Analysis and research analysis of series battery pack voltage measurement methods

　　At present, the operating power systems of power plants and substations mostly use DC power supply. The DC power supply system is a very important secondary equipment in power plants and substations. Its main task is to provide relay protection, circuit breaker opening and closing and other controls. To provide reliable DC operating power and control power, it requires a battery system. Practical experience shows that among all parameters that characterize a battery, the terminal voltage of the battery best reflects the current condition of the battery. The charging and discharging process of the battery can be judged based on the terminal voltage, and whether the current voltage exceeds the allowable limit voltage. It can also determine the uniformity of the battery pack. Therefore, it is very important to measure the terminal voltage of the battery.

　　2. Analysis and comparison of different terminal voltage measurement methods

　　The key to monitoring the working status of the battery lies in the collection of battery terminal voltage and current signals. Since there are a large number of batteries in a series battery pack, the voltage of the entire battery pack is very high, and there is a potential connection between each battery, so direct measurement is difficult. In the process of researching the battery monitoring system. Many methods have been proposed for measuring the terminal voltage of a single cell in a series battery pack. To sum up, there are mainly the following:

　　1. Common mode measurement method

　　Common mode measurement is based on the same reference point, using precision resistors to attenuate the voltage of each measurement point in equal proportions, and then subtracting them in sequence to obtain the voltage of each cell. The circuit of this method is relatively simple, but the measurement accuracy is low. For example, for a 24-cell battery with a nominal voltage of 12V and a test system with a single-cell battery test accuracy of 0.5%, the absolute error of the single-cell battery test is ±60mV. The absolute error accumulated by 24V cells in series can reach 1.44V. Obviously, its relative The error can reach 12V, which often causes false alarms in emergency power supply monitoring systems, so it cannot meet the requirements of emergency power supply monitoring systems. This method is only suitable for occasions where the number of batteries connected in series is small or the measurement accuracy is not required.

　　2. Differential mode measurement method

　　Differential mode measurements are made by strobing a single cell through electrical or electronic components. When there are a large number of batteries connected in series and the requirements for measurement accuracy are high, the differential mode measurement method should generally be used.

　　2.1 Relay switching to extract voltage

　　The traditional and mature test method is to use a relay and a large electrolytic capacitor for isolation. The basic test principle is: first close the relay to the battery side to charge the electrolytic capacitor; during measurement, close the relay to the measurement circuit side. , isolate the electrolytic capacitor from the battery. Since the electrolytic capacitor maintains the voltage signal of the battery, the test part only needs to measure the voltage on the electrolytic capacitor to obtain the corresponding single battery voltage. This method has the advantages of simple principle and low cost. However, due to the defects of relays such as slow mechanical action and low service life, the detection device implemented based on this principle is unsatisfactory in terms of speed, service life, and working reliability. In order to solve the above problems, the mechanical relay can be replaced by an optocoupler relay. This eliminates the need for external electrolytic capacitors and improves reliability. The speed and service life will also meet the requirements, but the relative cost will be greatly increased. Use photoelectric isolation devices and large electrolytic capacitors to form a sampling and holding circuit to measure the voltage of a single cell in the battery pack. The disadvantages of this circuit are: the voltage on the capacitor can change during the A/D conversion process, resulting in lower accuracy, and the charging and discharging time of the capacitor and the action delay of devices such as transistors and isolation cores determine the long sampling time.

　　2.2V/F conversion contactless sampling and extraction of voltage

　　The schematic diagram of the V/F conversion method is shown in Figure 1. Its working principle is as follows: signal acquisition adopts the V/F conversion method, single batteries are sampled separately, and the terminal voltage of a single battery is taken and divided (reduces power consumption). ) is used as the input of V/F conversion. The dispersion of the voltage dividing resistor can be adjusted through the V/F conversion circuit. The V/F conversion signal output is sent to the analog switch through the optoelectronic isolation device, and the processor collects the frequency signal by controlling the analog switch. The data acquisition circuit and the data processing circuit adopt photoelectric isolation and transformer isolation technology to achieve electrical isolation between them. However, the disadvantages of using V/F conversion as the A/D converter are slow response speed, poor linearity and low accuracy in the small signal range.

　　Figure 1 Principle diagram of V/F conversion method

　　2.3 Floating ground technology measures battery terminal voltage

　　Since the total voltage of the battery packs connected in series reaches 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 float automatically when measuring different battery voltages to ensure normal measurement. The schematic is shown in Figure 2. Each time it works, the analog switch is first gated so that the potential signal at both ends of the battery under test is connected to the test circuit. On the one hand, this signal enters the differential amplifier; on the other hand, it enters the window comparator, and is compared with the fixed potential in the window comparator. Compared with Vr, it can be identified from the switching state of the window comparator output whether the potential of the current measurement ground (GND) is too high, too low or just right (relative to Vr). If it is right, you can start the A/D for measurement. If it is too high or too low, the ground (GND) potential is controlled by floating control through the controller. Since the ground potential often changes due to on-site interference, this method cannot accurately control the ground potential in real time, thus affecting the measurement accuracy of the entire system.

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