Hybrid Vehicle Battery Internal Resistance Testing Method

In hybrid vehicles, the power battery typically uses high-power batteries composed of multiple lithium or nickel-metal hydride cells connected in series. These batteries are characterized by high charge and discharge power, low loss, and high instantaneous discharge current. Internal resistance is a very important technical parameter for power batteries as it measures the ease of ion and electron transmission inside the battery. It significantly affects the battery’s charging and discharging power, current, heat generation, and energy loss.

Battery Internal Resistance Characteristics

1. Battery Internal Resistance Equivalent Circuit

Battery internal resistance includes both ohmic resistance and polarization resistance. Ohmic resistance is the resistance of the electrode materials, electrolytes, separators, and the contact resistance between various parts of the battery. Polarization resistance refers to the internal resistance caused by the polarization during the electrochemical reactions between the positive and negative electrodes.

In the equivalent circuit model, R1 represents the battery’s ohmic resistance, while R2 and R3 represent the polarization resistance. The ohmic resistance is generally a constant that does not change significantly and is mainly determined by factors such as the battery’s material system, internal structure, and connection method. On the other hand, the polarization resistance is more sensitive to external factors. For example, the charge and discharge current, operating temperature, and state of charge (SOC) all impact this resistance.

2. AC Internal Resistance

Battery internal resistance can be measured using various methods based on different principles. According to the testing principles and methods, battery internal resistance can be classified into AC internal resistance and DC internal resistance.

If a 1 kHz sine wave AC current signal is applied to the battery’s positive and negative terminals, the values of R2 and R3 are generally small and can be neglected at this frequency. The internal resistance measured using this method is closer to the ohmic resistance R1.

AC internal resistance testing is based on this principle. By applying a 1 kHz sine wave current signal to the battery’s positive and negative terminals and measuring the voltage drop and current signal, the battery’s impedance can be calculated. Therefore, the AC internal resistance is approximately equal to the ohmic resistance R1. In battery production lines, internal resistance testers are commonly used to test individual cells, and the internal resistance measured by such testers is the AC internal resistance.

3. DC Internal Resistance

Hybrid vehicle power battery packs are composed of multiple cells connected in series, typically with a voltage range of 200–400 V. Due to the high voltage of the battery pack, direct current (DC) testing methods are generally used for safety and equipment reasons.

In DC internal resistance testing, a constant current pulse is applied to the battery. This current pulse typically lasts for a short time.

DC internal resistance testing is typically conducted in a laboratory setting using specialized charge-discharge equipment. The DC internal resistance can reflect the true internal resistance state of the battery pack under specific conditions. It includes both the ohmic resistance and polarization resistance and provides insights into the battery’s charging and discharging capacity as well as its aging status. Therefore, DC internal resistance is a crucial technical indicator for evaluating battery packs.

4. Factors Affecting Battery Internal Resistance

The internal resistance of a battery is closely related to external load conditions and the battery’s operating environment. It is a dynamic variable influenced by several factors, including:

Operating temperature
Battery SOC status
Charging and discharging current size
Duration of charging and discharging cycles

For power battery packs, internal resistance is usually measured under a specific condition, such as a fully charged state at room temperature, unless otherwise stated.

DC Internal Resistance (DCR) Testing Method

Hybrid vehicle batteries are typically high-power batteries that operate under high power and frequent pulse charging and discharging conditions. Therefore, when testing the internal resistance of a hybrid vehicle battery under laboratory conditions, the test parameters such as current size, duration, battery SOC, and operating temperature should simulate the actual usage conditions of the battery in the vehicle.

