The last line of defense in the field of high-reliability power supply

1. Introduction

As the core component of the DC power supply system, the battery plays a key role in storing electric energy, coping with abnormal grid 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 security. At present, online monitoring and status evaluation of batteries are gradually being paid attention to by people, and are increasingly widely used in industries such as electricity and communications. However, the key technology used in online monitoring and status evaluation of batteries - internal resistance measurement technology - is not understood by people, and there is still a vague understanding.

2. Online evaluation of battery status

Currently, there are two methods for online evaluation of battery (lag) status: floating charge voltage monitoring method and battery internal resistance monitoring method.

The float charge voltage monitoring method was widely used in the past, but theoretical analysis and a large number of experiments have shown that the float charge voltage has nothing to do with the (deteriorated) state of the battery and its expected service life, so it is no longer used to evaluate the battery state.

Theoretical analysis and a large number of experiments have proved that the battery (outdated) state and expected service life are closely related to its internal resistance. The battery monitoring equipment and battery status analysis equipment currently used at home and abroad all use the battery internal resistance as the main indicator, combine the battery internal resistance change rate and historical data, and establish an expert system to conduct online evaluation of the battery (outdated) state and estimate its service life.

Power station batteries often use large-capacity batteries with extremely small internal resistance, ranging from tens to hundreds of micro-ohms. Even the tightness of the joints will not affect the measurement results. In addition, the battery works online because the charging device produces certain ripple interference. Therefore, traditional resistance measurement technology is difficult to meet the requirements. Micro-resistance precision measurement technology should be used to measure the battery internal resistance in order to respond to such tiny changes in the battery internal resistance at the micro-ohm level.

3. Internal resistance model of battery

Figure 1 (a) is a simplified equivalent circuit of a battery, where RC1 and RC2 are the polarization resistances of the positive and negative electrodes, C1 and C2 are the double-layer capacitances 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 are the polarization resistances of the battery, and C is the equivalent value of the double-layer capacitance of the two electrodes. The connection part of the battery is mainly ohmic resistance, while the electrode active material part has both ohmic resistance and polarization resistance.

(1) Ohmic resistance: It is composed of the resistance of plates, buses, poles, electrolytes, diaphragms, etc., which obey Ohm's law.

(2) Polarization resistance: including concentration polarization resistance and electrochemical polarization resistance. They are composed of diffusion polarization resistance and charge transfer resistance. They are caused by electrode kinetics and material transfer and do not obey Ohm's law.

Concentration polarization: When current passes through the battery, it causes changes in the electrolyte concentration near the surface of the positive and negative electrodes, thereby generating concentration polarization electromotive force η+ and η-, the magnitude of which is related to the current, temperature, electrode reaction rate, electromigration, and diffusion rate.

Electrochemical polarization: When current passes through a battery, the electrode process is hindered due to the retardation of a certain step in the electrode process, causing the electrode potential to leave the equilibrium electrode potential. Its magnitude is related to factors such as current size, temperature, and the actual effective surface area of ​​the electrode.

4. Factors affecting battery internal resistance

The factors that affect the internal resistance of the battery are:

(1) Battery degradation: As the battery is used for a longer time, the battery capacity decreases and the internal resistance gradually increases due to factors such as battery water loss, corrosion of the plates and connecting strips, sulfation of the plates, deformation of the plates, and shedding of active substances.

(2) Battery charge: For the same battery, the internal resistance varies greatly due to the different discharge levels, such as the depth of the battery electrolyte, the thickness of the reactant on the electrode surface, and the porosity on the electrode surface. The deeper the discharge level, the greater the internal resistance of the battery.

Therefore, when evaluating the battery's obsolescence, a unified internal resistance standard should be established for batteries of the same model from the same manufacturer. The battery should be fully charged before internal resistance measurement, and the battery's obsolescence should be evaluated against the standard. If the battery is not fully charged, the evaluation becomes meaningless because it is impossible to distinguish whether the increase in internal resistance is caused by the battery's obsolescence or the battery's charge state.

(3) Temperature: As the temperature increases, the diffusion of reactants, charge transfer, electrode kinetics and material transfer become easier, thus reducing the internal resistance of the battery.

(4) Battery model: Batteries of different manufacturers, types and models have different internal resistances due to different material formulations of electrodes, electrolytes and diaphragms, different battery structures and different assembly processes.

(5) Measurement signal frequency: Currently, many battery internal resistance measurements actually measure the impedance of the battery. However, the battery is not a pure resistance property, but also contains capacitance. Therefore, the size of its impedance is related to the measurement signal frequency, making the battery internal resistance measurement result not objective.

In order to make the battery internal resistance measurement result objective, the influence of 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, the internal resistance measurement result is unique at any measurement signal frequency.

(6) Measurement time and measurement current: When a larger measurement current is used, at the moment of applying the measurement signal and turning off the measurement signal, since polarization, establishment and stabilization are a changing process, different measurement currents and different measurement times will result in different polarizations, making the battery internal resistance measurement result not objective.

