Monitoring technology and application of battery internal internal resistance in DC power supply system - News - Global IC Trade Starts Here 工业

1 Overview

Currently, valve-regulated lead-acid batteries are widely used in power supply and telecommunication power systems. Due to the unique structure of these batteries, it has become challenging yet essential to monitor their performance during operation and carry out targeted maintenance. Various battery monitoring systems are also extensively employed in power systems because of their high reliability demands. Different testing modes reveal different aspects of the battery's performance. Over the years of research and practical application, it has been proven that internal resistance detection is one of the most reliable testing methods. Different failure modes of the battery manifest differently in terms of internal resistance. Understanding the relationship between the battery's internal resistance and various failure modes, and properly analyzing the internal resistance data of valve-regulated lead-acid batteries, is crucial for effective battery testing and maintenance. In recent years, due to rising raw material costs, many domestic manufacturers of valve-regulated lead-acid batteries have adopted new production processes, leading to new considerations in analyzing the internal resistance data of these batteries. Selecting appropriate benchmarks for such battery internal resistance data is highly beneficial in assessing the performance of valve-regulated lead-acid batteries. Utilizing internal resistance data effectively for battery maintenance significantly extends the battery's lifespan, providing substantial safety and economic benefits.

2 Common Battery Failure Modes

For valve-regulated lead-acid batteries, typical mechanisms of performance degradation include water loss, plate corrosion, shedding of active materials, passivation caused by deep discharge, and recovery after deep discharge. Below are descriptions of several cases of performance degradation:

(1) Battery Water Loss

The loss of water in a lead-acid battery increases the specific gravity of the electrolyte, causing corrosion of the positive grid and reducing the active material, thus decreasing the battery's capacity and leading to failure.

In the later stages of charging a valve-regulated lead-acid battery, the oxygen released from the positive electrode reacts with the negative electrode to regenerate water:

O2 + 2Pb → 2PbO

PbO + H2SO4 → H2O + PbSO4

The negative electrode remains undercharged due to the action of oxygen, preventing the generation of hydrogen gas. The oxygen from the positive electrode is absorbed by the lead of the negative electrode, and the synthesis of water continues, a process known as cathode absorption.

In the cathode absorption process, since the generated water cannot overflow under sealed conditions, valve-regulated sealed lead-acid batteries do not require supplementary water maintenance, earning them the nickname "maintenance-free batteries." However, during charging, if the voltage exceeds 2.35V per cell, gas may escape. At this point, the battery generates a large amount of gas in a short time, which cannot be fully absorbed by the negative electrode. When the pressure exceeds a certain threshold, the one-way exhaust valve opens, allowing the escaping gas to pass through the acid filter pad, but ultimately, the loss of gas equals the loss of water. Thus, valve-regulated sealed lead-acid batteries have very strict charging voltage requirements and must never be overcharged.

(2) Sulfation of Negative Plate

The primary active material of the negative grid is sponge-like lead. During charging, the following reaction occurs in the negative grid:

PbSO4 + 2e = Pb + SO4-

An oxidation reaction occurs on the positive electrode:

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

The chemical reaction during the discharge process is the reverse of this reaction. When the charge of a valve-regulated sealed lead-acid battery is insufficient, PbSO4 forms on both the positive and negative grids. If PbSO4 persists for a long time, it loses its reactivity and ceases to participate in chemical reactions, a phenomenon known as sulfation of active substances. To prevent sulfation, the battery must always remain fully charged, and over-discharging should be avoided.

(3) Positive Plate Corrosion

Due to water loss in the battery, the specific gravity of the electrolyte increases, and the overly acidic electrolyte accelerates the corrosion of the positive electrode plate. Preventing electrode plate corrosion is essential to avoid the water loss phenomenon in the battery.

(4) Thermal Runaway

Thermal runaway refers to the cumulative enhancement of charging current and battery temperature during constant-voltage charging, gradually damaging the battery. The root cause of thermal runaway is excessive float voltage.

Under normal circumstances, the float charge is set to (2.23 ~ 2.25)V per cell at 25°C, which is more suitable. If this range is not followed, for example, using 2.35V per cell (25°C), thermal runaway may occur after four months of continuous charging; or using 2.30V per cell (25°C), thermal runaway may occur after six to eight months of continuous charging; if 2.28V per cell (25°C) is used, a significant capacity decline may occur within twelve to eighteen months, leading to thermal runaway. The direct consequence of thermal runaway is the swelling, leakage, and reduced capacity of the battery, ultimately resulting in battery failure.

3 Research on Internal Resistance Model of Valve-Regulated Lead-Acid Battery

Impedance analysis is a common method in electrochemical research and an indispensable tool for battery performance research and product design [10].

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

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