What Are The Performance Parameters of Lithium Battery Cells?

What is open circuit voltage?
Open circuit voltage refers to the potential difference between the positive and negative poles of the battery when the battery is not working, that is, when there is no current flowing in the circuit. Generally, the open-circuit voltage of the lithium-ion battery is about 4.1-4.2V after full charge and about 3.0V after discharge. By detecting the open-circuit voltage of the battery, the state of charge of the battery can be judged.

What is working voltage?
Working voltage, also known as terminal voltage, refers to the potential difference between the positive and negative poles of the battery in the working state, that is, when the circuit current flows through. In the working state of battery discharge, when the current flows through the battery, there is no need to overcome the resistance caused by the internal resistance of the battery, so the working voltage is always lower than the open-circuit voltage, which is the opposite when charging. The working voltage of the lithium-ion battery is about 3.6V.

What is discharge platform time?
Discharge platform time refers to the discharge time when the battery is fully charged to a certain voltage. For example, measure the 3.6V discharge platform time of an NCM battery, charge it at constant voltage until the voltage is 4.2V, and when the charging current is less than 0.02C, stop charging, i.e., charge it fully, and then put it aside for 10 minutes. When the discharge current at any rate is lowered to 3.6V, the discharge time is the discharge platform time of the current. Because some electric appliances using lithium ion batteries have voltage requirements, if the voltage is lower than the required value, it will not work. So discharge platform is one of the important criteria to measure battery performance.

What is the charge and discharge rate?
The charge and discharge rate refers to the current value required when the battery releases its rated capacity within a specified time. 1C is numerically equal to the rated capacity of the battery, which is usually represented by the letter C. If the rated capacity is 10Ah (1C), then 10A is 1C, 5A is 0.5C, and so on.

What is the self-discharge rate?
Self-discharge rate, also known as charge retention capacity, refers to the battery in the open circuit status, and the battery storage capacity under certain conditions. It is an important parameter to measure the performance of the battery, which is mainly affected by the manufacturing process materials and storage conditions of the battery.

What are charge efficiency and discharge efficiency?
Charging efficiency refers to the degree to which the electric energy consumed by the battery in the charging process is converted into the chemical energy that the battery can store. It is mainly affected by the battery process, formula and the working ambient temperature of the battery. Generally, the higher the ambient temperature, the lower the charging efficiency.

Discharge efficiency refers to the ratio of the actual amount of electricity discharged to the terminal voltage under certain discharge conditions to the rated capacity of the battery. It is mainly affected by discharge rate, ambient temperature, internal resistance and other factors. Generally, the higher the discharge rate, the lower the discharge efficiency. The lower the temperature, the lower the discharge efficiency.

What is battery cycle life?
Battery cycle life is the number of charges and discharges a battery undergoes under a certain charge and discharge system when the battery capacity decreases to a specified value.

What is battery internal resistance?
Battery internal resistance refers to the resistance of current flowing through the battery when the battery is working. It is composed of ohmic internal resistance and polarization internal resistance. A large battery resistance reduces the battery discharge voltage and shortens the discharge time. The internal resistance is mainly affected by the material, manufacturing process and structure of the battery. Battery internal resistance is an important parameter to measure battery performance.

What is the battery capacity?
The capacity of a battery can be rated or actual.
The rated capacity of lithium-ion battery is when it is charged for 3h and discharged to 2.75V at 0.2C under the charge condition of normal temperature, constant current (1C) and constant voltage (4.2V).
The actual capacity of the battery refers to the actual power released by the battery under certain discharge conditions, mainly affected by the discharge rate and temperature (so strictly speaking, the capacity of the battery should specify the charge and discharge conditions). Capacity unit :mAh, Ah(1Ah=1000mAh)

What are constant current and constant voltage charging?
The first stage is charged with constant current: when the voltage reaches a predetermined value, the second stage is charged with constant voltage, and the current gradually decreases. When the charging current drops to zero, the battery is fully charged. This is the most commonly used charging method for lithium batteries at present. The voltage waveform of constant current and constant voltage charging current is shown in the figure below.

Charging characteristics of lithium batteries
Lithium-ion batteries require high voltage accuracy, with an error of no more than 1%. At present, the battery with a rated voltage of 3.7V is commonly used, and the charging termination voltage of the battery is 4.2V, so the allowable error range is 0.042V. Lithium-ion batteries are usually charged in constant current and constant voltage modes. Charging starts at a constant current stage, at which the battery voltage is low and the charging current is stable and constant. As the charging continues, the battery voltage gradually rises to 4.2V, at this time the charger should immediately turn to constant voltage charging, charging voltage fluctuation should be controlled by 1%, charging current gradually decreases. When the current drops to a certain range, it enters the trickle charging stage. Trickle charging is also called maintenance charging. In the maintenance charging state, the charger continues to add a charge to the battery at a certain charging rate, and finally makes the battery in a sufficient state.

Discharge characteristics of lithium batteries
The discharge characteristic curve of lithium-ion batteries is shown in the figure. Lithium battery discharge, firstly, the discharge current can not be too large, too large current will lead to internal heating, and may cause permanent damage. Second, the battery voltage can not be lower than the discharge termination voltage. If the termination voltage still continues to discharge, it will cause over-discharge, which will also cause permanent damage to the battery. The variation of battery voltage has a great difference under different discharge rates. The higher the discharge rate, the lower the battery voltage at the corresponding remaining capacity. The rated capacity can be released when the battery voltage drops to 2.75V with a 0.2C discharge rate. At the 1C discharge rate, 98.4% of rated capacity can be released.

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