The failure mechanism of lead-acid batteries

by:Power Kingdom     2021-04-28
The failure study of lead-acid batteries is of great significance to the safe operation of the power supply system. We will discuss this issue briefly so that readers have a general understanding of this issue.  1.1 Water loss in the battery   Lead-acid battery water loss will increase the specific gravity of the electrolyte, cause corrosion of the positive grid of the battery, reduce the active material of the battery, and reduce the capacity of the battery and cause it to fail.   The difficulty of sealing lead-acid batteries is the electrolysis of water during charging. When charging reaches a certain voltage (generally above 2.30V/cell), oxygen is released on the positive electrode of the battery, and hydrogen is released on the negative electrode. On the one hand, the released gas brings out the acid mist and pollutes the environment. On the other hand, the moisture in the electrolyte is reduced, and it is necessary to add water for maintenance at intervals. The valve-regulated lead-acid battery is a product developed to overcome these shortcomings. Its product features are:    (1) The use of multiple high-quality grid alloys improves the over-potential of gas release. That is, ordinary battery grid alloy releases gas when it is above 2.30V/cell (25℃). After using high-quality multi-element alloys, gas will be released when the temperature is above 2.35V/single (25°C), thereby reducing the amount of gas released.   (2) Allow the negative electrode to have excess capacity, that is, 10% more capacity than the positive electrode. In the later stage of charging, the oxygen released from the positive electrode contacts the negative electrode and reacts to regenerate water, that is, O2+2Pb→2PbO, PbO+H2SO4→H2O+PbSO4, so that the negative electrode is under-charged due to the role of oxygen, and therefore does not produce hydrogen. The oxygen in the positive electrode is absorbed by the negative lead, and the process is further transformed into water, which is the so-called cathode absorption.   (3) In order to allow the oxygen released from the positive electrode to circulate to the negative electrode as soon as possible, a new ultra-fine glass fiber separator that is different from the microporous rubber separator used in ordinary lead-acid batteries must be used. The porosity is increased from 50% of the rubber separator to more than 90%, so that oxygen can easily circulate to the negative electrode and be reconstituted into water. In addition, the ultra-fine glass fiber board has the function of adsorbing sulfuric acid electrolyte, so the valve-regulated sealed lead-acid battery adopts a lean-liquid design, and even if the battery is dumped, there is no electrolyte overflow.   (4) The sealed valve-controlled acid filtering structure is adopted to prevent acid mist from escaping, achieving the purpose of safety and environmental protection.  In the above cathode absorption process, since the water produced cannot overflow under the sealed condition, the valve-regulated sealed lead-acid battery can be exempted from water supplement maintenance. This is also the origin of the valve-regulated sealed lead-acid battery called the non-dimensional battery.  Valve-regulated sealed lead-acid batteries are equipped with acid filter pads, which can effectively prevent acid mist from escaping. However, it is conditional for the sealed battery not to escape gas, that is: no gas should escape during the storage period; no gas should escape if the charging voltage is below 2.35V/cell (25℃); no gas should escape during the discharge period . But when the charging voltage exceeds 2.35V/cell, the gas may escape. Because a large amount of gas is generated in the battery body for a short period of time to be absorbed by the negative electrode, when the pressure exceeds a certain value, it will start to exhaust through the one-way exhaust valve. Although the exhaust gas is filtered out by the filter acid pad, the acid mist is filtered out. The battery loses gas, so the valve-regulated sealed lead-acid battery has very strict requirements on the charging voltage and cannot cause overcharging. 1.2 Negative plate The main active material of the negative grid plate of a sulfated battery is spongy lead. When the battery is charged, the negative grid plate undergoes the following chemical reaction: PbSO4+2eu003dPb+SO4-, and oxidation reaction occurs on the positive electrode: PbSO4+2H2Ou003dPbO2+ 4H++SO4-+2e, the chemical reaction in the discharge process is the reverse reaction of this reaction. When the charge of the valve-regulated sealed lead-acid battery is insufficient, there will be Pb on the positive and negative grids of the battery, and PbSO4 If it exists for a long time, it will lose its activity and can no longer participate in the chemical reaction. This phenomenon is called sulfation of active material. Sulfation reduces the active material of the battery, reduces the effective capacity of the battery, and also affects the gas absorption capacity of the battery. The battery has failed.   In order to prevent the formation of sulfation, the battery must always be kept in a fully charged state. 1.3 Corrosion of the positive electrode plate Due to the loss of water in the battery, the electrolyte's specific gravity will increase. Excessive electrolyte acidity will aggravate the corrosion of the positive electrode plate, which will increase the porosity of the positive electrode plate, reduce the relative electrolyte, and reduce the active material of the electrode plate, and the battery capacity will be lower. . To prevent corrosion of the electrode plates, attention must be paid to prevent battery loss from occurring.  1.4 Thermal runaway   Thermal runaway refers to a cumulative increase in the charging current and battery temperature when the battery is charged at a constant voltage, and the battery is gradually damaged. The root cause of thermal runaway is:    Ordinary flooded lead-acid batteries are filled with liquid between the positive and negative plates, and there is no gap, so the oxygen generated by the positive electrode cannot reach the negative electrode during the charging process, so the negative electrode is not depolarized. It is easy to produce hydrogen gas and escape the battery with oxygen.  Because the heat cannot be dissipated by the way of loss of water, the heat generated during the overcharging of VRLAB batteries is more than that of flooded lead-acid batteries. Thermal runaway is more likely to occur.  
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