The meaning of the single cell voltage, the detection technology of the single cell voltage

First, the introduction

With the development of pure electric vehicles and hybrid vehicles, the series battery pack, which is an important energy storage device, is a key factor affecting the performance of the whole vehicle.

Extending battery life and improving battery efficiency is the key to commercialization and practical use of electric vehicles. Because of the existence of bucket effect in series with the battery monomer battery with the worst performance in overall performance depends on the battery pack, in order to be able to have effective management to the energy use of batteries in series, the need of real-time monitoring series battery monomer battery status. In the characterization of the parameters of the battery status, battery terminal voltage can reflect the working status, most so accurately every single battery voltage in the battery pack is very important.

Second, the existing monomer battery detection method

At present there are many monomer battery voltage measurement method, the main can be summarized as the partial pressure resistance step-down, floating ground survey, analog switch gating several methods, such as the following these methods as a analysis:

1, the resistance partial pressure method

Resistance partial pressure method mainly through resistance attenuation to the partial pressure will be the actual voltage measurement chip acceptable voltage range, and then to convert the modulus.U1 voltage corresponding BT1, from BT1 to BTN-1, the voltage between the Un corresponds to the voltage of battery pack, as shown in figure 1.This method measurement, low cost, long life, but there are cumulative error, and impossible to erase. Increase as the number of single cells, monomer battery voltage measurement error will increase with the increase of common-mode voltage.

2, floating method

A floating ground technology is used to measure the voltage of the battery, the window comparator will automatically judge whether the current low potential right. If appropriate measure directly start the analog-to-digital conversion; If too high or too low, through the microcontroller via mathematical model to control the floating low potential to low potential under suitable condition. The plan because of the low potential often change by the interference, can't accurate control of low potential affect the measurement result of the whole system.

3, the method of analog switch

Through analog switch selection of analog switch was used in measuring channels, each channel using an operational amplifier linear sampling circuit. Is selected to measure the channel, analog switch output the voltage follower into AD converter to convert the modulus. The method according to the size of the total voltage in series battery pack, select the appropriate magnification, not partial pressure resistance network or change the low potential can directly measure arbitrary a battery voltage, measure and convenient. But this method needs large Numbers of op-amp and precision matching resistance, high cost, and high resistance dispersion can lead to measurement results of dispersion.

Literature [4] build measurement circuit switch matrix is presented in this paper, the scheme of low cost, high accuracy, but requires absolute value circuit. Literature [5] by the combined method of relay operational amplifier, can overcome the problem of temperature drift, but to adopt the method of analog switch, also need a lot of operational amplifier and the relay, and relay will have life.

Third, new monomer battery voltage detection method

1, the overall plan

Due to the differential amplifier can overcome the disturbance of common mode signal, and only the difference signal processing. Using the ends of each monomer battery switch matrix, derivation terminal voltage measurements can be made and not affected by other cells. The scheme overall structure as shown in figure 2, when SB1 and SB2 closed, all the other switch off, the ends of the battery BT1 potential respectively connected to the differential amplifier positive side and negative side, after differential amplifier amplification modulus conversion into AD converter; When SB2 and SB3 closed and all the other switch off when battery BT2 at both ends of the potential difference amplifier positive side and negative side, so on all monomer battery voltage of the battery pack can be measured.

Figure  topology

The maximum voltage of each lithium-ion battery is 5V, which can be obtained from Figure 3. The input potential of the first INA117's 3-pin is 40V. Similarly, the input voltage of the 16th INA117 has a minimum input voltage of -40V. The output voltages of the first to eighth INA117s are positive, and the output voltages of the ninth to sixteenth INA117s are negative, so multiple analog switches and A/D converters are required to input positive and negative voltages. Multi-select an analog switch selects MUX16, which is 16-select 1 positive and negative voltage input analog switch, so 16-cell battery only needs 1 MUX16. However, due to the limited IO port of the single-chip microcomputer, the article uses a 74LS154 to expand the IO port, using only the single-chip microcomputer. Four IO ports can control the MUX16 to individually strobe a single-cell lithium-ion battery for voltage sampling.

1.3 A/D converter

Monitoring the battery pack does not require sampling the voltage of each battery with a very high sampling speed. The sampling of 16 battery voltages shares one A/D converter [4]. The measurement voltage input to each section of the battery is connected to the A/D converter through a multi-selection analog switch MUX16. According to the update cycle and voltage requirements of the battery voltage, the voltage conversion value error transmitted by the A/D converter to the single chip microcomputer is up to 10mV. Choose Maximus MAX1272.

MAX1272 is a fault protection, can choose the input range of 12 through software serial analog to digital converter, using SPI three wire communication protocols, + 5 V power supply, analog input voltage range 0 ~ 10 V, 0 ~ 5 V, + / - 10 V, + 5 V itself + 4.096 V reference voltage.When using internal + 4.096 V reference voltage, ideally the corresponding digital output analog voltage input, as shown in table 1.

