Hybrid Electric Vehicle Battery Internal Resistance Test Method

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Hybrid Electric Vehicle Battery Internal Resistance Test Method [Copy link]

In hybrid electric vehicles, the power battery generally adopts a power type battery, which is composed of multiple lithium batteries (or nickel-metal hydride batteries) connected in series. It has the characteristics of high charge and discharge power, low loss and large instantaneous discharge current. Internal resistance is a very important technical parameter of power batteries. As a parameter to measure the difficulty of ion and electron transmission inside the battery, it has an important influence on the battery's charge and discharge power, charge and discharge current, battery heating and loss.

Battery internal resistance characteristics

1. Battery internal resistance equivalent circuit

Battery internal resistance includes ohmic internal resistance and polarization internal resistance. Ohmic internal resistance is composed of the resistance of electrode materials, electrolyte, diaphragm and the contact resistance of various parts. Polarization resistance refers to the internal resistance caused by polarization when the positive electrode and the negative electrode undergo electrochemical reaction.

Figure 1 Schematic diagram of battery internal resistance equivalent circuit

Figure 1 is a schematic diagram of the battery internal resistance equivalent circuit. Among them, R1 is the battery ohmic internal resistance, R2 and R3 are polarization resistances. The battery ohmic internal resistance can be roughly understood as a constant with relatively small changes. It is mainly related to factors such as the battery's material system, internal structure and connection form. Relatively speaking, polarization resistance is greatly affected by external factors.
Factors such as battery charge and discharge current, working environment temperature and battery SOC state have an impact on it.

2. AC internal resistance

For battery internal resistance testing, the test results obtained using different testing methods and different testing principles are different. According to the definition of testing methods and testing principles, battery internal resistance can be divided into two categories: AC internal resistance and DC internal resistance.

In Figure 1, if a 1kHZ sinusoidal AC current signal is input to the positive and negative electrodes of the battery, the R2 and R3 values of the battery at this frequency are generally small and can be ignored. The battery internal resistance tested using this AC current signal is close to the ohmic internal resistance R1.

AC internal resistance test is based on this principle. A 1kHz sinusoidal AC current signal is input to the positive and negative electrodes of the battery, and then the battery impedance is calculated by detecting the voltage drop and current signal at both ends of the battery. Therefore, the AC internal resistance of the battery can be approximately considered as the ohmic internal resistance R1. In the battery production line, an internal resistance meter is generally used to test single cells. The battery internal resistance measured by the internal resistance meter is the AC internal resistance.

3. DC internal resistance

The power battery pack (or battery pack) for hybrid electric vehicles is composed of multiple single cells connected in series, and the voltage is generally between 200 and 400 V. For the power battery pack, due to the high voltage, considering various factors such as safety and equipment, the battery internal resistance is generally tested by inputting a DC current signal to the battery pack. The battery internal resistance tested in this way is called DC internal resistance. The
DC current signal input during the DC internal resistance test is generally a constant current pulse current, and the current signal generally lasts for a short time.

The battery DC internal resistance test is generally carried out under the conditions of a battery laboratory using dedicated charging and discharging equipment. The battery DC internal resistance can better reflect the actual internal resistance state of the battery pack under specific conditions. It not only includes the battery ohmic internal resistance, but also the battery polarization internal resistance. It can not only reflect the battery's charging and discharging capabilities, but also reflect the degree of aging during the battery's use. Therefore, the battery DC internal resistance is an important technical indicator for evaluating battery packs.

4. Factors affecting battery internal resistance

The internal resistance of the battery is closely related to the external load and the battery's operating environment. It is a dynamic variable value. There are many factors that affect the internal resistance of the battery, mainly including: ① working environment temperature; ② battery SOC
status; ③ battery charge and discharge current; ④ charge and discharge duration.

For power battery packs, the internal resistance of the battery is generally the internal resistance under certain conditions. If no specific conditions are specified, it generally refers to the internal resistance of the battery when it is fully charged at room temperature.

Direct current internal resistance (DCR) test method Hybrid vehicle batteries are power-type batteries that often operate in high-power, frequent pulse charge and discharge states. Therefore, when hybrid vehicle batteries are tested for internal resistance under laboratory conditions, the selection of test parameters such as charge and discharge current, charge and discharge duration, battery SOC state, and operating environment temperature must be similar to the actual operating conditions of the battery in the vehicle.

The test method for the internal resistance of hybrid vehicle batteries is described in detail in Article 7.2 of GB/T 31467.1-2015. The specific test method and steps are as follows:

1) The charge and discharge pulse current adopts constant current.

2) Discharge for 18 s, charge for 10 s, and rest for 40 s in between.

3) The ambient temperatures were set to 40 ℃, room temperature, 0 ℃, and –20 ℃ respectively.

4) The battery SOC states are set to 80%, 50%, and 20% respectively.

5) Record the battery pack voltage and corresponding charge and discharge current at each moment.

