What is the internal resistance of lithium-ion batteries (LiFePO4-LFP)?

The internal resistance of a lithium-ion battery is an important parameter to measure the internal charge transfer and ion migration capabilities of the battery. It directly affects the performance of the battery, including charge and discharge efficiency, power output, thermal management and aging rate. The main methods for testing battery internal resistance include direct current internal resistance (DCIR), alternating current internal resistance (ACIR) and electrochemical impedance spectroscopy (EIS).

DC internal resistance (DCIR)

DC internal resistance testing is usually accomplished by applying a small DC pulse current to the battery and measuring the change in battery voltage response. Using Ohm's law (V = IR), the internal resistance of the battery can be calculated. DCIR is often used to quickly assess a battery's internal resistance and health. Short pulses of high-rate current are applied to the battery, the voltage and current are measured before and after the pulse, and Ohm's law (I = V/R) is applied to obtain the results. The calculation formula for DCIR is:

DCIR = (V1–V2) / (I2–I1)

In the IEC 61960 standard, a 0.2C discharge pulse is applied to the battery for 10 seconds, and the V 1 and I 1 values are measured. Then, give another 1C discharge pulse within 1 second, and measure the V2 and I2 values. Then, calculate the DCIR using the above formula.

AC internal resistance (ACIR)

The AC internal resistance test is performed by applying a small amplitude AC signal to both ends of the battery, and then measuring the AC response of the battery. Through this method, the impedance mode value (|Z|) and phase angle (θ) of the battery can be obtained. ACIR testing is usually performed at a certain frequency (1kHz) to obtain battery impedance information. AC current is usually around Iac = 100 mA at 1k Hz, and the impedance at 1 kHz is calculated as Vac/Iac. When measuring impedance, there may be a phase shift between Iac and Vac. For simplicity, the real part of the impedance Vac/Iac (dc) is called ACIR. In practical applications, a 1 kHz sinusoidal current load on the battery is unlikely. Therefore, this measurement of ACIR does not reflect how the battery will behave in real-world applications.

There are several reasons to use 1 kHz:

(1) The high frequency of 1 kHz makes the impact of low-frequency electrochemical processes on the measurement negligible and can capture the ohmic resistance component;

(2) 1 kHz is also low enough that any parallel capacitance or inductance of the battery and the capacitance and inductance of the test wire will not significantly affect the measured resistance value;

(3) At 1 kHz, the measured resistance value is less dependent on the battery state of charge (SoC) or temperature than at lower frequencies;

(4) Instruments that measure at 1 kHz are more accurate, reliable, and cost-effective than instruments that operate at lower and higher frequencies.

Electrochemical Impedance Spectroscopy (EIS)

Electrochemical impedance spectroscopy is a more complex method of testing battery internal resistance. It obtains the impedance information of the battery at different frequencies by applying an AC voltage signal within a certain frequency range and measuring the AC current response of the battery. EIS can provide detailed information on the electrochemical process inside the battery, including electrode process, electrolyte impedance, ion migration, etc. EIS is often used during the research and development phase to gain a deep understanding of the internal mechanisms of a battery.

The difference between the three

Test signal: DCIR uses DC signals, while ACIR and EIS use AC signals.

ACIR only looks at the response at one frequency to determine the internal resistance at that frequency, but EIS tests the impedance over a wide frequency range by sweeping the AC signal from mHz to 30 kHz and beyond, which can reveal much more about the battery under test. information. The difference between ACIR and EIS is shown in the following table:

DCIR and ACIR comparison

ACIR test results are highly repeatable; ACIR measurement speed is fast, and the test time is in the millisecond range; by matching with automated equipment, efficient batch measurements can be achieved; the ACIR tester is small in size, light in weight, and the test process is simple and energy-saving, which can be achieved Fast measurement, especially suitable for battery inspection and unit group testing.

Compared with ACIR, the reproducibility of DCIR test results of batteries is relatively low, battery DCIR measurement takes longer, and the charge/discharge test results of the battery need to be calculated; DCIR testers, that is, battery charging and discharging equipment, are more reliable than ACIR tests The instrument is larger, heavier, and consumes more power; but the DCIR will be closer to the internal resistance of the battery during actual operation.

What is the general internal resistance of lithium-ion batteries such as LFP and NFC?

Generally, ACIR has become the standard method for manufacturers to evaluate battery resistance to compare which battery has higher resistance. So, what is the current internal resistance of the battery? I extracted the internal resistance values from the battery specification books of various models of some manufacturers, hoping to have an overall understanding of the battery internal resistance, as shown in the following table:

The battery material systems in the table are all LiFePO4-LFP, and the cells are all prismatic batteries. Taking the battery capacity data in the table as the x-axis and the battery internal resistance as the y-axis, draw the chart as shown in Figure 4. The data of each manufacturer is represented by a different color because BXX Blade Battery uses the DC method to test the internal resistance. The data Not plotted on the chart. It can be seen from the figure that as the battery capacity increases, the internal resistance of the battery gradually decreases as a whole. When the battery capacity is higher, the area of the electrode sheet is usually larger, and the resistance is inversely proportional to the electrode area. Therefore, the higher the capacity of the battery, the lower the internal resistance.

In order to compare the internal resistance of batteries with different capacities, the internal resistance of the battery is normalized by capacity, that is, internal resistance capacity. The higher the value, the greater the internal resistance of the battery and the worse the battery performance. The normalized internal resistance of various types of batteries from different manufacturers is shown in Figure 5 below. Because the manufacturing years of each type of battery are also different, the internal resistance testing methods are also different, and the internal resistance value range of each type is relatively large. The normalized internal resistance range of CAXX's batteries is 35~65 mΩAh, the normalized internal resistance range of ETC's batteries is 45~50 mΩ*Ah, and the normalized internal resistance range of EVE's batteries is 45~70 mΩ* Ah. The internal resistance in the CAXX battery specification mainly gives a large range without specific data. The middle value of the internal resistance range is mainly used here, so the reference value is not great. Other manufacturers have larger ranges. Overall, the normalized internal resistance range of LFP batteries is 35~70 mΩ*Ah.

In addition, based on the battery capacity and mass, the energy density of the battery is calculated simply by multiplying the capacity * 3.2V by the mass, and the results are shown in Figure 6. Overall, the batteries of EXX and CATX have higher energy densities.

In short, according to the data in the battery specifications provided by various manufacturers, the current normalized internal resistance range of LFP commercial batteries is 35~70 mΩ*Ah.