Lithium-ion Battery Charge and Discharge Characteristics
Understanding the fundamental principles of lithium-ion battery charge and discharge processes is crucial for optimizing performance, ensuring safety, and extending battery life in various applications.
Key Insight
The battery charge process significantly impacts overall battery performance, safety, and longevity. Proper management of charging parameters is essential for optimal battery operation.
Two-Stage Charging Method
For lithium-ion batteries, considering safety, reliability, and battery charge efficiency, a two-stage charging method is typically employed. This approach balances the need for efficient charging while maintaining battery integrity.
First Stage: Constant Current with Voltage Limitation
The initial phase of the battery charge process uses a constant current while limiting the maximum voltage. This stage allows for efficient energy transfer into the battery while preventing overvoltage conditions that could damage the cells.
The optimal constant current value varies depending on the battery's cathode material and manufacturing process, typically ranging from 0.2C to 0.3C. In situations requiring rapid battery charge, higher rates such as 1C, 2C, or even higher may be utilized, though this can affect long-term battery health.
Second Stage: Constant Voltage with Current Limitation
Once the battery reaches the predetermined voltage limit during the first stage, the battery charge process switches to maintaining a constant voltage while allowing the current to gradually decrease. This stage ensures the battery reaches full capacity without overcharging.
The maximum voltage limit for battery charge varies depending on the specific cathode material used in the lithium-ion battery. This critical parameter is carefully determined by the battery manufacturer to ensure safety and performance.
Figure 3-11: Basic Charge-Discharge Voltage Curve of Lithium-ion Batteries
The charge and discharge curves shown use a C/3 current rate throughout the entire cycle
There are two primary differences between various types of lithium-ion batteries that affect their performance characteristics:
First Stage Constant Current Value
The optimal constant current value for the initial battery charge stage varies based on the battery's cathode material and manufacturing process. Typically, currents range from 0.2C to 0.3C for standard charging. In fast battery charge scenarios, higher rates such as 1C, 2C, or even higher may be employed, though these can impact battery lifespan and safety margins.
Constant Current Duration
Different lithium-ion batteries exhibit significant variations in the duration of the constant current phase during battery charge. This directly affects the proportion of total capacity that can be achieved during this stage. From an electric vehicle application perspective, a longer constant current phase reduces total battery charge time, which is highly beneficial for practical use.
Discharge Characteristics
During discharge, lithium-ion batteries maintain relatively stable voltage in the early and middle stages, with only a gradual decline. However, in the later stages of discharge, the voltage drops rapidly, as shown in segment DE of Figure 3-11.
Effective control must be implemented during this final discharge phase to prevent over-discharging, which can cause irreversible damage to the battery cells and significantly reduce their lifespan.
Under the same discharge current, the battery voltage will experience a sharp drop as illustrated in Figure 3-13. However, because the voltage remains at a relatively high level during most of the discharge cycle, the overall discharge energy remains substantial.
During the initial discharge phase, energy dissipated across the battery's internal resistance causes the battery's temperature to rise. This increased temperature enhances the activity of the lithium-ion battery's active materials, resulting in a slight voltage increase and therefore greater energy output. During the middle and later stages of discharge, the battery voltage decreases, leading to a corresponding reduction in energy output per unit time.
At a given temperature and with the same discharge termination voltage, different end-of-discharge currents result in variations in achievable capacity and energy. Generally, under normal temperature conditions, lower currents allow for greater capacity and energy output. For example, in discharge tests, a 0.2C rate yields 3.2% more capacity and energy compared to a 1C rate. This characteristic highlights the importance of matching discharge rates to application requirements, just as with battery charge rates.
Factors Affecting Battery Charge Characteristics
1. Influence of Charging Current on Battery Charge Characteristics
Taking an NCM lithium-ion battery with a rated capacity of 242Ah as an example, tests were conducted at 20°C with SOC=0% using different charging rates. The results are presented in Table 3-1, with corresponding charging curves shown in Figure 3-12.
| Charge Rate | Charge Time (h) | Total Capacity (Ah) | Efficiency (%) |
|---|---|---|---|
| 0.2C | 5.2 | 241.8 | 99.9 |
| 0.5C | 2.3 | 240.5 | 99.4 |
| 1C | 1.4 | 238.2 | 98.4 |
| 2C | 0.8 | 232.5 | 96.1 |
| 3C | 0.6 | 225.3 | 93.1 |
Figure 3-12: Charging Curves at Different Charge Rates
2. Influence of Depth of Discharge on Battery Charge Characteristics
Under constant temperature conditions of 20°C, discharge tests were conducted on an NCM lithium-ion battery with a rated capacity of 66.2Ah. The battery was discharged at a 0.5C rate to different depths of discharge (DOD) ranging from 10% to 100%, corresponding to SOC levels from 90% to 0%.
Voltage, current, and capacity data were recorded during the discharge process. After a 60-minute rest period, the battery charge process was initiated at 0.5C in constant current (CC) mode. When the cut-off voltage was reached, the system switched to constant voltage (CV) battery charge mode. The process was terminated when the current dropped below 0.05C, with all relevant voltage, current, and capacity data recorded throughout the battery charge cycle.
| DOD (%) | SOC (%) | Charge Time (h) | Charge Capacity (Ah) |
|---|---|---|---|
| 10 | 90 | 0.3 | 6.8 |
| 30 | 70 | 0.8 | 20.1 |
| 50 | 50 | 1.4 | 33.5 |
| 70 | 30 | 1.9 | 46.9 |
| 100 | 0 | 2.6 | 66.2 |
Figure 3-13: Lithium-ion Battery Charge Current Curves Under Different DOD Conditions
(Curves from left to right represent increasing discharge capacity)
3. Influence of Temperature on Battery Charge Characteristics
Battery charge behavior varies significantly across different ambient temperatures. Tests were conducted on an NCM lithium-ion battery with a rated capacity of 66.2Ah using a constant current, voltage-limited battery charge method. Charge parameters were recorded with charge current cut-off thresholds of 1.3A and 3.3A, as presented in Table 3-3.
| Temperature (°C) | Cut-off Current (A) | Charge Time (h) | Charge Capacity (Ah) |
|---|---|---|---|
| -10 | 1.3 | 5.2 | 58.3 |
| 3.3 | 3.8 | 52.1 | |
| 0 | 1.3 | 4.1 | 62.5 |
| 3.3 | 3.0 | 57.8 | |
| 25 | 1.3 | 2.8 | 66.2 |
| 3.3 | 2.1 | 63.5 | |
| 45 | 1.3 | 2.5 | 65.8 |
| 3.3 | 1.9 | 62.9 |
Figure 3-15: Discharge Energy vs. Discharge Capacity Curves at Different Temperatures
Summary of Key Findings
- The two-stage battery charge method (constant current followed by constant voltage) optimizes both safety and efficiency for lithium-ion batteries.
- Charge current significantly impacts battery charge time and efficiency, with higher currents reducing time but slightly decreasing overall capacity.
- Depth of discharge directly influences subsequent battery charge requirements, with deeper discharges requiring more time and capacity to fully recharge.
- Temperature has a substantial effect on battery charge performance, with optimal results typically observed in moderate temperature ranges (around 25°C).
- Discharge characteristics show stable voltage in early stages, with rapid voltage drop in later stages requiring careful monitoring to prevent over-discharge.
- Both battery charge and discharge rates affect overall energy output, with lower rates generally yielding higher capacity and energy at the cost of longer cycle times.