6) According to the calculation formula of discharge internal resistance, the battery internal resistance measured at different test durations is obtained. The test durations are 0.1 s, 2 s, and 10 s. The calculation method of charging internal resistance is similar to that of discharging internal resistance, so it will not be described here.

Research on hybrid vehicle battery internal resistance test method based on vehicle test

1. Battery operating data that can be collected by vehicle testing

The commonly used hybrid vehicle battery internal resistance test method under laboratory conditions was introduced above. In actual engineering applications, sometimes there are no battery laboratory test conditions. For example, the battery has been installed on the whole vehicle for vehicle testing. How to obtain the battery internal resistance parameters under vehicle test conditions is a practical problem faced by many engineers.

In the vehicle test, the real-time operating data of each vehicle system (such as engine, motor, battery) can be collected in real time through data acquisition equipment. The following is an example of a hybrid vehicle vehicle operating test. The vehicle test lasts for 30 minutes (1800 seconds). After post-data processing, the data sampling frequency is 10 Hz. The collected battery operating data include battery voltage, current, temperature and SOC, etc. This data reflects the charge and discharge state of the battery in actual application on the vehicle. The battery voltage data curve collected during the test is shown in Figure 2, and the vehicle speed curve is shown in Figure 3.

Figure 2 Battery voltage curve

Figure 3 Speed curve

2. Derivation of the battery internal resistance calculation formula based on operating condition data

From the above data, it can be seen that in actual working conditions, the battery of a hybrid vehicle is pulse-discharged and pulse-charged in most time periods, and the pulse duration is mostly 1 to 2 seconds. The power system connection of a hybrid vehicle is shown in Figure 4, including power batteries, motor control, drive motor, generator and engine.

Figure 4 Connection diagram of the power system of a hybrid vehicle (40 seconds)

Among them, the battery discharge current direction is set to be positive, and the charging current direction is set to be negative. VB is the battery open circuit voltage, VO is the terminal voltage of the battery output to the external load, and R is the battery internal resistance. Assuming that
the battery pack output voltage and current at time t1 are V1 and I1 respectively, and at time t2 are V2 and I2 respectively, according to the circuit principle, it can be obtained:

V 1=V ocv-I 1×R （1）

V 2=V ocv-I 2×R （2）

According to the above formula, the relationship between the battery internal resistance R and the voltage and current in the time period t 1 to t 2 is:

R=-(V 1-V 2)/(I 1-I 2) （3）

In formula (3), if I 1 = 0 at time t 1, then R = (V 1 - V 2) / I
2, which is consistent with the calculation formula of the national standard. The difference is that the national standard test uses constant current discharge and constant current charging, and the current is 0 at the starting time.

3. Calculation method of battery internal resistance based on operating condition data

According to formula (3), combined with the actual battery data under the vehicle test conditions, the internal resistance of the battery under certain conditions (certain time, certain temperature, certain SOC) can be calculated.

In most periods of actual working conditions, the duration of the battery charge and discharge of a hybrid vehicle is 1 to 2 seconds. Based on this feature, the battery internal resistance algorithm for a duration of 1 second is as follows:

1) Data selection: First select a battery charge and discharge condition data under certain conditions (a certain temperature, a certain SOC), and first calculate the battery internal resistance of n 1 s during this period, for example: R1, R2, R3...Rn (n value depends on the actual situation). Since the data acquisition frequency is 10 Hz, 10 groups of data can be collected at a time interval of 1 s, so R1=-(V1-V11)/(I1-I11), R2=-(V2-V12)/(I2-I12）...Rn=-(VnVn+10)/(InIn+10）

2) When processing data, some abnormal data should be eliminated, such as negative values or data that is too large or too small, and then the average value of the calculation results can be taken to obtain the 1 s battery internal resistance under certain conditions (certain temperature, certain SOC).

If you want to calculate the 2 s internal resistance of the battery under the operating data, the above interval data is taken as 20. Similarly, this method can also be used to calculate the 10 s internal resistance of the battery under the operating data. At this time, the battery operating data during acceleration from 0 to 100 km/h can be selected.

4. Examples

The calculation example of the battery internal resistance of a hybrid electric vehicle's actual operating data is shown in the table. From left to right are the real-time collected battery operating data, including time, SOC, battery voltage, current and temperature. The rightmost column is the 1 s battery internal resistance calculated by the above method. In order to obtain better accuracy, after eliminating abnormal data, the calculation result is averaged.

Example of calculating the internal resistance of a battery for 1 s

in conclusion

1) Battery internal resistance has a great impact on battery performance. Battery internal resistance testing is an important task in the development of hybrid electric vehicles. Under battery laboratory conditions, hybrid electric vehicle battery internal resistance is generally tested according to the national standard GB/T 31467.1-2015.

2) In the absence of a battery laboratory, the internal resistance of the battery can be calculated by collecting specific operating data of the hybrid vehicle battery. Based on actual application results, the method described in this article can meet general engineering application needs.

Reference: [1] Liu Junqi, Li Fangke, Wang Lijun, et al. Research on internal resistance test method of hybrid vehicle battery[J]. Automobile Manufacturing, 2022(2):3.