction. Generally, the thermal design of lead-acid batteries can control the temperature rise. After a large amount of gas evolution of the lead-acid battery, the oxygen is compounded into water on the negative plate, and the calorific value is much greater than that during charging. The sealed lead-acid battery expects the negative plate to have good oxygen circulation capability, however, the oxygen circulation will generate heat. Therefore, the oxygen cycle is a double-edged sword. The advantage is that water loss is reduced, and the disadvantage is that the battery will heat up.

Under the condition of constant voltage charging, the oxygen cycle current also participates in the charging current, so the rate of decrease of the charging current is slowed down. The heat of the lead-acid battery will cause the charging current to drop more slowly, or even increase the current. Under the action of the battery heating, the charging current increases the heating once the current rises. In this way, the charging current will always rise to the current limit value. The battery generates high heat and accumulates heat until the battery case undergoes thermal softening and deformation. When the battery is thermally deformed, the internal air pressure is high, so the battery swells. This is a battery thermal runaway that damages the battery. Once the lead-acid battery is seriously swollen, the problems of acid leakage and gas leakage also appear, and the lead-acid battery will fail acutely. There are many reasons that induce battery bulge. If the charging voltage is high and the gas evolution is large, thermal runaway will occur. If a group of batteries or a single-cell battery is seriously behind, and the constant voltage value of charging remains unchanged, other single-cell batteries will also have a relatively high charging voltage, which will also cause thermal runaway problems. In order to reduce the probability of thermal runaway of the battery, many charger manufacturers reduce the constant voltage value to 43 volts, which will inevitably lead to undercharging.

Another cause of lead-acid battery charging and heating is vulcanization. The vulcanization directly leads to an increase in the internal resistance of the battery, which further causes the lead-acid battery to generate heat during charging, and the heat increases the oxygen circulating current. Therefore, the probability of thermal runaway occurs in severely vulcanized batteries. very large. It is proved from the analysis of the failure mode of lead-acid batteries of electric bicycles that 90% of the failed batteries are accompanied by serious water loss. Gel batteries lose less water than ordinary batteries, so their life should be longer than ordinary batteries. The internal self-discharge of the gel battery is not larger than that of the ordinary battery during storage, which can be proved by the comparison of the capacity drop after storage. Under the same internal pressure condition of lead-acid battery, the gas evolution of gel battery is less than that of ordinary battery. And every time the valve is opened, the gas will take away part of the heat. The valve opening of colloidal lead-acid battery is less than that of ordinary lead-acid battery, and it has less water loss. . The internal temperature of the battery increases, the self-discharge is also large, and the heat generated is higher. Therefore, under the condition of high ambient temperature in summer, due to the decrease of gas evolution level, the gas evolution amount is the closest, and the temperature rise is also high. In this way, the probability of colloidal lead-acid batteries entering thermal runaway is much greater.

4. The active material falls off and the plate softens

The active ingredient of the active material of the positive plate of the lead-acid battery is lead oxide. Lead oxide is divided into α-PbO2 and β-PbO2. Among them, α-PbO2 has hard physical properties and relatively small capacity. Plate area and support plate; β-PbO2 is attached to the skeleton composed of α-PbO2, and its charging capacity is much stronger than that of α-PbO2. After lead oxide is discharged, lead sulfate is formed, and lead sulfate is reduced to lead oxide during charging. In a strong acid environment, lead sulfate can only generate β-PbO2, and the shedding of active substances is α-PbO2 shedding. There are many reasons for the shedding of active substances:

1. The active material distribution of the lead-acid battery plate is uneven, resulting in different expansion tension and falling off during discharge.

2. When the lead-acid battery is over-discharged and under-voltage, β-PbO2 is greatly reduced, and α-PbO2 will participate in the discharge reaction to generate lead sulfate.

