Failure modes and repair methods of lead-acid batteries (1/3)

At present, the battery is still mainly lead-acid battery according to its capacity. Lead acid battery has the advantage of large capacity, which can not be replaced by other batteries at present. In addition, its high current discharge characteristics also determine its advantages in starting battery. However, as a heavy metal, lead has certain toxicity in addition to its cost, which has different degrees of harm to the environment and human body. Therefore, prolonging the life of lead-acid batteries is not only to reduce the operating costs, but also to meet the needs of environmental protection. It is also an important issue to expand the application field of lead-acid batteries. Therefore, the research on repairing lead-acid battery and prolonging its service life will not only reduce the sales volume of lead-acid battery, but also increase the pollution to the environment.

In order to understand the repair of lead-acid battery, we must first understand the failure mode of lead-acid battery. Then, according to different failure modes, repair methods are discussed.

1. Failure mode of lead acid battery

Due to the differences in the types, manufacturing conditions and use methods of the plates, the final causes of battery failure are different. To sum up, the failure of lead-acid battery has the following conditions:

1.1 Corrosion deformation of positive plate

At present, there are three kinds of alloys used in production: traditional lead antimony alloy, antimony content is 4% - 7%, low antimony or ultra-low antimony alloy, antimony content is 2% or less than 1% mass fraction, containing tin, copper, cadmium, sulfur and other deformation agents; lead calcium series, actually lead calcium tin aluminum quaternary alloy, calcium content is 0.06% - 0.1% mass fraction. The positive grid made of the above-mentioned alloys will be oxidized into lead sulfate and lead dioxide during the charging process of the battery, which will eventually lead to the loss of the supporting active material and make the battery invalid; or the formation of the corrosion layer of lead dioxide will cause the lead alloy to produce stress and make the grid grow and deform. When the deformation exceeds 4%, the whole plate will be destroyed, and the active material and grid will be destroyed Poor contact and falling off, or short circuit at the bus bar.

1.2 The active material of positive plate falls off and softens.

In addition to the active material shedding caused by the growth of the grid, the binding between PbO2 particles is also relaxed and softened with the repeated charging and discharging, and the particles fall off from the grid. A series of factors, such as grid manufacturing, assembly tightness and charge discharge conditions, have an impact on the softening and shedding of active materials in the positive plate.

1.3 Irreversible sulfation

When the battery is over discharged and stored in the state of discharge for a long time, its negative electrode will form a kind of coarse lead sulfate crystal which is difficult to accept charging. This phenomenon is called irreversible sulfation. Some methods can be used to recover the slight irreversible sulfation. If it is serious, the electrode will be invalid and cannot be charged.

1.4 Premature loss of capacity

When the low sb or Pb Ca alloy is grid alloy, the capacity of the battery will drop suddenly in the initial use period (about 20 cycles), which will lead to the failure of the battery.

1.5 Heavy accumulation of antimony on active substances

Antimony on the positive grid is partially transferred to the surface of the active material of the negative plate along with the cycle. Since the overpotential of H + reduction on antimony is about 200mV lower than that on lead, the charging voltage decreases when antimony accumulates, most of the current is used for water decomposition, and the battery fails to charge normally.

The content of antimony in the negative active material of lead-acid battery was tested when the charging voltage was only 2.30v. It was found that the content of antimony in the surface layer of negative active material was 0.12% - 0.19%. For some batteries, such as submarine batteries, hydrogen evolution is limited. The content of antimony in the negative electrode of batteries with hydrogen evolution exceeding the standard was tested, and the average antimony content was 0.4%.

1.6 Thermal failure

For the battery with less maintenance, the charging voltage should not exceed 2.4V per cell. In practical use, for example, in the automobile, the voltage regulator may be out of control, the charging voltage is too high, so the charging current is too large. The heat generated will increase the temperature of the battery electrolyte, resulting in the decrease of the internal resistance of the battery, which will strengthen the charging current. The excessive temperature rise and current of the battery strengthen each other, and eventually can not be controlled, resulting in the failure of the battery due to deformation and cracking. Although hotRunaway is not a common failure mode of lead-acid battery, but it is also common. Attention should be paid to the phenomenon of high charging voltage and battery heating when using.

1.7 Corrosion of negative bus bar

In general, there is no corrosion problem in the negative grid and bus bar, but in the valve regulated sealed battery, when the oxygen cycle is established, the upper space of the battery is basically filled with oxygen, and the bus bar is more or less the electrolyte in the diaphragm and climbs up to the bus bar along the electrode ear. The alloy of the bus bar will be oxidized and lead sulfate will be formed. If the electrode alloy of the bus bar is not selected properly, there will be slag inclusions and cracks in the bus bar, and the corrosion will deepen along these gaps, resulting in the separation of the electrode lug and the bus bar and the failure of the negative plate.

1.8 Short circuit caused by diaphragm perforation

Some kinds of separators, such as PP (polypropylene) diaphragm, have larger pore diameter, and the PP fuse will shift in the process of use, resulting in large pores. Active substances can pass through the large pores in the process of charging and discharging, resulting in micro short circuit and battery failure.

(From magazine UPS Application - April 29, 2019)

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