As can be seen from Table 1, the highest digit of the MAX1272 output is the sign bit, and the remaining 11 bits are the data. Negative numbers are given in the form of complements.

Reference voltage of + 4.096 V, 1 LSB = 1.2207 mV.

The maximum quantization error of the MAX1272, plus the effects of nonlinearity, offset, etc., the total error is about 5mV. The accuracy of the INA117 is high. Under normal conditions, the error is within 1mV. Therefore, using INA117 and MAX1272 combination, can meet the series of lithium ion batteries battery monitoring system at the request of the voltage error under 10MV. Need higher voltage accuracy, need to choose A higher resolution A/D converter.

MAX1272 wiring diagram as shown in figure 4.

In figure 4 MAX1272 used the internal reference voltage, VREF 6 feet and the indirect 2.2 mu of tantalum capacitor and 0.1 F u F ceramic capacitors.

Both PCBs are required to be as close as possible to the MAX1272 when wiring the PCB.

1.4 temperature monitoring

For the series battery pack, the traditional temperature measurement method mostly uses the analog temperature sensor to measure, and is vulnerable to the external environment during the data collection and transmission process, so that the measured result error is large, and when there are many measurement points, The connection is more complicated. In this paper, the single-chip microcomputer and single-bus digital temperature sensor DS18B20 are used to solve the above problems.

Using an external 5 v power supply, the bus can be articulated chip DS18B20, and can simultaneously precise temperature conversion, without the need for an external drive circuit. Temperature measurement range and 55 ~ + 125 ℃;Temperature measuring accuracy: in 10 ~ + 85 ℃ within the scope of the accuracy of plus or minus 0.5 ℃;In the process of temperature acquisition, microcontroller chip DS18B20 command word, should be at the same time also need to be read by DS18B20 temperature were collected. Therefore, the single chip microcomputer controller I/O must be set to have two-way data transmission ability.

This detection system every section of the lithium ion battery on the bus to hook up a piece of DS18B20, set up eight temperature monitoring, while detecting temperature at 8 o 'clock. In the practical application by the single chip microcomputer software to judge the need to display the temperature value: when the temperature above 10 ℃, the eight temperature point of the highest temperature value; When the temperature is below 10 ℃, the display eight temperature point the lowest temperature value and achieve effective reasonable temperature monitoring results.

1.5The fan and heating control circuit

For heat dissipation problem of the battery, fan control circuit was designed, based on measuring the battery temperature value judgment, decision fan is turned on or off. When temperature is too high, the microcontroller will signal on the fan.

Circuit shown in figure 6, FAN for low electricity at ordinary times, transistor conduction, 9014 at this time relay without action;9014 when FAN for high electricity at ordinary times, transistor conduction, makes the relay contact us absorption, FAN start work under 24 v power supply voltage of the power supply.

For tandem lithium-ion battery packs with complex application environments, in addition to the case of excessive temperature, consider the case of too low temperatures. Since the battery operates in an environment where the temperature is too low, the lithium ion activity is deteriorated, the embedding and extraction ability is lowered, and it is easy to deposit on the surface of the graphite crystal to form lithium metal. The formed lithium metal reacts irreversibly with the electrolyte.

If the lithium ion battery is operated at a low temperature for a long period of time, the capacity of the battery will be significantly reduced. Therefore, a heater control circuit is designed as needed, such as a fan control circuit.

2, the performance of the monitoring system

Measurement proved that using INA117, 16 choose 1 analog switch MUX16, MAX1272, 51 series MCU and DS18B20 the lithium ion battery monitoring system for monitoring 16 section 3.7 V lithium ion battery, voltage measurement error completely within 10mv. Temperature, due to the DS18B20 accuracy is higher, the temperature error within 1 ℃.Voltage and temperature measurement are up to par, the system reliable operation. When series lithium-ion battery pack any section battery voltage < 2.2 V, the microcontroller call light alarm with sound and light alarm, and problems of the battery.

When the battery voltage of any one of the series lithium-ion battery packs is >5V, the single-chip microcomputer calls the severe alarm program for sound and light alarm. If the temperature value exceeds the allowable range of the preset temperature value, the series lithium ion battery monitoring system performs an audible and visual alarm. Both the fan and the heating control circuit can normally start the control circuit according to the set temperature. When the temperature is lower than 5 °C, the heating control circuit is started; when the temperature is higher than 50 °C, the fan control circuit is activated.

3, conclusion

The series lithium-ion battery test system uses a high common-mode rejection ratio differential operational amplifier INA117 to solve the common problem. The monitoring voltage error is plus or minus 10MV. To further improve the detection accuracy, a high-level A/D converter can be selected. When testing, the lithium-ion battery is connected in series to the detection module, and the wiring must be correct. Depending on the application, several detection systems can be connected in series to detect more series lithium-ion battery packs, but ensure that the common-mode voltage does not exceed the maximum protection common-mode voltage range of the INA117.

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