Testing and Analysis of Three-Electrode Lithium Battery's Direct Current Internal Resistance
1 Introduction
This research focuses on a 51Ah prismatic lithium-ion battery in a ternary system. A comprehensive analysis was conducted based on various aspects, including battery state of charge (SOC), pulse current, pulse duration, testing temperature, and operating conditions.
The findings can serve as a reference for direct current internal resistance (DCIR) testing of ternary system batteries in practical applications.
2 Experimental Section
2.1 Test Subject
The selected test sample is a prismatic lithium-ion battery in the NCM system, with an individual capacity of 51Ah and a nominal voltage of 3.65V. The testing equipment used includes the Arbin charge-discharge system and a high/low-temperature chamber.
2.2 Test Methods
(1) Capacity Test:
The capacity test of the battery is conducted in a temperature chamber at 25±2°C, following the test procedure below:
① Allow a 2-minute rest.
② Discharge the battery to 2.0V using a 1C (1C=51A) current.
③ Allow a 2-minute rest.
④ Charge the battery at a constant current and constant voltage with a 1C current until it reaches 4.2V.
⑤Repeat steps ① to ④, taking the capacity of the second discharge as the actual capacity of the battery.
(2) Discharge DCIR Test:
① Let the fully charged battery stand still for 1 hour.
② Adjust the battery to the target SOC using a 1C current.
③ Let it rest for 30 minutes and record the voltage V0 at this time as the OCV for the corresponding SOC.
④ Discharge with a discharge current I1 for t seconds and record the voltage V1 at time t seconds.
⑤ The calculation formula for DCIR during discharge is as follows:
DCIR_discharge = (V0 – V1) / I1 (1).
(3) Charging DCIR Test:
① Let the empty battery stand still for 1 hour.
② Charge it to the target SOC using a 1C current.
③ Let it rest for 30 minutes and record the voltage V0 at this time as the OCV for the corresponding SOC.
④ Charge with a charging current I2 for t seconds and record the voltage V1 at time t seconds.
⑤The calculation method for charging DCIR is as follows:
DCIR_charge = (V1 – V0) / I2 (2).
3 Results and Discussion
3.1 Influence of Different SOC on DCIR
DCIR tests for charging and discharging at different SOC levels were conducted according to the test methods described in 2.2. The discharging pulse current was I1=5C, and the charging pulse current was I2=3C, with a pulse duration of t=10s. The test results are shown in Figure 1(a) and Figure 1(b).
From the test results, it can be observed that the different SOC states have a significant impact on the DCIR test results. Both charge DCIR and discharge DCIR data show a decreasing trend as SOC increases, with DCIR stabilizing above 20% SOC.
The reason for this can be attributed to the internal reaction processes within the battery. At low SOC, the charge transfer impedance is higher, and as SOC gradually increases, the charge transfer impedance decreases, leading to a gradual reduction in DCIR.
3.2 Impact of Current Magnitude on DCIR
Following the method described in Section 2.2, the test samples were adjusted to a 50% SOC state. Pulses of discharge were applied at currents of I1 = 1C, 1.5C, 2C, 3C, and 5C, and pulses of charge were applied at currents of I2 = 1C, 1.5C, 2C, and 3C.
The DCIR values at 10 seconds were calculated and compared to study the influence of pulse current magnitude on the DCIR test results. The test results are shown in Figure 2.
The test results for discharge DCIR and charge DCIR obtained at 50% SOC showed a gradual decrease as the current increased for discharge pulse currents in the range of 1-5C and charge pulse currents in the range of 1-3C.
In contrast, reference to other literature on ternary system batteries showed that charge DCIR increased with higher currents, while discharge DCIR decreased with higher currents.
This inconsistency with the results of this study indicates that the trend of DCIR with changing current is also influenced by the battery’s system design.
3.3 Impact of Pulse Duration on DCIR
To verify the effect of pulse duration on DCIR, testing was conducted for DCIR during pulse discharge and pulse charge with pulse durations ranging from 1 to 30 seconds, following the testing method outlined in 2.2.
