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Analysis of the three characteristics of lithium-ion batteries

by:dcfpower     2021-03-11
1. The capacity characteristics of the battery

The relationship curve between the discharge voltage and the capacity of the battery under different rates is shown in Figure 3.

Figure 3 The relationship between discharge voltage and capacity under different rates

It can be seen from the figure that the lithium battery pack's The voltage curve can be divided into 3 stages: 1) The battery voltage drops rapidly in the initial stage, the greater the discharge rate, the faster the voltage drops; 2) The battery voltage enters a stage of slow changes, this period of time is called the battery platform Area, the smaller the discharge rate, the longer the platform area lasts, the higher the platform voltage, the slower the voltage drop. In the actual use of lithium-ion batteries, it is desirable that the battery work in the platform area as much as possible; 3) When the battery is nearly discharged, the battery load voltage begins to drop sharply until it reaches the discharge cut-off voltage. From the results of the capacity test, the curve relationship between the discharge current and the capacity can also be obtained at the same time, as shown in Figure 4.

Figure 4  The relationship curve between different discharge current and capacity

It can be seen from the figure that the size of the battery discharge current will directly affect the actual capacity of the battery. The greater the discharge current, the corresponding reduction in battery capacity, which indicates that the greater the discharge current, the shorter the time to reach the termination voltage. So when talking about battery capacity, you should specify its discharge current (discharge rate).

Second, battery open circuit voltage characteristics

Open circuit voltage test [6] The relationship curve between open circuit voltage of lithium battery pack and battery SOC is shown in Figure 5.

Figure 5 'OCV-SOC curve during battery charging and discharging

It can be seen from the figure that the trend of the battery's OCV-SOC curve is basically the same as that of the battery discharge voltage curve. In the middle range of SOC (20%<SOC<80%), the OCV of the battery changes very little, and the battery is in the plateau area; while in the two ends of SOC range (SOC<10% and SOC>90%), the OCV changes The OCV-SOC curve of the entire lithium iron phosphate battery is flat in the middle, and the head and tail ends are steep. The open circuit voltage method uses this stable correspondence to estimate the SOC. The OCV-SOC relationship curve of lithium ion battery is less affected by temperature, discharge rate, and aging degree factors [7], but there will be a certain difference between the two characteristic curves in the two states of charge and discharge.

3. Battery internal resistance characteristics

Figure 6 shows the ohmic internal resistance of a lithium iron phosphate battery during charging and discharging.

Figure 6  Battery internal resistance change curve

The ohmic internal resistance curve of a lithium iron phosphate battery presents the following characteristics: In Figure 6, a wider SOC Within the range, that is, in the range of SOCu003d[10%, 100%], the ohmic resistance of the battery changes very little, while in the lower SOC range, the ohmic resistance increases significantly as the SOC decreases. The reason is that the battery's internal chemical substance activity decreases at the end of battery discharge; in the entire SOC range, the charging ohmic internal resistance is generally greater than the discharging ohmic internal resistance. This is because the discharge of lithium-ion batteries is a spontaneous reaction and is easier; charging is performed by an external power source. , It is more difficult to insert lithium ions into the negative electrode. It should be noted that the internal resistance of the battery changes very complicatedly and is affected by factors such as temperature, depth of discharge, charge and discharge rate, and number of cycles. Different battery cells of the same type are also related to the inconsistency of the battery and the different working environment.

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