Research on VRLA battery simulated power outage test

1 Introduction

With the widespread application of lead-acid batteries in the production field, the service life of batteries in harsh environments has become a standard for many users to judge battery performance. In today's society where electricity supply is tight, simulated power outage life detection can better meet practical needs, accurately judge the service life of batteries in actual environments, and better measure battery product performance and product quality.

2 Test methods

Randomly select 4 12V80AH lead-acid batteries from 50 sampled mother batteries. Before starting the test, the test battery is fully charged. Under 25℃ environment, 4 batteries are connected in series, and the actual capacity of the 10-hour rate is not less than the rated capacity (10hr-10.8V, C ₁₀ ≥80AH). Then perform the test as follows (25℃):

3 Test results

From the above two figures, we can see that after the simulated power outage test, the battery loses water more and more seriously with the increase of the number of cycles, the internal resistance of the battery gradually increases, the battery charging acceptance gradually decreases, the capacity also gradually decreases, and finally fails. The reasons for battery failure are analyzed and discussed below.

4 Analysis and discussion

The causes of battery failure were analyzed and it was found that the main influencing factors are: degradation of positive electrode active material and loss of bonding with the grid, gas recombination efficiency, internal resistance of the battery, battery water loss, stratification of the electrolyte and the influence of the separator, thermal runaway, and acid density.

4.1 Degeneration of positive electrode active material and loss of bonding with the grid

During discharge, the conversion of PbO ₂ to PbSO ₄ occurs via a dissolution-deposition mechanism ^[1 , ^2] . During recharging, PbO _{2 is redeposited in a slightly different morphology from the PbO 2} present before discharge , which may cause changes in the morphology of the positive electrode active material as the cycle progresses. It is assumed that the neck region connecting the PbO ₂ particles gradually thickens, resulting in the eventual loss of bonding between the particles.

As the number of cycles increases, the specific surface area of ​​the active material decreases, and the crystal particles also increase with the increase in the number of cycles, causing ß-PbO ₂ to gradually lose contact with the grid. And as the active material expands, the conductivity between PbO _{2 particles decreases, so the expansion increases the resistance between the active materials, causing PbO 2} to soften, lose discharge capacity, and reduce battery capacity. The deeper the discharge, the greater the trend of active material expansion and capacity loss.

During the discharge process, a non-conductive layer or a low-conductive layer is formed at the interface between the grid and the active material, causing high resistance at the interface between the grid and the active material. This high-resistance layer generates heat during the charge and discharge cycle, causing the positive plate near the grid to expand actively, resulting in a decrease in the positive electrode capacity.

4.2 Effect of gas recombination efficiency

Since the gas recombination efficiency cannot reach 100%, there is always a small amount of lead sulfate in the negative electrode, which makes the negative electrode in a non-fully charged state for a long time, forming irreversible lead sulfate. In the early stage of discharge, small particles of lead sulfate crystals grow larger, and grow larger through the dissolution-recrystallization process when standing. As the cycle progresses, the lead sulfate crystal particles on the negative plate become larger and larger, the content becomes higher and higher, the negative plate potential gradually shifts to the positive, and the capacity gradually decreases, leading to the end of the battery life.

4.3 Influence of internal resistance

With the increase of cycle test, the corrosion of the positive grid and negative plate connecting strip of the battery reduces the metal channel of the battery and increases the metal damping; the growth of the grid makes the effective material (paste) loose with the grid; part of the active material is sulfided, resulting in a decrease in active material and an increase in the resistance of the paste. The sulfidation consumes part of the sulfuric acid, making the resistivity of the electrolyte larger; the drying of the electrolyte increases the resistance of the conductive channel between adjacent grids of the battery internal resistance. These factors lead to the aggravation of ohmic polarization, electrochemical polarization, and ion concentration polarization in the battery, which aggravates the gas escape and temperature rise of the battery during charging. Gas escape creates pressure in the plate, making the active material on the surface of the plate easy to fall off; the temperature rises, the polarization voltage rises, and the voltage drop increases. At the same time, when the battery is always in an undercharged state, not only will irreversible sulfation be formed inside the battery plate, but also a high resistance barrier layer will be formed between the active material and the grid, which will increase the internal resistance of the battery and reduce the capacity. When the internal resistance of the battery increases by about 25%, it indicates that the battery has a potential fault; when the internal resistance increases by about 50%, the battery has a serious fault; when the internal resistance increases by 100% or more, the battery fails ^[3] .

