The three main characteristics of lithium-ion batteries were analyzed.
I. Battery Capacity Characteristics
Figure 3 shows the relationship between battery discharge voltage and capacity at different discharge rates.
As can be seen from the figure, the voltage curve of a lithium-ion battery during the entire discharge process can be divided into three stages:
1) The battery's terminal voltage drops rapidly in the initial stage; the higher the discharge ratio, the faster the voltage drops.
2) The battery voltage enters a phase of slow change. This period is called the battery's plateau region. The lower the current flow, the longer the plateau region lasts. In practical use of lithium-ion batteries, the battery should ideally operate within this plateau region.
3) When the battery is nearing the end of its discharge, the battery load voltage begins to drop sharply until it reaches the discharge cutoff voltage. The relationship between discharge current and capacity can also be obtained from the capacity test results, as shown in Figure 4.
As the diagram shows, the battery's discharge current directly affects its actual capacity. A higher discharge current results in a lower battery capacity, meaning a larger discharge current leads to a shorter time to reach the termination voltage. Therefore, when discussing battery capacity, the discharge current (discharge rate) should be specified.
Second generation. Battery open-circuit voltage characteristics.
The relationship between the open-circuit voltage and the state of charge (SOC) of a lithium-ion battery is shown in Figure 5.
As can be seen from the figure, the OCV-SOC curve of the battery shows the same trend as the battery's discharge voltage curve. In the middle range of SOC (20%-90%), the rate of change of OCV is relatively large. The overall OCV-SOC curve of the lithium iron phosphate battery is flat in the middle region, but steep at both the beginning and end.
The ocv-soc relationship curve of lithium-ion batteries is less affected by factors such as temperature, discharge rate and aging degree[7], but its characteristic curves are different in the two states of charging and discharging.
3. Battery internal resistance characteristics
The ohmic resistance curve of lithium iron phosphate batteries exhibits the following characteristics: Within a wide SOC range (Figure 6), from SOC=100% (10%), the battery's ohmic resistance changes very little. However, as the SOC interval decreases, the ohmic resistance decreases substantially due to the internal chemical activity of the battery during discharge. Throughout the entire SOC range, the internal resistance during charging is generally greater than that during discharging. This is because discharging a lithium-ion battery is a spontaneous reaction and occurs relatively easily. Charging is accomplished through an external power source, making it difficult for lithium ions to intercalate into the negative electrode. It is important to note that the battery's internal resistance is very complex and is affected by factors such as temperature, depth of discharge, charge/discharge rate, and cycle count. Different battery cells of the same type also vary depending on the manufacturer and operating environment.
