You must know the reasons for the failure of lead-acid batteries and how to fix them

You must know the reasons for the failure of lead-acid batteries and how to fix them

The current best way to store electricity from solar cells is with lead-acid batteries. Because lead-acid batteries have many excellent properties, especially the characteristics of large capacity and high current discharge, they cannot be replaced by other batteries for the time being. However, lead is poisonous, expensive, and has a short service life. These outstanding problems have not yet been well resolved. If the cause of the failure of the lead-acid battery can be found, the service life of the lead-acid battery can be prolonged, so that the cost can be relatively reduced, and it is also conducive to environmental protection. In addition to lead-acid batteries, the performance of lithium batteries is not bad and can be comparable to them. Click here to open to learn about high-quality lithium battery products.

  1. Reasons for the failure of lead-acid batteries

(1) The corrosion deformation of the positive plate The positive grid made of lead alloy will be oxidized to lead sulfate and lead dioxide during the charging process of the battery. After the battery is discharged, if it is not charged for a long time, the active material in the battery will soon lose its activity, causing an irreversible chemical reaction inside the battery, and eventually the active material will lose its function and the battery will fail. In addition, the formed lead dioxide corrosion layer causes stress to the positive grid, which grows and deforms. Once this deformation exceeds 4%, the entire plate will be destroyed, and the active material will fall off due to poor contact with the grid.

(2) Falling off and softening of the active material of the positive plate In addition to the falling off of the active material caused by the growth of the grid, as the charge and discharge are repeated, the bonding between the lead dioxide particles also relaxes and softens, and falls off the grid. A series of factors, such as the manufacture of the grid, the tightness of the assembly, and the charging and discharging conditions, all have an impact on the softening and shedding of the active material of the positive plate.

(3) When an irreversible sulfated battery is over-discharged and stored in a discharged state for a long time, its negative electrode will form a coarse lead sulfate crystal that is difficult to accept charging. This phenomenon is called irreversible sulfation. Slight irreversible sulfation can still be recovered by some methods. In severe cases, the electrode fails and cannot be charged.

(4) Premature loss of capacity For grid alloys made of low ladder or lead-calcium, the capacity suddenly drops in the early stage of battery use (about 20 cycles), making the battery ineffective. The reason is that with the increase of the number of cycles on the positive grid, a part of it will be transferred to the surface of the active material of the negative plate, resulting in a decrease in the charging voltage, most of the current is used for water splitting, and the battery cannot be charged normally and thus fails. Therefore, the ladder content of the active material of the negative electrode of the lead-acid battery with a charging voltage of only 2.3V should be tested.

(5) In actual use of thermally-failed lead-acid batteries, if the charging voltage is too high, the charging current will be too large, the heat released will be too much, and the temperature of the electrolyte in the battery will increase, resulting in a decrease in the internal resistance of the battery. . The drop in internal resistance in turn increases the charging current. That is, if the charging current is too large, the internal resistance decreases; if the internal resistance decreases, the charging current increases, and the temperature rise and current of the battery are too large, which strengthen each other and eventually become uncontrollable, causing the battery to deform, crack and fail. Therefore, when charging, do not make the charging voltage too high and the battery heating should not be too serious.

(6) Corrosion of the negative busbar If oxygen circulation is established in the valve-regulated sealed battery, the upper space of the battery will be filled with oxygen. When the electrolyte rises along the tab to the busbar, the alloy of the busbar will be oxidized to form sulfuric acid. lead. If the busbar electrode alloy is not properly selected, and the busbar has slag inclusions and gaps, the corrosion along the gap will deepen, causing the tabs to separate from the busbar and the negative plate to fail. However, under normal circumstances, there is no corrosion problem on the negative grid and busbar.

(7) Diaphragm perforation causes short circuit. Individual varieties of diaphragms (made of polypropylene) have larger pore sizes, and the polypropylene fuse will be displaced during use, resulting in large pores through which active substances can pass through during charging and discharging. Large holes, causing micro short circuits and making the battery useless.

  1. Factors affecting the life of lead-acid batteries

The failure of lead-acid batteries is the result of a combination of many factors, which depends not only on the internal factors of the plate, such as the composition of the active material, crystal form, porosity, plate size, grid material and structure, etc., but also on a series of external factors. Factors such as discharge current density, electrolyte concentration and temperature, depth of discharge, maintenance conditions and storage time, etc.

(1) The main external factors

①Discharge depth The discharge depth is the degree to which the discharge starts and stops during use. 100% depth means full capacity release. The life of lead-acid batteries is greatly affected by the depth of discharge. The key point of design consideration is deep cycle use, shallow cycle use or floating charge use. If a shallow cycle battery is used for a deep cycle, the lead-acid battery will quickly fail. Because the positive active material lead dioxide itself is not firmly bonded to each other, lead sulfate is generated during discharge, and it returns to lead dioxide during charging. The molar volume of lead sulfate is larger than that of lead oxide, so the volume of the active material expands during discharge. If 1 mol of lead oxide is converted into 1 mol of lead sulfate, the volume will increase by 95%. In this way, the repeated shrinkage and expansion will gradually loosen the mutual bonding between the lead dioxide particles, which is easy to fall off. If only 20% of the active material of 1 mol of lead dioxide is discharged, the degree of shrinkage and expansion will be greatly reduced, and the damage of the binding force will be slow. Therefore, the deeper the discharge depth, the shorter the cycle life.

