What are charge and discharge characteristics?
Charge and discharge characteristics
Lithium-ion batteries typically employ a two-stage charging method to ensure safety, reliability, and charging efficiency. The first stage is constant current with voltage limiting, and the second stage is constant voltage with current limiting. The maximum voltage limit for charging a lithium-ion battery varies depending on the cathode material. The basic charge/discharge voltage curves of a lithium-ion battery are shown in Figure 3-11. The curves in the figure use a charge/discharge current of C/3. For different lithium-ion batteries, the main differences are twofold:
1) The optimal constant current value for the first stage varies depending on the battery's cathode material and manufacturing process. Generally, a current range of 0.2C to 0.3C is used. In cases of rapid power consumption, 1C, 2C, or even higher rates can be employed.
2) Different lithium-ion batteries exhibit significant differences in constant current duration, and the proportion of capacity that can be charged by constant current to the total capacity also varies considerably. From the perspective of practical electric vehicle applications, a longer constant current duration results in a shorter total charging time, which is more beneficial for applications.
Lithium-ion battery voltage is stable and decreases slowly in the early and middle stages of discharge, but drops rapidly in the later stages, as shown in segment DE of Figure 3-11. Effective control is crucial during this stage to prevent over-discharge and irreversible damage to the battery.
Factors affecting charging characteristics
(1) Effect of charging current on charging characteristics Taking a certain NCM lithium-ion battery with a rated capacity of 242A·h as an example, under the conditions of SOC=0% and constant temperature of 20℃, different charging rates were used for charging. The parameter results are shown in Table 3-1 and the charging curve is shown in Figure 3-12.
Table 3-1 Charging Parameters for Different Charging Rates
| Current/A(Rate) | CC-CV①Total Time | Constant CurrentTime/s | Total ChargedCapacity/A·h | Total ChargedEnergy/W·h | Constant CurrentCharged Capacity/A·h | Constant VoltageCharged Energy/W·h | 170A·hTime/s | 170A·hCurrent/A |
|---|---|---|---|---|---|---|---|---|
| 4.84/(0.02C) | 182220 | 182220 | 245.74 | 942.54 | 245.74 | 942.54 | 127400 | 4.85 |
| 12.1/(0.05C) | 72318.5 | 72318.5 | 243.70 | 935.37 | 243.70 | 935.37 | 50400 | 12.11 |
| 24.2/(0.1C) | 36206.8 | 35800 | 243.20 | 935.77 | 241.03 | 926.69 | 25200 | 24.24 |
| 48.4/(0.2C) | 18317.5 | 17560 | 241.08 | 933.32 | 236.32 | 912.16 | 12600 | 48.44 |
| 80.7/(0.33C) | 11443.6 | 10490 | 243.50 | 946.27 | 235.29 | 910.08 | 7590 | 80.76 |
| 121/(0.5C) | 7936.6 | 6900 | 243.92 | 952.95 | 232.09 | 900.85 | 5110 | 121.09 |
① CC, Constant Current; CV, Constant Voltage.
As shown in Table 3-1, the constant current time gradually decreases with increasing charging current, and the capacity and energy that can be charged under constant current also gradually decrease. Taking the charging and discharging capacity of 1/2 (i.e., SOC=50%) as the standard, the required charging time decreases with increasing charging current; the time required for 0.1C is approximately 5 times that for 0.5C. Under this condition, the current difference for continued charging is small, so the charging time for the last 30A·h is not significantly different. Therefore, within the battery's allowable charging current, increasing the charging current, although reducing the capacity and energy that can be charged under constant current, helps to reduce the overall charging time. In practical battery pack applications, the maximum allowable charging current of the lithium-ion battery can be used for charging, and after reaching the voltage limit, constant voltage charging can be performed. This reduces charging time while ensuring charging safety. However, increasing the charging current will also lead to an increase in energy loss due to the battery's internal resistance. The energy consumed in the internal resistance is calculated according to equation (3-4).
Where E is the energy consumed by the internal resistance;
r is the battery's internal resistance;
t is the charging time variable;
I is the charging current;
t₁ and t₂ are the charging start and end times.
Extensive testing has demonstrated that the internal resistance of lithium-ion batteries changes within 0.4 mΩ during charging. Therefore, equation (3-4) shows that the energy consumption due to battery internal resistance is essentially linearly related to charging time, but quadratically related to the charging current. During the constant current charging stage, the magnitude of the charging current is the primary factor influencing internal resistance energy consumption; a higher charging current results in greater energy consumption. During the constant voltage, low current stage, charging time becomes the primary factor influencing internal resistance energy consumption; a longer charging time results in greater energy consumption. Considering the entire charging process, since the charging current has a quadratic relationship with internal resistance energy consumption and is the main factor affecting it, a higher charging current results in greater internal resistance energy consumption. In practical battery applications, a suitable charging current should be selected by comprehensively considering both charging time and efficiency.