3. The expansion tension of the sulfide crystal growing on the electrode plate will also cause the active material to fall off. Once the positive plate softens, the supporting porous structure is destroyed, and the pores of the positive plate are compacted by the pressure of the battery plate, which reduces the real area involved in the reaction, and the lead-acid battery capacity decreases. In this way, preventing overdischarge, suppressing and eliminating sulfidation are important measures to control the softening of the positive plate. When discharging, each discharge, more or less, always have a little bit of α-PbO2 involved in the reaction.

Therefore, for a normally used lead-acid battery, without losing water, vulcanization, or overdischarge, the battery life depends on the softening of the positive plate. Battery capacity is affected by active material and utilization. Electric vehicle lead-acid batteries have a certain shape and size, and the quality of the plates has been limited to a certain extent. Only by improving the utilization rate of active substances can the capacity be increased. To increase the capacity of lead-acid batteries, it is necessary to increase the porosity, increase the content of PbO2 and the proportion of sulfuric acid, but these measures will accelerate the softening of the positive plate, resulting in accelerated decline of the life of the lead-acid battery, and the active material will expand and shrink during the charging and discharging process (especially It is the positive plate), the deeper the discharge depth, the greater the expansion and shrinkage of the active material, which accelerates the softening of the active material. Therefore, when the initial capacity is too large, it directly affects the life of the lead-acid battery.

5. Short circuit

The short circuit of the lead-acid battery refers to the connection of the positive and negative groups inside the lead battery. In order to increase the capacity of the lead-acid battery, the number of plates in the lead-acid battery of electric vehicles is generally increased by increasing the number of plates, which makes the separator relatively thinner than the separators of other batteries, and the lead sulfate crystals of the negative plate grow up. After charging, a small amount of lead sulfate is left in the separator. Once the lead sulfate left in the separator is reduced to lead, and accumulates too much, the lead-acid battery will have a micro-short circuit. This phenomenon is called "lead branch bridge". . Micro-short circuit lightly produces the single-cell voltage lag, and when serious, single-cell short-circuit occurs. The active material on the plate expands and falls off, which will also cause the positive and negative plates to be connected.

6. Equilibrium problems

Many lead-acid batteries can get better results in the single test. However, for series-connected lead-acid batteries, due to the original configuration errors such as capacity difference and open circuit voltage difference, the battery with high voltage during charging will be damaged. When water loss increases, the battery with low voltage will be undercharged, and when discharging, the battery with low voltage will be overdischarged, resulting in lead-acid battery vulcanization. With the cycle of charge and discharge, the vulcanized monomer of the lead-acid battery is more easily vulcanized, and this difference is enlarged, which ultimately affects the life of the entire battery pack.

7. Unable to charge

The termination discharge voltage of a 12V lead-acid battery is 10.5 volts. If it is forcibly discharged below the termination voltage, the lead-acid battery has a great chance of losing its ability to recharge. There is a protection device in the controller of the electric vehicle. When the lead-acid battery reaches the terminal voltage, the protection device will forcibly disconnect the circuit, but if the protection device drifts upward, or the battery voltage rises after the power failure, the protection device cannot be judged correctly.

8. Lead-acid battery self-discharge

The phenomenon that a fully charged lead-acid battery is left unused and gradually loses its power is called self-discharge. Self-discharge is inevitable, and under normal circumstances, the daily discharge rate should not exceed 0.35%~0.5%. The main reasons for the self-discharge of lead-acid batteries: (1) There are impurities in the electrode plate or electrolyte, and a potential difference is generated between the impurities and the electrode plate or between different impurities, which becomes a local battery, and a circuit is formed through the electrolyte solution to generate a local current. Discharge lead-acid batteries. (2) The separator is broken, resulting in a short circuit of the positive and negative plates. (3) There is electrolyte or water on the surface of the lead-acid battery shell, which becomes a conductor between the poles, causing the lead-acid battery to discharge. (4) The active material falls off too much and is deposited on the bottom of the battery, short-circuiting the plate and causing discharge.

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