In order to make the battery internal resistance measurement result objective, the internal resistance measurement should be performed with a smaller signal current as much as possible. According to experiments, the measurement current should be ≤0.05C10, where C10 is the capacity of the battery at a 10-hour discharge rate. This makes the internal resistance measurement result independent of the measurement time and the measurement current, and the internal resistance measurement result is unique.

5. Internal resistance measurement technology of batteries using AC discharge method

The battery measurement technology of AC discharge method is a further development based on the battery internal resistance measurement technology of AC injection method. This method combines the advantages of AC injection method and DC discharge method. Its principle is that the CPU controls the intelligent load through D/A to make the battery discharge to the intelligent load, generating a low frequency (frequency less than 100Hz) and amplitude of about 0.01C10~0.05C10 sine wave AC signal (effective signal, frequency is f0, angular velocity ω0=2πf0): I=I0Sin(ω0t), where C10 is the capacity of the battery at a 10-hour discharge rate. Correspondingly, the voltage response generated on the battery is: U=U0Sin(ω0t+φ), and the impedance is: Z(ω)= U0/I0×ejφ.

The principle diagram of battery internal resistance measurement by AC discharge method and the waveform of measurement signal are shown in Figures 2 and 3.

(1) MOS tube: The function of the MOS tube is that the CPU controls the MOS tube through D/A, so that the battery discharges to the load and generates a sinusoidal wave excitation signal with a specific frequency and stable amplitude.

(2) Multi-way switch: The multi-way switch is controlled by the CPU to switch signals to measure the internal resistance of each battery in the battery pack.

(3) Coupling capacitor: The function of coupling capacitor is to isolate DC and allow AC signal to pass smoothly. To ensure the accuracy of the measurement circuit, the coupling capacitor must ensure strict matching.

(4) Programmable bandpass filter: When the battery is working online, the ripple current of the charging device may be quite large. The ripple current of some UPS power supplies is several amperes or even tens of amperes, which is much larger than the measurement signal. If filtering is not used, the amplifier at the subsequent stage will be saturated. The design of the bandpass filter allows signals with frequencies close to the measurement signal frequency to pass, while signals with other frequencies cannot pass. In this way, the amplifier at the subsequent stage can effectively amplify the weak measurement signal.

(5) High-speed synchronous A/D converter: This device is a high-speed synchronous A/D converter that realizes high-speed synchronous sampling of current signals and voltage signals, and converts analog signals into digital signals while ensuring the strict phase relationship between current signals and voltage signals.

(6) DSP: Although most of the interference signals are removed by the pre-filter, there are still a lot of interference signals sampled together with the effective signals. If they are not processed, the measurement accuracy will be seriously affected. Since only the signal with a frequency of f0 is a valid signal, the digital computing capability of DSP is used to extract the signal part with a frequency of f0 from the current and voltage sampling signals using the FFT algorithm for calculation.

After the current and voltage sampling signals are sent to the DSP, the DSP processes the signals as follows:

(a) Perform FFT transformation on the current and voltage sampling signals to calculate the frequency spectrum distribution of the current signal and voltage signal respectively;

(b) Extract the current signal and voltage signal with frequency f0 respectively:

Current signal: I=I0Sin(ω0t+φ1)

Voltage signal: U=U0Sin(ω0t+φ2)

(c) Calculate the impedance, internal resistance and phase difference of the battery:

The impedance is: Z(ω)= U0/I0×ejφ.

The phase difference is: φ=φ2-φ1

Battery internal resistance R=|Z(ω)|×Cosφ

(d) The results are sent to the CPU for further display and storage for further analysis.

(7) CPU: It uses Philips' ARM chip LPC2478 to control and manage each unit and communicate with other devices.

Characteristics of battery internal resistance measurement by AC discharge method:

(1) Safe and reliable. The main working circuit of the battery is not connected to any device. The measurement circuit is designed with a 10KΩ current limiting resistor and a fuse. The measurement circuit is designed with high resistance. The battery working circuit and the measurement circuit are safe and independent and do not affect each other. The battery monitoring equipment can be replaced when the battery is working online.

(2) The discharge current is small and does not damage the battery. The discharge current is 0.01C10~0.05C10, which does not impact the battery, will not cause grid deformation and active material shedding, and has no effect on the battery life.

(3) Strong anti-interference performance, suitable for real-time online monitoring of working batteries. The programmable bandpass filter is used for filtering, and the digital signal processing technology is used to process the signal, which effectively eliminates the influence of the ripple of the DC charging device on the measurement. It has good anti-interference performance and is suitable for real-time online monitoring of working batteries.

(4) High test accuracy, accurate status assessment and prediction. Bandpass filter + multi-stage high-precision operational amplifier + digital signal processing make the battery internal resistance test accuracy much higher than the traditional DC discharge method and AC injection method battery internal resistance measurement technology. Accurate online evaluation of battery (aging) status and life prediction require the response to such tiny changes in battery internal resistance as micro-ohm level. The online test accuracy of battery internal resistance should be within 2% and the repeatability should be within 1%, which is currently unachievable with the traditional DC discharge method and AC injection method.

(5) The test result is the real internal resistance of the battery, which has nothing to do with the measurement time, signal frequency, and test current. It is objective and convenient for horizontal comparison of data.