Monitoring Technology and Application of Battery Internal Internal Resistance in DC Power System

1 Overview

At present, valve-regulated lead-acid batteries are widely used in electric power supply and communication power supply. Due to the special structure of the valve-regulated lead-acid battery, the performance of the battery is reliably detected during operation, and the battery is maintained in a targeted manner. It has become difficult but urgent. Various battery monitoring systems are also widely used from the high reliability requirements of power system operation. However, different test modes reflect different performances of the battery. Years of research and application have shown that internal resistance detection is one of the most reliable test methods. The different failure modes of the battery reflect the internal resistance differently. Understand the relationship between the internal resistance of the battery and various failure modes, and reasonably analyze the internal resistance data of the valve-regulated lead-acid battery, which is better for the battery. Test and maintain. In recent years, due to the rising prices of raw materials, many domestic valve-regulated lead-acid battery manufacturers have adopted many new production processes, which has brought about new changes in the analysis of internal resistance data of new process batteries. Reasonable selection of such battery internal resistance data benchmarks is of great help in judging the performance of valve-regulated lead-acid batteries. Reasonable use of internal resistance data to maintain the battery has a great effect on extending the service life of the battery, and is of great significance for obtaining maximum safety and economic benefits.

2 common battery failure modes

For valve-regulated lead-acid batteries, the usual performance deterioration mechanisms include: water loss of the battery, corrosion of the plate group, shedding of the active material, passivation caused by deep discharge, and recovery after deep discharge. Several cases of performance deterioration are described below.

(1) Battery water loss

Loss of water in the lead-acid battery will cause the specific gravity of the electrolyte to increase, causing corrosion of the positive grid of the battery, and reducing the active material of the battery, thereby reducing the capacity of the battery and failing.

In the later stage of valve-regulated lead-acid battery charging, the oxygen released from the positive electrode contacts the negative electrode, reacts, and regenerates water.

O2 + 2Pb→2PbO

PbO + H2SO4→H2O +PbSO4

The negative electrode is in an undercharged state due to the action of oxygen, so that no hydrogen gas is generated. The oxygen of this positive electrode is absorbed by the lead of the negative electrode, and the process of synthesizing water is further developed, that is, the so-called cathode absorption.

In the above cathode absorption process, since the generated water can not overflow under the condition of sealing, the valve-regulated sealed lead-acid battery can be spared from the supplementary water maintenance, which is also the origin of the valve-regulated sealed lead-acid battery called the dimension-free battery. However, during charging, when the charging voltage exceeds 2.35 V/cell, it is possible to cause gas to escape. Because at this time, the battery body generates a large amount of gas in a short time and can not be absorbed by the negative electrode. When the pressure exceeds a certain value, it starts to be exhausted through the one-way exhaust valve, and the exhausted gas filters out the acid mist through the filter acid pad, but after all, The loss of gas in the battery is also equal to the loss of water. Therefore, the valve-regulated sealed lead-acid battery has a very strict charging voltage requirement and must not be overcharged.

(2) Sulfation of negative plate

The main active material of the negative grid of the battery is sponge-like lead. When the battery is charged, the following reaction occurs in the negative grid:

PbSO4 + 2e = Pb + SO4-

Oxidation reaction occurs on the positive electrode:

PbSO4 + 2H2O = PbO2 + 4H+ + SO4- + 2e

The chemical reaction that occurs during the discharge process is the reverse reaction of this reaction. When the charge of the valve-regulated sealed lead-acid battery is insufficient, PbSO4 exists on the positive and negative grids of the battery. PbSO4 will lose its activity for a long time. Participation in chemical reactions, this phenomenon is called sulfation of active substances. In order to prevent the formation of sulphation, the battery must always be kept in a fully charged state, and the battery must not be over-discharged.

(3) Positive plate corrosion

Due to the water loss of the battery, the specific gravity of the electrolyte is increased, and the acidity of the excessively strong electrolyte increases the corrosion of the positive electrode plate, and the corrosion of the electrode plate must be prevented to prevent the water loss phenomenon of the battery.

(4) Thermal runaway

Thermal runaway means that the battery has a cumulative enhancement of charging current and battery temperature during constant voltage charging, and gradually damages the battery. The root cause of thermal runaway is that the float voltage is too high.

Under normal circumstances, the float charge is set to (2.23 ~ 2.25) V / monomer (25 ° C) is more suitable. If you do not work according to this float range, but use 2.35V / monomer (25 ° C), thermal runaway may occur after 4 months of continuous charging; or 2.30V / monomer (25 ° C), continuous charging (6 ~ 8) months may occur thermal runaway; if 2.28V / monomer (25 ° C), there will be a serious capacity decline in consecutive (12 ~ 18) months, which leads to thermal runaway. The direct consequence of thermal runaway is that the battery casing is bulged, leaking, and the battery capacity is reduced, eventually failing.

3 Research on internal resistance model of valve-regulated lead-acid battery

Impedance analysis is a common method in electrochemical research and a necessary means for battery performance research and product design [10].

Figure 3-1 shows the equivalent circuit of the common lead-acid battery impedance.

Figure 1 Battery impedance equivalent circuit

In Figure 1, Lp and Ln are positive and negative inductances; Rt.p and Rt.n are electrode ion migration resistances; Cdl.p and Cdl.n are plate electric double layer capacitances; Zw.p and Zw.n are Warburg impedances. It is determined by the diffusion rate of ions in the electrolyte and the porous electrode; RHF is the aforementioned ohmic resistance.

In the literature [4], the Warburg impedance is expressed as an impedance ZW composed of a series of resistors and capacitors.

λ - Warburg coefficient, indicating the diffusion mass transfer characteristics of reactants and products; ω - angular frequency

The impedance of the battery includes ohmic resistance and positive and negative impedance:

Zcell = Zp + Zn + RHF (2)

The battery impedance is a complex impedance that is related to the test frequency if other conditions are constant.

The internal resistance of a normal situation refers to the internal resistance value at a fixed frequency. For a general VRLA battery, most of the frequencies below 100 Hz are used. In practical use, the complex impedance is often referred to as internal resistance.

4 internal resistance online measurement method

The VRLA battery used in the standby occasion generally has a large capacity, ranging from tens of Ah to several thousand Ah, and the internal resistance of the battery is small. Because the resistance is low, the amplitude of the voltage induced by the positive and negative terminals of the battery is very small. It is difficult to accurately measure the internal resistance, especially when there is charging ripple and load dynamic change at the battery end during online measurement. A common internal resistance test method is briefly described below.

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