AC Excitation Charging for Fast Charging of Electric Vehicles at Low Temperatures

By Laura ThomsonJun 7 2022

Electric vehicles are viewed as a solution to the era of energy conservation and environmental preservation since they solve the problems of energy loss and pollution. At low temperatures, however, lithium-ion batteries have certain limitations, such as weak electrolyte ionic conductivity, slow chemical reaction rate, and poor conductivity of the SEI coating on the surface of negative graphite particles.

Image Credit: ALDECA studio/Shutterstock.com

At low temperatures, the diffusion coefficient of lithium-ion in anode graphite particles is particularly low, resulting in slow charging.

It is also simple for lithium-ions to concentrate on a negative surface during charging and produce lithium metal at low temperatures, causing irreversible damage to the battery after a lengthy period of lithium metal dendrite development.

As a result, it is important to look into a new technique for dealing with the aforementioned challenges in the case of low-temperature batteries.

As a way of charging at low temperatures, warm air heating, liquid heating, film heating, and circulating high-temperature gas heating have all been examined.

Internal self-heating, internal resistance heating, convection heating, pulse heating, and alternating current (AC) excitation heating are some examples of internal heating systems.

Warm air heating is easy and inexpensive, although it is inefficient compared to liquid heating. The liquid heating method is comparable to circulating high-temperature gas heating. However, due to the greater liquid thermal conductivity, the liquid heating effect is superior to gas heating.

The heat exchange between the liquid and the batteries warms the batteries in the liquid heating technique. Oil, glycol, and water are the most often employed heat transfer media. This approach involves submerging the batteries in liquid or a pipeline around the battery.

However, this approach necessitates a highly airtight battery or battery box, which increases the challenge of battery construction.

Methodology

For lithium-ion batteries to charge fast in low-temperature situations, they must have a short charging time and a rapid temperature increase without losing battery life or safety.

To prevent lithium plating, the negative area of lithium-ion batteries should have a substantial solid-liquid potential difference during charging, but the overall polarization voltage should be low.

Furthermore, the upper and lower cutoff voltages must be higher than the voltage at the lithium-ion battery’s two ends.

The effectiveness of the AC excitation charging method was evaluated using the charging speed and temperature increase speed of the LG INR18650HG2 battery. Table 1 lists the features of the test battery.

Table 1. The parameters of the test battery. Source: Guo, et al., 2022

Figures 1 and 2 show the experimental devices in action. The electricity was provided via a thermal chamber and an inverter.

Figure 1. The BTS600 and thermal chamber. Image Credit: Guo, et al., 2022

Figure 2. The AC inverter. Image Credit: Guo, et al., 2022

The BTS 600 was used to calibrate the battery’s capacity using uniform current and constant voltage charge and discharge. An inverter provided the alternating current electricity. A computer was utilized to regulate the charging and discharging of the batteries and to collect the data using a technique.

Temperature setting, capacity testing, SOC calibration, and charging tests at low temperatures were all part of the test. During the charging and discharging processes, numerous parameters were monitored and recorded.

4.2 V and 2.5 V were the cut-off voltages during the charging and discharging periods, respectively. A temperature sensor was installed in the center of the battery surface to detect temperature changes. The voltage and current were measured by sensors and reported in real-time by the computer.

The battery was charged using an AC incentive approach at 0 °C and 5 °C.

The battery must be kept static for more than two hours in each environment to guarantee that the battery temperature in the laboratory approaches the exterior ambient temperature.

Using the 0% SOC battery as an example, as seen in Figure 3, varied temperature increase rates were observed at various external temperatures. Firstly, the battery temperature rose quicker at 5 °C than at 0 °C. Secondly, when the temperature climbed, the rate of rising remained steady.

Figure 3. The temperature in the case of charging starting at 0% SOC at 0 °C and 5 °C respectively. Image Credit: Guo, et al., 2022

Finally, in the 5 °C ambient condition, the final battery temperature was greater than the 0 °C battery temperature.

Figure 4 shows that the battery voltage increased significantly at the start of charging. As charging progressed, the rate of growth grew progressively. When charging began at 0% SOC, the 5 °C DC charging time was 200 seconds quicker than the 0 °CDC charging time, and the 5 °C AC charging time was 100 seconds faster than the 0 °C DC charging time.

This suggests that the AC charging approach outperformed the DC charging strategy.

Figure 4. The voltage changes in the case of charging starting at 0% SOC at 0 °C and 5 °C, respectively. Image Credit: Guo, et al., 2022

Results

The battery’s starting capacity was 25%, as indicated in Figure 5, and it was charged in a 5 °C external environment. The temperature change graph on the left shows that the battery temperature rises significantly during the initial charging stage.

Figure 5. The temperature and voltage changes in the case of charging starting at 25% SOC and 5 °C. Image Credit: Guo, et al., 2022

It is clear that the voltage increased rapidly at the start of charging. On average, the voltage curve of AC charging was higher than that of DC charging. The charging pace was slow in the beginning, the voltage grew slowly, and the curve was flat.

In contrast, the charging rate increased and the slope of the curve increased in the latter charging phases, which might be attributed to a rise in temperature and a decrease in the internal resistance of the battery.

The temperature and voltage variations in Figures 6, 7, and 8 are identical to those in Figure 5.

When the initial battery capacity is 25% and the outside temperature is 0 °C, the AC charging time is 1300 seconds, and the temperature rises by 8 °C, respectively, as illustrated in Figure 6.

Figure 6. 0 °C and 25% SOC temperature and voltage change diagram. Image Credit: Guo, et al., 2022

Figure 7 shows that when the initial battery capacity was 50%, the external temperature was 5 °C, and the charging current was 0.8 c, the AC full charging time was 1300 seconds and the DC full charging time was 1500 seconds. The charging speed has been boosted by 13%.

Figure 7. 5 °C and 50% SOC Temperature and voltage change diagram. Image Credit: Guo, et al., 2022

The AC full charging time was 4300 seconds and the DC full charging time was 4900 seconds when the initial battery capacity was 0%, the battery exterior temperature was set to 0 °C, and the current user was set to 0.5 c, as shown in Figure 8. The charging speed has already been boosted by 12%.

Figure 8. 0 °C and 0% SOC temperature and voltage change diagram. Image Credit: Guo, et al., 2022

Table 2 shows the particular temperature changes as well as the charging time.

Table 2. Temperature and time. Source: Guo, et al., 2022

Conclusions

The AC excitation charging method was used to study a low-temperature rapid charging strategy in the study. To monitor temperature change, voltage change, and charge power, the system performed DC and AC charging and discharging tests with various starting SOCs at various temperatures.

The AC excitation charging approach is 20% quicker than the CCCV method. Compared to the CCCV approach, the AC excitation charging strategy exhibits a variable voltage rise. As a result, the temperature increases quicker than with the CCCV approach, with an average temperature increase of 2–4 °C.

Therefore, the suggested charging technique, which is superior to the current CCCV approach, can greatly cut charging time at a reasonable temperature rise.

The AC excitation charging strategy might be employed for quick charging of electric automobiles in low-temperature conditions in the future.

Journal Reference:

Guo, S., Han, Z., Wei, J., Guo, S. and Ma, L. (2022) A Novel DC-AC Fast Charging Technology for Lithium-Ion Power Battery at Low-Temperatures. Sustainability. 14(11), 6544 Available Online: https://www.mdpi.com/2071-1050/14/11/6544/htm

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