The method for testing hybrid vehicle battery internal resistance is outlined in detail in GB/T 31467.1-2015, Article 7.2. The specific steps are as follows:

A constant current is used for pulse charge and discharge.
Discharge lasts for 18 seconds, charging lasts for 10 seconds, and there is a 40-second rest period in between.
Ambient temperature is set at 40°C, room temperature, 0°C, and -20°C.
The battery’s SOC is set at 80%, 50%, and 20%.
Record the battery pack voltage and corresponding charge and discharge current at each moment.
Calculate the internal resistance based on the discharge resistance formula, with testing durations of 0.1 s, 2 s, and 10 s. The calculation for charge resistance is similar to the discharge resistance.

Research on Battery Internal Resistance Testing Methods Based on Vehicle Testing

In practical engineering applications, it is not always feasible to test the battery under laboratory conditions, especially when the battery is already installed in a vehicle. In such cases, the challenge lies in how to obtain the battery’s internal resistance parameters under actual vehicle testing conditions. During vehicle testing, real-time data from various systems, such as the engine, electric motor, and battery, can be collected using data acquisition equipment.

Consider the example of a hybrid vehicle’s vehicle condition test. The test lasts for 30 minutes (1,800 seconds), with a sampling frequency of 10 Hz. The collected data includes battery voltage, current, temperature, and SOC, reflecting the charging and discharging state of the battery during the vehicle’s actual operation. The collected data allows for the calculation of the internal resistance of the battery under real-world conditions.

Battery Internal Resistance Calculation Formula Based on Vehicle Data

From the collected data, it can be observed that, during actual driving conditions, the battery typically operates under pulse charging and discharging for most of the time, with pulse durations of 1–2 seconds.

Let’s define the discharge current direction as positive and the charging current direction as negative. $V_B$ represents the battery’s open-circuit voltage, while $V_O$ is the voltage output from the battery to the external load, and $R$ is the internal resistance.

At time $t_1$ , the battery’s voltage and current are denoted as $V_1$ and $I_1$ , and at time $t_2$ , the values are $V_2$ and $I_2$ . Based on circuit theory, the following equations can be derived:

$V1=Voc−I1×R(1)V_1 = V_{oc} – I_1 \times R \quad (1)$ $V2=Voc−I2×R(2)V_2 = V_{oc} – I_2 \times R \quad (2)$

From equations (1) and (2), the battery’s internal resistance $R$ can be calculated as:

$-\frac{V_1 – V_2}{I_1 – I_2} \quad (3)$

If at time $t_1$ , $I_1 = 0$ , then:

$\frac{V_1 – V_2}{I_2}$

This formula is consistent with the national standard method, where constant current discharge and charge are applied, with the initial current set to zero.

Battery Internal Resistance Calculation Method Based on Vehicle Data

Using the formula (3) and the actual data collected during vehicle testing, the internal resistance of the battery can be calculated under specific conditions (such as time, temperature, and SOC). For instance, if we want to calculate the internal resistance for a 1-second pulse duration, the calculation is as follows:

Data Selection: First, select a segment of charging and discharging data under specific conditions (temperature and SOC), and calculate the internal resistance for each 1-second period, such as $R1,R2,R3,…,RnR_1, R_2, R_3, \dots, R_n$ .
Data Processing: After removing any outliers or abnormal data, calculate the average of the results to determine the battery’s internal resistance under these conditions.

This method can be used to calculate the internal resistance for pulse durations of 2 seconds or 10 seconds, depending on the application.

Example Calculation of Battery Internal Resistance

An example of the internal resistance calculation for a hybrid vehicle battery is presented. The table below shows the real-time collected data, including time, SOC, battery voltage, current, and temperature, with the final column representing the calculated 1-second internal resistance. By eliminating any outliers, the average value is obtained for better accuracy.

Conclusion

Battery internal resistance significantly affects battery performance, and testing internal resistance is a crucial part of hybrid vehicle development. Under laboratory conditions, hybrid vehicle battery internal resistance is generally tested according to the national standard GB/T 31467.1-2015.
In situations where laboratory testing is not possible, internal resistance can be calculated by collecting specific operating condition data from the hybrid vehicle’s battery. The method described here can meet the requirements for most engineering applications.