(2) Effect of Depth of Discharge on Charging Characteristics Under a constant temperature of 20℃, a discharge test was conducted on an NCM lithium-ion battery with a rated capacity of 66.2 A·h. The battery was discharged at a rate of 0.5C to different depths of discharge (DOD) (10%→100%), corresponding to a State of Charge (SOC) of 90%→0%. Voltage, current, and capacity data were recorded during the discharge process. After resting for 60 minutes, the battery was charged at a rate of 0.5C (CC). When the cutoff voltage was reached, the charging mode was switched to constant voltage (CV). When the current was less than 0.05C, the process was stopped, and voltage, current, and capacity data were recorded. The relevant data are shown in Table 3-2. The charging current curves of the lithium-ion battery under different depths of discharge conditions are shown in Figure 3-13.
Table 3-2 Charging Test Parameters at Different Depth of Discharge
| SOC | DOD | Discharge | Charge | Equal-CapacityCharged Energy①/W·h | Equal-CapacityDischarged Energy②/W·h | ChargingTime/min | Constant CurrentTime/min | Constant CurrentCharged Capacity/A·h | Average Charging Timeper Unit Capacity③/min | ||
|---|---|---|---|---|---|---|---|---|---|---|---|
| Capacity/A·h | Energy/W·h | Capacity/A·h | Energy/W·h | ||||||||
| 80.00 | 20.00 | 13.35 | 54.03 | 13.48 | 55.88 | 27.94 | 27.02 | 41.13 | 33.50 | 12.32 | 3.05 |
| 70.00 | 30.00 | 20.02 | 80.16 | 19.99 | 82.08 | 27.36 | 26.72 | 59.23 | 50.83 | 18.69 | 2.96 |
| 60.00 | 40.00 | 26.69 | 105.62 | 26.61 | 108.19 | 27.05 | 26.41 | 77.72 | 68.50 | 25.19 | 2.92 |
| 50.00 | 50.00 | 33.36 | 130.42 | 33.27 | 133.61 | 26.72 | 26.08 | 96.02 | 86.67 | 31.87 | 2.89 |
| 40.00 | 60.00 | 40.04 | 154.61 | 39.95 | 158.50 | 26.42 | 25.77 | 114.18 | 104.83 | 38.55 | 2.86 |
| 30.00 | 70.00 | 46.71 | 178.38 | 46.61 | 182.97 | 26.14 | 25.48 | 132.28 | 123.00 | 45.22 | 2.84 |
| 20.00 | 80.00 | 53.38 | 201.73 | 53.26 | 207.07 | 25.88 | 25.22 | 150.40 | 141.00 | 51.84 | 2.82 |
| 10.00 | 90.00 | 60.05 | 224.45 | 59.92 | 230.62 | 25.62 | 24.94 | 168.47 | 159.17 | 58.52 | 2.81 |
① Equal-Capacity Charged Energy: Energy charged under the same SOC change (e.g., 10%). For example: if the charging capacity at 90% DOD is 30W·h, the equal-capacity charged energy is 30W·h; if the charging capacity at 80% DOD is 50W·h, the equal-capacity charged energy is 25W·h.
② Equal-Capacity Discharged Energy: Energy discharged under the same SOC change (e.g., 10%).
③ Average Charging Time per Unit Capacity /min: Charging Time / Charging Capacity.
From Table 3-2 and Figure 3-13, the following conclusions can be drawn:
1) With increasing depth of discharge, the charging time increases, but the average charging time per unit capacity decreases, meaning the increase in charging time is not proportional to the depth of discharge.
2) With increasing depth of discharge, the proportion of constant current charging time to total charging time increases, and the proportion of constant current charging capacity to required charging capacity increases. In reality, these characteristics are mainly caused by two factors: first, a deeper depth of discharge requires a longer time to fully charge the battery; second, a deeper depth of discharge corresponds to a lower voltage range, resulting in less energy being charged into the battery under the same current and charging time conditions.
(3) Influence of Temperature on Charging Characteristics Lithium-ion batteries were charged under different ambient temperatures. Taking a 66.2 A·h NCM lithium-ion battery as an example, a constant current and voltage limiting method was used. Charging parameters were recorded with the charging current limit being 1.3 A and 3.3 A, as shown in Table 3-3. Under the same discharge current, the battery voltage will experience a sharp drop, as shown in Figure 3-13. However, because the voltage remains relatively high, the discharge energy is still high. In the initial stage of discharge, the energy consumed by the battery's internal resistance increases the battery's temperature, enhances the activity of the lithium-ion battery's active materials, and raises the battery voltage, thus increasing the energy that can be released. In the middle and later stages of discharge, the battery voltage decreases, and the energy released per unit time decreases accordingly.
At the same temperature and with the same discharge termination voltage, different discharge termination currents will result in differences in the capacity and energy released. Generally, under normal temperature conditions, the lower the current, the greater the capacity and energy released. As in the discharge experiment mentioned above, 0.2C releases 3.2% more capacity and energy than 1C.