Since the DCIR differences above 50% SOC were relatively small, for the purpose of comparison, only data below 50% SOC is presented here. The test results are shown in Figure 3(a) and Figure 3(b).
The test results show that when the pulse duration is in the range of 1 to 5 seconds, both charge and discharge DCIR exhibit a linear increasing trend.
When the pulse duration is between 5 and 30 seconds, the growth trend of DCIR slows down, gradually deviating from linearity.
This phenomenon is due to the increase in mass transfer impedance inside the battery with the increase in pulse duration, gradually becoming dominant, leading to the deviation of DCIR change from linearity.
Furthermore, as the SOC state increases, DCIR decreases, and the rate of increase of DCIR with time is smaller. This phenomenon is related to the lower charge transfer impedance at high SOC levels.
3.4 Temperature Effects on DCIR
Reference to section 2.2, DCIR tests were conducted at different temperatures, and the pulse current levels were adjusted according to the charge and discharge capacity at different temperatures. The test results are shown in Figure 4.
It can be observed that as the temperature decreases, both discharge DCIR and charge DCIR gradually increase.
This is because as the temperature decreases, the electrolyte viscosity increases, and ion mobility decreases, leading to a slower chemical reaction rate.
As a result, the battery’s ohmic resistance and polarization resistance both increase, leading to an increasing trend in the measured DCIR.
3.5 Impact of Operating Conditions on DCIR
Considering that lithium-ion batteries operate under various real-world conditions, an analysis of the changes in DCIR under different operating conditions was conducted.
The test samples were cycled at 1C charge-discharge rates in environments at 25°C, 35°C, and 45°C, with each cycle comprising 100 cycles. In accordance with the testing method outlined in section 2.2, the batteries were adjusted to 50% SOC, subjected to a 1C, 30s pulse discharge, and the discharge DCIR data at 30 seconds was calculated using equation (1) to observe the variation of DCIR with the number of cycles.
Figure 5(a) and Figure 5(b) present the degradation trends under different temperatures and the variation of discharge DCIR during the cycling process.
From the graph, it can be observed that the capacity degradation trends during cycling differ at the three temperatures, and the changes in DCIR during cycling show markedly different patterns. Moreover, as the temperature increases, DCIR increases more rapidly with the number of cycles. This is because battery capacity degradation during cycling is mainly due to three factors:
(1)Secondary reactions inside the battery leading to the loss of active lithium.
(2)Decomposition of active materials, structural cracking, and electrode delamination within the battery.
(3)Increasing contact resistance due to the gradual thickening of the solid electrolyte membrane on the surface of the active material, reduced membrane conductivity, and other factors.
The increase in DCIR is a result of the cumulative effect of these three factors.
Secondary reactions within the battery, electrode deterioration rates, and other factors all intensify as the temperature rises, causing DCIR to increase more rapidly.
In summary, DCIR test results for square lithium-ion power batteries with a ternary system were tested and analyzed under different conditions. Based on the test results, it can be concluded that:
- DCIR gradually decreases as SOC increases, and DCIR stabilizes above 20% SOC.
- The test results for both discharge DCIR and charge DCIR at 50% SOC showed a gradual decrease as the current increased when using discharge pulse currents from 1 to 5C and charge pulse currents from 1 to 3C. However, different battery designs may exhibit different trends.
- For pulse times ranging from 1 to 5s, DCIR exhibited a linear growth trend. While for pulse times from 5 to 30s, the DCIR growth trend gradually deviated from linearity.
- As the temperature decreases, the DCIR of the battery gradually increases.
- The growth trend of DCIR under different usage conditions varies.
4. Conclusion
Through DCIR testing under different conditions, factors affecting the DC internal resistance of square lithium-ion batteries with a ternary system were analyzed.
Based on the test results, it can be concluded that there are numerous factors influencing the DC internal resistance of lithium-ion batteries.
Therefore, in practical applications, DC internal resistance testing should be conducted under relevant conditions based on the specific requirements. Besides, attention should be paid to the consistency of the test conditions to ensure the reliability of the test results.