②Overcharge degree When overcharging, a large amount of gas is precipitated. At this time, the active material of the positive plate is impacted by the gas, and this impact will promote the shedding of the active material; Charging will shorten the application period.

③The influence of temperature The life of lead-acid batteries increases with the increase of temperature. At 10~35℃, for every 1℃ increase, add 5~6 cycles, at 35~45℃, for every 1℃ increase, the life can be extended for more than 25 cycles. Above 50℃, it will be caused by the loss of anode vulcanization capacity. Reduce lifespan.
Battery life increases with temperature over a range of temperatures because capacity increases with temperature. If the discharge capacity does not change, the depth of discharge decreases when the temperature increases, so the life is extended.

Relationship between battery discharge capacity and temperature
Relationship between battery discharge capacity and temperature

④Influence of sulfuric acid concentration The increase in acid density is beneficial to the capacity of the positive plate, but the self-discharge of the battery increases and the corrosion of the grid accelerates, which also promotes the looseness of lead dioxide. life expectancy decreased.

⑤ Influence of discharge current density As the discharge current density increases, the life of the battery decreases, because under the conditions of high current density and high acid concentration, the lead dioxide of the positive electrode is loosened and shed.
⑥Effect of water loss For open batteries, water loss is a normal phenomenon, but for sealed batteries, it should not occur under strict control. However, sometimes the constant voltage value of charging is too high, and the sealed battery may lose water.

⑦ Influence when the grid is a low alloy When the grid is a low-ladder alloy, the capacity of the battery will suddenly drop in the initial period of use (about 20 cycles), which will cause the battery to fail. Almost every cycle the battery capacity will drop by 5%, and the rate of capacity drop is faster and earlier.

(2) Solution

①Control the content of tin in the positive plate. For deep cycle batteries, a tin content of 1.5% to 2% is basically used.
② Lead-acid batteries should not be left idle for too long.
③The content of electrolyte (concentrated sulfuric acid) should not be too high.
④ Avoid too many overcharging times, and the initial charging current is continuously too low.
⑤ Reduce deep discharge.
⑥Increase assembly pressure.
⑦ The higher the utilization rate of active materials, the greater the discharge capacity, but do not increase the battery capacity through excessive utilization of active materials. Batteries with early capacity losses can be recovered.
First, the initial charging current should be increased, and then a small current should be used to supplement the charging; secondly, the fully charged battery should be stored at 40~60℃. Repeatedly use a small current for several times of discharge (the discharge will be very slow after the battery voltage reaches half of the nominal voltage), and the capacity of the battery can be recovered.

(3) Precautions For the phenomenon of battery capacity decline, be sure to identify whether it occurs in the first 20 cycles. If the battery has a capacity drop in the middle and late stages, the above method will destroy the positive plate of the battery, resulting in softening of the positive plate.

Lead-calcium alloy series batteries often have several battery capacity drops for no reason, mainly caused by battery imbalance. The sufficient voltage of lead-calcium alloy series batteries is relatively high, generally 12V, and the charging voltage is greater than 16V. When the voltage of the charger is too low, it is easy to cause battery imbalance. When assembled together, the self-discharge of each cell of the battery cannot be absolutely equal. The battery with a larger self-discharge cannot be fully charged every time the constant voltage charger is used. The larger the relative area of ​​the plate in contact with the electrolyte, the larger the self-discharge. The grid with small self-discharge can be fully charged every time. When it is fully charged and then overcharged a little, gas evolution reaction occurs, gas is generated, the contact surface of the electrode plate is relatively reduced, and the self-discharge is reduced. At the same time, the charging voltage increases.

The self-discharge of cells with small self-discharge and high voltage is getting smaller and smaller, and can be fully charged every time, while the self-discharge of cells with large self-discharge is getting bigger and bigger, and it cannot be fully charged each time, and the smaller the power is used, the longer the battery is. If it is not enough, it will vulcanize and fail. The root of the problem is that the constant voltage charger cannot be used. When using a constant voltage charger, the above phenomenon will occur if the constant voltage value is too low. If the constant voltage value is too high, the battery will run out of control. The best way is to use a variety of currents. , Multi-stage chargers with various voltages, and at the end of charging, there should be a long charging process with higher voltage and lower current to balance the battery power.

Overcharging often requires large current and high voltage, and both large current and high voltage will form a strong side reaction and damage the positive plate of the battery, and will also cause water loss of the battery. The overcharge repair can be achieved by the pulse method. The basic principle is as follows: the use of high voltage and high current pulses overcomes the decline in battery acceptance caused by various reasons. Due to the use of pulses, after the high current pulse disappears, the battery itself (or imposed conditions) depolarization ability without serious side effects. Due to the birth of this overcharge repair method, damage-free overcharge can be realized. After several years of experiments, this method has greatly extended the cycle life of lead-acid batteries.

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