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The voltage range of lithium-ion batteries and the discharge curve of lithium batteries

2020-09-27 09:20

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1. Voltage range of lithium-ion battery


The voltage range of lithium-ion batteries is 3.0 to 4.2V, and the nominal voltage is 3.7V.


Lithium-ion batteries have a handy feature in terms of electricity metering, that is, when discharging, the battery voltage will gradually decrease with the passage of current, and there is a steep slope, which provides us with another approximate method of electricity metering. The way of measuring the battery voltage is the same as measuring the height of the water in the water tank, and the remaining water can be roughly estimated. Still, the battery voltage is much more complicated than measuring the height of the calm water in the water tank.


Using voltage to estimate the remaining capacity of a battery has the following instabilities:


1. The more massive the discharge current, the lower the voltage, and the highest voltage appears when there is no current;


2. Under the same capacity, the lower the temperature, the lower the battery voltage;


3. As the cycle progresses, the discharge platform of lithium-ion batteries tends to degenerate, and the discharge platform decreases, so the capacity represented by the same voltage also changes;


4. Lithium-ion batteries with different degrees of different manufacturers have slightly different discharge platforms;


5. Li-ion batteries with varying materials of the electrode have other discharge platforms, and the discharge platforms of cobalt lithium and manganese lithium are also entirely different.


2. Flow curve of lithium battery

The discharge curve refers to the angle of the voltage, current and capacity of the battery overtime during the discharge process. The information contained in the charge and discharge curve is very rich, including the relationship between power, energy, working voltage and voltage plateau, electrode potential and state of charge. The primary data recorded in the discharge test is the time evolution of current and voltage, and many parameters can be obtained from these raw data.


The basic principle of the discharge test: After knowing the battery voltage, we begin to analyze the discharge curve of the lithium battery. The discharge curve reflects the state of the electrode, which is the superposition of the state changes of the positive and negative electrodes. During the charge and discharge test, the device imposes a specific load on the battery. It records the evolution of voltage and current over time, according to the set data recording conditions. Process.


The voltage curve of a lithium battery during the entire discharge process can be divided into three stages


1. In the initial stage, the terminal voltage drops rapidly, the higher the discharge rate, the faster the voltage drops;


2. The battery voltage enters a phase of slow change, called a period, the smaller the flow, the longer the duration in plateau areas, the higher the platform voltage, the slower the voltage drop;


3. When the battery is nearly discharged, the load voltage of the storm begins to drop sharply until it reaches the discharge cut-off voltage.


When a lithium battery is discharged, its working voltage always changes with time. The curve is drawn with the battery working voltage as the ordinate, discharge time or capacity, or state of charge, or discharge depth as the abscissa, is called the discharge curve. To understand the discharge characteristic curve of the battery, it is necessary to understand the voltage of the battery in principle.