4.4 Effect of battery water loss

When testing 4 batteries in series, when the consistency of the batteries is poor, it is easy to cause cumulative capacity failure. There are many links in the battery manufacturing process that will cause capacity differences, such as differences in the weight of each grid, the density and weight of the lead paste, the weight of the electrolyte, the formation current, and the chemical composition of the active material. As long as one battery has a low capacity, after dozens of charging cycles, the cumulative difference will be very large. Each time the battery is charged, it will gasify prematurely, causing chronic loss of electrolyte. The loss of electrolyte causes the capacity failure of VRLA. At the same time, the reasons for water loss may include: low gas recombination efficiency; water evaporation in the battery shell; too low opening pressure of the safety valve; corrosion of the positive plate and self-discharge of the negative plate. A large number of experiments have shown that for every 10% decrease in electrolyte, the battery capacity decreases by about 20%; for a 20% decrease in electrolyte, the battery capacity decreases by about 50%. When the electrolyte decreases by about 15%, the battery is considered to be invalid and scrapped ^[4] .

4.5 Effect of electrolyte stratification and separator on battery life

AGM ultra-fine glass fiber felt separator is a good thermal insulation material. The heat generated is not easy to dissipate and the temperature rise is obvious. Due to the thermal expansion and contraction of the separator and the elastic fatigue of the separator, micro cracks are generated between the separator and the plate, poor contact, increased internal resistance, easy to cause thermal runaway and dry-up, and reduced battery life. When the lead dendrites generated by the negative plate penetrate the separator and the positive plate, a through short circuit is formed; when the battery is fully discharged and placed in a state of reduced electrolyte density, the lead on the negative plate dissolves in the electrolyte, deposits in the pores of the separator, and forms a penetration short circuit with the positive plate. Both short circuits can terminate the battery life prematurely.

The stratification of the electrolyte in AGM batteries not only results in the active material on the top not being able to release its due capacity due to insufficient sulfuric acid, but also the excessively high sulfuric acid concentration at the bottom accelerates grid corrosion and the sulfation of the active material, thus shortening the battery life.

4.6 Occurrence of Thermal Runaway

The main reason for thermal runaway is plate drying. Voltage and ambient temperature are the main causes of water loss. After dozens of cycles, the battery consistency becomes poor and lagging batteries appear. Some batteries are over-discharged, and the charging voltage of some batteries often exceeds 2.45V/cell, resulting in water loss, and ultimately causing single-cell short circuit and plate drying ^[5] .

4.7 Effect of acid density on battery life

The density of acid directly affects the open circuit voltage, charge acceptance and battery capacity of the battery. In the deep cycle simulated power outage process, the density of acid affects the corrosion rate and softening rate of the positive plate and the sulfation rate of the negative plate. In the discharge state, very low concentrations of acid are harmful to the grid, and very high concentrations of acid increase the probability of gassing and sulfation of active substances.

5 Ways to Extend Battery Life

The capacity recovery and life of batteries have serious economic impacts on battery manufacturers and users. The following methods can help extend the battery life and improve capacity recovery:

(1) Suitable electrolyte additives have become one of the main ways to improve the performance of lead-acid batteries ^[6] . Additives in the electrolyte can enhance the conductivity of the electrolyte, improve the capacity recovery performance and recharge acceptance of the battery after over-discharge, inhibit dendrite short circuit, increase the capacity of the battery and inhibit early capacity loss, prevent the softening and shedding of active materials, and slow down the corrosion of the grid.

(2) Nanographite sol is directly added to the electrode or made into an activator and added to the battery. The content of carbon fiber in the positive electrode active material is 0.2-0.3% (mass percentage), and _the content of Na2SO4 _is 2.0% (mass percentage) ^[7] . It can effectively improve the rapid capacity decline and short service life of lead-acid batteries, reduce the internal resistance of batteries, improve the electrode structure, prevent the sulfation of lead-acid batteries, improve the conversion efficiency of battery electrical energy and chemical energy, and improve battery performance.

(3) When the battery is being formed, the current is appropriately increased, the pH value of the electrolyte is reduced, and effective cooling measures are adopted to suppress the temperature rise of the battery. By changing the formation conditions to change the α-PbO ₂ and β-PbO ₂ of the positive plate , the purpose of increasing capacity and extending life can be achieved.

(4) Improve the capacity uniformity of single cells. The capacity difference should be distributed within a very small range, with a maximum of no more than 4%, to avoid over-discharge of lagging cells.

(5) After the battery is assembled, it must be overcharged at a constant current to ensure that each cell in the battery pack is fully charged without being affected by the original charge state of each cell.

(6) The temperature in the laboratory must be balanced to ensure that all cells have the same charging efficiency to prevent the production of high-temperature batteries with low charging efficiency and high oxygen recombination efficiency.

(7) Rationally configure the alloy composition to prevent the formation of a barrier layer between the grid alloy and the active material. Use high tin (1.0-1.5%) alloy, which can basically overcome the PCL effect that the alloy is prone to produce and is suitable for deep cycles.

6 Conclusion

VRLA batteries are widely used, but battery life and quality are issues that users are very concerned about. This article systematically discusses the various factors that affect the service life of VRLA batteries and methods to extend their service life. The above content helps to extend the service life of VRLA batteries and improve the reliability and availability of the system.

References:
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