The characteristics of lithium-ion battery

The features of battery capacity

The relation curve between discharge voltage and capacity in different discharge rate is shown in figure 3.

Figure 3 the relation curve between discharge voltage and capacity in different discharge rate

In the whole course of discharging, the voltage curve of lithium-ion battery can be divided into three stages:

1) In the initial stage, voltage falls fast, the higher discharge rate, the faster the voltage drops;

2) The voltage comes into a phase of slow change, which is called a battery of platform area. The smaller the discharge rate, the longer the duration of the platform, and the higher the voltage, the more slowly the voltage drops. During the usage of lithium-ion battery, it should be at this platform stage as much as possible.

3) When battery capacity is near finished, the load voltage falls sharply until discharge cut-off voltage is reached. During the capacity test, we can get the relation curve of the discharge current and capacity, as shown in figure 4.

Figure 4 different relation curve of the discharge current and capacity

Can be seen from the graph, the size of the battery discharge current, will directly affect the actual capacity of the battery. Discharge current, the greater the battery capacity decreases, this suggests that the larger the discharge current, the shorter the time of arrival in termination voltage experience. So, when it comes to battery capacity, shall indicate the discharge current, discharge ratio).

The characteristics of battery open circuit voltage

Open circuit voltage test [6] for lithium ion battery open circuit voltage and the curve of the battery SOC is shown in figure 5.

Figure 5 battery charging and discharging OCV - SOC curve

Can be seen from the diagram, battery OCV - SOC curve and the battery discharge voltage curve trend is basically the same. In the middle of the SOC interval (20% < SOC < 80%), battery OCV change is very small, the battery to the courts; In the SOC at both ends of the interval (SOC < 10% and SOC > 10%), OCV the rate of change is bigger, the whole lithium iron phosphate batteries Flat OCV - SOC curve of the middle area, the appearance of the steep on both ends of the head to tail, open circuit voltage method is the use of this stable corresponding relation between SOC estimation.

Lithium ion battery OCV - SOC relation curve affected by temperature, discharge ratio, aging degree of smaller [7], but in the condition of charging and discharging two, there are certain differences between the two-characteristic curve.

The characteristics of battery internal resistance

Figure 6 iron phosphate the lithium battery Ohm internal resistance when charging and discharging.

Figure 6 battery internal resistance curve

The curve of LiFePO4 battery ohmic resistance presents the following characteristics: in a broad SOC of figure 6, the SOC = [10%-100%] range, the change of battery ohm resistance is small. And in the lower SOC interval, along with the decrease of SOC, ohm resistance was substantially increasing. This is because in the end of battery discharge, the activity of internal chemical material decreases. Within the whole SOC, charging ohm internal resistance is greater than the discharge ohm internal resistance. This is because the lithium-ion battery discharge is a spontaneous reaction, which is easier. Charge is done by an external power supply, making the lithium-ion battery into a cathode, which is more difficult. To be sure, battery internal resistance change is very complex, affected by temperature, depth of discharge, charge-discharge rate, cycles and other factors. The same type of different monomer battery, it also can be related to different situation of ex-factory and working environment.

The page contains the contents of the machine translation.