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| Analysis and Research on Voltage Measurement Method of Series Panasonic Battery Pack |
| Time：2019-03-01 Source： Views：1673 |
I. Preface
At present, the operation power supply systems of power plants and substations mostly use direct current power supply. The direct current power supply system is a very important secondary equipment of power plants and substations. Its main task is to provide reliable direct current operation power supply and control power supply for relay maintenance, circuit breaker switching and other control. It requests Panasonic battery system. The theoretical experience shows that the terminal voltage of Panasonic battery best represents the current state of the battery among all the parameters that characterize the battery. The charging and discharging process of the battery can be judged according to the terminal voltage, and whether the current voltage can exceed the allowable limit voltage or not. It can also distinguish the homogeneity of the battery group. Therefore, it is very important to measure the terminal voltage of the battery.
Analysis and comparison of voltage measurement methods at different terminals
The key to monitor the working state of storage battery lies in the collection of voltage and current signals at the end of storage battery. Because of the large number of batteries in series batteries, the high voltage of the whole batteries and the potential connection between each batteries, it is difficult to measure directly. In the process of discussing the battery monitoring system. Many methods for measuring the terminal voltage of a single battery in series batteries have been proposed. Generally speaking, there are several main types:
1. Common mode measurement
Common-mode measurement is relative to the same reference point, using fine resistance to attenuate the voltage of each measurement point in equal proportion, and then subtract each battery voltage in turn. The circuit of this method is simple, but the measuring accuracy is low. For example, for 24 batteries with nominal voltage of 12V, the test precision of a single battery is 0.5%. The absolute error of a single battery test is +60mV. The absolute error of a 24V battery test can reach 1.44V. Obviously, its relative error can reach 12V. This often causes false alarm in the emergency power supply monitoring system, so it can not meet the requirements of the emergency power supply monitoring system. This method is only suitable for places where the number of series batteries is small or the measurement accuracy is not high.
2. Differential Modulus Measurement Method
Differential mode measurement is to stop measuring by electrically or electronically selected single battery. When the number of series batteries is large and the measurement accuracy is high, the differential mode measurement method should be generally adopted.
2.1 Relay Switching Extraction Voltage
The traditional comparatively mature test method is to isolate the relay and the large electrolytic capacitor. Its Basic test principle is: first, the relay is closed to the side of the battery to charge the electrolytic capacitor; when measuring, the relay is closed to the side of the measuring circuit, and the electrolytic capacitor and the battery are separated. Because the electrolytic capacitor insists on the voltage signal of the battery, so, The test part only needs to measure the voltage on the electrolytic capacitor to get the corresponding single battery voltage. This method has the advantages of simple principle and low cost. However, due to the defects of slow mechanical action and low service life of relays, it is difficult to satisfy the requirements of speed, service life and reliability of relays. In order to deal with the above problems, mechanical relays can be replaced by optocoupler relays, which improves the reliability without additional electrolytic capacitors, and the speed and service life will also meet the requirements, but the relative cost will be greatly improved. Sampling is made up of photoelectric isolator and large electrolytic capacitor. Circuit is used to measure the voltage of single battery in battery pack. The defect of this circuit is that the voltage on capacitor can change during A/D conversion, which makes the accuracy lower, and the defect of capacitor charging and discharging time and the long sampling time of action delay resolution of transistor and isolator core.
2.2 V/F Conversion Contactless Sampling Extraction Voltage
The principle diagram of V/F conversion method is shown in Fig. 1. Its working principle is as follows: signal acquisition adopts V/F conversion method, single battery adopts V/F conversion method, and single battery adopts separate sampling method. The terminal voltage of single battery is taken as input of V/F conversion after partial voltage (reducing power consumption). The distribution of dividing resistance can be adjusted by V/F conversion circuit to output V/F conversion signal through photoelectric isolator and sent to analog switch. The device collects frequency signal by controlling analog switch. Data acquisition circuit and data processing circuit use photoelectric isolation and transformer isolation technology to complete the electrical isolation between the two. But the disadvantage of using V/F conversion as A/D converter is slow response speed, poor linearity and low precision in small signal range.
2.3 Floating Ground Technology for Measuring Battery End Voltage
Because the total voltage of Panasonic battery pack connected in series is tens of volts or even hundreds of volts, which is much higher than the normal working voltage of the analog switch, it is necessary to make the ground potential automatically float with the measurement of different battery voltage to ensure the normal stop of measurement. The schematic diagram is shown in Figure 2. In each operation, the analog switch is selected to connect the potential signals at both ends of the tested battery into the test circuit. On the one hand, this signal enters the differential amplifier; on the other hand, it enters the window comparator. Compared with the fixed potential Vr in the window comparator, the switching state output from the window comparator can identify that the current measuring ground (GND) is too high, too low or positive. OK (related to Vr). If it happens, it can start A/D stop measurement. If it is too high or too low, it is controlled by the controller to ground (GND) potential line floating. Because the ground potential is often changed by the onset of field interference, this method can not control the ground potential stop accurately in real time, so it affects the measurement accuracy of the whole system.
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