Failure modes and repair methods of lead-acid batteries (2/3)

2. Factors affecting the life of lead acid battery

The failure of lead-acid battery is the result of many factors, which are determined by the internal factors of the plate, such as the composition of active substances. The crystal form, porosity, plate size, grid material and structure also depend on a series of external factors, such as discharge current density, electrolyte concentration and temperature, discharge depth, maintenance condition and storage time. Here are the main external factors.

2.1 Discharge depth

The discharge depth refers to the extent to which the discharge starts and stops in the process of use. 100% depth refers to the full discharge capacity. The life of lead-acid battery is greatly affected by the depth of discharge. The key point of design consideration is deep circulation, shallow circulation or floating charge. If the shallow cycle battery is used in the deep cycle, the lead-acid battery will fail quickly.

Because the positive active material lead dioxide itself is not firmly combined with each other, lead sulfate is generated during discharge, and then recovered to lead dioxide when charging. When the molar volume of lead sulfate is larger than that of lead oxide, the volume of active material expands during discharge. If one mole of lead oxide is converted into one mole of lead sulfate, the volume increases by 95%. In this way, repeated contraction and expansion will make the binding between PbO2 particles gradually relaxed and easy to fall off. If only 20% of the active material in one mole of PbO2 is discharged, the degree of shrinkage and expansion will be greatly reduced, and the destruction of binding force will be slower. Therefore, the deeper the discharge depth is, the shorter the cycle life will be.

2.2 Overcharge level

When overcharge, a large amount of gas is precipitated, and the active material of the positive plate is impacted by the gas, which will promote the active material to fall off; in addition, the positive grid alloy is also subject to severe anodic oxidation and corrosion, so the battery life will be shortened when overcharged.

2.3 Effect of temperature

The life of lead-acid battery increases with the increase of temperature. In the range of 10 ℃~ 35 , each increase of 1 will increase about 5-6 cycles, and between 35 and 45 , the service life can be prolonged by more than 25 cycles; if the temperature is higher than 50 , the service life will be reduced due to the loss of the capacity of the negative electrode.

The battery life increases with the increase of temperature in a certain temperature range, because the capacity increases with the increase of temperature. If the discharge capacity is constant, the discharge depth decreases and the solid life increases with the increase of temperature.

2.4 Influence of sulfuric acid concentration

The increase of sulfuric acid density is beneficial to the capacity of positive plate, but the self discharge of battery increases, the corrosion of grid is accelerated, and the loose shedding of lead dioxide is also promoted. With the increase of sulfuric acid density in the battery, the cycle life decreases.

2.5 Influence of discharge current density

With the increase of discharge current density, the life of the battery decreases, because under the condition of high current density and high acid concentration, the positive lead dioxide is loose and detached.

Another failure mode is water loss. For open battery, water loss is normal maintenance, for sealed battery, it should not appear under strict control. Therefore, water loss is not included in the failure mode. The problem of water loss of sealed batteries focuses on electric bicycles. Because the constant voltage of charging is too high.

(From magazine UPS Application - April 29, 2019)

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