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1、 Capacity characteristics of batteries

The relationship curve between discharge voltage and capacity of the battery at different rates obtained from capacity testing is shown in Figure 3.

Figure 3 Relationship curve between discharge voltage and capacity at different magnifications

From the graph, it can be seen that the voltage curve of lithium-ion batteries can be divided into three stages throughout the entire discharge process:

1) In the initial stage, the terminal voltage of the battery drops rapidly, and the larger the discharge rate, the faster the voltage drops;

2) The battery voltage enters a stage of slow change, which is called the platform area of the battery. The smaller the discharge rate, the longer the platform area lasts, the higher the platform voltage, and the slower the voltage drop. In the actual use of lithium-ion batteries, it is advisable to expect the battery to work in the platform area as much as possible;

3) As the battery level approaches full discharge, the load voltage of the battery begins to drop sharply until it reaches the discharge cut-off voltage. From the results of capacity testing, the curve relationship between discharge current and capacity can also be obtained, as shown in Figure 4.

Figure 4 Relationship curves between different discharge currents and capacities

From the graph, it can be seen that the size of the battery discharge current directly affects the actual capacity of the battery. The larger the discharge current, the smaller the battery capacity, indicating that the larger the discharge current, the shorter the time it takes to reach the termination voltage. So when it comes to battery capacity, its discharge current (discharge rate) should be indicated.

2、 Open circuit voltage characteristics of batteries

The relationship curve between the open circuit voltage of lithium-ion batteries and the battery SOC obtained from the open circuit voltage test [6] is shown in Figure 5.

Figure 5 OCV-SOC curve during battery charging and discharging

From the graph, it can be seen that the OCV-SOC curve of the battery has a similar trend as the discharge voltage curve of the battery. In the middle range of SOC (20% ＜ SOC ＜ 80%), the OCV of the battery changes very little, and the battery is in the platform area; In the two end range of SOC (SOC<10% and SOC>90%), the rate of change of OCV is relatively large. The OCV-SOC curve of the entire lithium iron phosphate battery shows a flat middle region and steep ends. The open circuit voltage method uses this stable correspondence to estimate SOC.

The OCV-SOC relationship curve of lithium-ion batteries is less affected by factors such as temperature, discharge rate, and aging degree [7], but there may be some differences between the two characteristic curves in the two charging and discharging states.

Measuring the discharge curve of a battery is one of the basic methods for studying battery performance. Based on the discharge curve, it is possible to determine whether the battery's performance is stable and the maximum current allowed for stable operation. This article provides a detailed and comprehensive introduction to the basic knowledge of the discharge curve of lithium-ion batteries. Due to the limited level of the author, any errors in the text are welcome for criticism and correction. This article is long and over 10000 words long, mainly including:

1. Battery voltage

1.1 Electrode potential of lithium-ion battery materials

1.2 Open circuit voltage of the battery

1.3 Battery polarization

1.4 Working voltage of the battery

Basic principles of discharge testing

2.1 Discharge Test Mode

2.2 Information contained in the discharge curve

2.3 Basic form of discharge curve

Differential processing of discharge curves

When a lithium-ion battery is discharged, its working voltage always changes continuously over time. The curve drawn using the battery's working voltage as the vertical axis, discharge time, capacity, state of charge (SOC), or discharge depth (DOD) as the horizontal axis is called the discharge curve. To understand the discharge characteristic curve of a battery, it is necessary to first understand the voltage of the battery from a theoretical perspective.

1. Battery voltage

The electrode reaction to form a battery must meet the following conditions: the process of losing electrons (i.e. oxidation process) and the process of obtaining electrons (i.e. reduction reaction process) in chemical reactions must be separated in two different regions, which is different from general redox reactions; The electrons required for the oxidation-reduction reaction of the active substance in the two electrodes must be transferred through the external circuit, which is different from the micro battery reaction in the metal corrosion process. The voltage of a battery is the potential difference between the positive and negative electrodes, and the specific key parameters include open circuit voltage, working voltage, charging and discharging cut-off voltage, etc.

1.1 Electrode potential of lithium-ion battery materials

The electrode potential refers to the electrical effect exhibited by a solid material immersed in an electrolyte solution, which is the potential difference between the surface of the metal and the solution. This potential difference is called the potential of the metal in this solution or the electrode potential. Simply put, electrode potential represents the trend of an ion or atom acquiring electrons and being reduced.

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