The last line of defense in the field of high-reliability power protection - Power Circuits - Circuit Diagram
1 Introduction
As a core component of the DC power system, the battery plays a key role in the storage of electrical energy, coping with abnormal grid conditions and special working conditions, and maintaining the normal operation of the system. It is the last line of defense in the field of high reliability power protection. At present, battery online monitoring and status assessment are gradually being valued by people, and they are more and more widely used in power and communication industries. However, the key technology used in battery online monitoring and status evaluation, internal resistance measurement technology, is not known. There is still a vague understanding.
2. Battery status online evaluation
At present, the online evaluation of the battery (backward) state has a floating charge voltage monitoring method and a battery internal resistance monitoring method.
The float voltage monitoring method has been widely used in the past, but theoretical analysis and a large number of experiments have proved that the float voltage is not related to the (backward) state of the battery and the expected service life, so it is no longer used to evaluate the state of the battery.
Theoretical analysis and a large number of experiments have proved that the battery (backward) state and expected service life have a close relationship with the internal resistance. At present, the battery monitoring equipment and battery state analysis equipment used at home and abroad are based on the internal resistance of the battery as the main indicator, combined with the battery. The rate of change of resistance and historical data, an expert system is established, and the battery (backward) status is evaluated online, and its service life is expected.
Power station batteries often use large-capacity batteries, and their internal resistance is extremely small, ranging from tens to hundreds of micro-ohms. Even the tightness of the joints will not affect the measurement results, and the battery works online because the charging device produces a certain pattern. Wave interference, so the traditional resistance measurement technology is difficult to meet the requirements, the micro-resistance precision measurement technology should be used to measure the internal resistance of the battery, in order to reflect the slight change of the battery internal resistance micro-European level.
3. Internal resistance model of battery
Figure 1 (a) shows the simplified equivalent circuit of the battery. In the figure, RC1 and RC2 are the polarization resistance of the positive and negative electrodes, C1 and C2 are the electric double layer capacitance of the positive and negative electrodes, and RΩ is the ohmic resistance of the battery. Figure (a) can be further simplified to (b), where R and is the polarization resistance of the battery, and C is the equivalent value of the two-electrode double layer capacitance. The battery connection portion is mainly an ohmic resistance, and the electrode active material portion has both an ohmic resistance and a polarization resistance.
(1) Ohmic resistance: It consists of resistors such as plates, bus bars, poles, electrolytes, diaphragms, etc., which obey Ohm's law.
(2) Polarization resistance: including concentration polarization resistance and electrochemical polarization resistance, which are composed of diffusion polarization resistance and charge transfer resistance, which are caused by electrode dynamics process and mass transfer, and they do not obey Ohm's law.
Concentration polarization: After the current passes through the battery, it causes the concentration of the electrolyte near the surface of the positive and negative electrodes to change, which in turn produces a concentrated polarization electromotive force η+, η-, its size and current magnitude, temperature, electrode reaction rate, electromigration, diffusion. Speed ​​related.
Electrochemical polarization: When a current passes through the battery, the electrode process is prevented from proceeding from the equilibrium electrode potential due to the retardation of a certain step of the electrode process. Its size is related to the current magnitude, temperature, and the true effective surface area of ​​the electrode.
4. Factors affecting the internal resistance of the battery
The main factors affecting the internal resistance of the battery are:
(1) The backwardness of the battery: As the battery life increases, the battery capacity is caused by factors such as water loss of the battery, corrosion of the plates and connecting strips, sulfation of the plates, deformation of the plates and falling off of the active materials. When it is reduced, the internal resistance of the battery gradually becomes larger.
(2) The charge capacity of the battery: the same battery, the degree of discharge is different, because the battery electrolyte depth, the thickness of the reaction material on the electrode surface, and the porosity of the electrode surface are significantly different, so that the internal resistance of the battery varies greatly. The deeper the discharge, the greater the internal resistance of the battery.
Therefore, when evaluating the backward state of the battery, the same manufacturer's battery of the same model should establish a uniform internal resistance standard. The battery should be fully charged and then measured for internal resistance. The battery should be evaluated against the backward state of the battery if it is not fully charged. Since the internal resistance caused by the backward state of the battery is not distinguished, or the internal resistance caused by the state of charge of the battery is increased, the evaluation is meaningless.
(3) Temperature: As the temperature increases, the diffusion of the reactants, charge transfer, electrode kinetics, and mass transfer are easier to perform, and the internal resistance of the battery is reduced.
(4) Types of batteries: Different manufacturers, different types, different types of batteries, due to different material formulas of electrodes, electrolytes, diaphragms, different battery structures, different assembly processes, the internal resistance of the battery is different.
(5) Measurement signal frequency: At present, many battery internal resistance measurements actually measure the impedance of the battery, while the battery is not purely resistive, but also contains the capacitance. Therefore, the impedance is related to the frequency of the measurement signal, so that the internal resistance measurement result of the battery is not It is objective.
In order to make the internal resistance measurement result of the battery objective, the influence of the battery capacitance on the measurement result should be removed by analytical method according to the phase relationship between the measurement signal current and voltage, so that the measurement result is independent of the measurement signal frequency, that is, at any measurement signal frequency. Next, the internal resistance measurement results are unique.
(6) Measurement time and measurement current size: In the case of using a large measurement current, at the moment when the measurement signal is applied and the measurement signal is turned off, since the polarization and establishment and stabilization are a changing process, different measurement currents are different. The measurement time and polarization are different, so that the internal resistance measurement result of the battery is not objective.
In order to make the internal resistance measurement result of the battery objective, the internal resistance should be measured with a small signal current. According to the experiment, the measured current should be ≤0.05C10, where C10 is the capacity of the battery under the 10-hour discharge rate. The internal resistance measurement result is independent of the measurement time and the measurement current magnitude, and the internal resistance measurement result is unique.
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