Basic concepts related to lithium batteries
Open circuit voltage: The voltage across the positive and negative terminals of the battery when there is no load. Under normal circumstances, the open circuit voltage of a lithium-ion battery is about 4.1-4.2V after it is fully charged, and the open circuit voltage after discharge is about 3.0V. By detecting the open circuit voltage of the battery, the state of charge of the battery can be judged.
Working voltage: The voltage at both ends of the positive and negative terminals when the battery is under load. In the working state of battery discharge, when the current flows inside 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, and the opposite is true when charging. The discharge working voltage of a lithium-ion battery is around 3.6V.
Nominal voltage: the average voltage of the whole process when the battery is discharged at 0.2C.
Nominal capacity: the discharge capacity when the battery is discharged at 0.2C.
Internal resistance: The resistance between the positive and negative terminals of the battery. Including ohmic internal resistance and polarization internal resistance.
Rated capacity: Lithium-ion battery stipulates that under normal temperature, constant current (1C) and constant voltage (4.2V) controlled charging conditions, when it is charged for 3 hours and then discharged to 2.75V at 0.2C, the amount of electricity released is its rated capacity.
Actual capacity: refers to the actual power released by the battery under certain discharge conditions, which is mainly affected by the discharge rate and temperature (so strictly speaking, the battery capacity should indicate the charge and discharge conditions). Capacity unit: mAh, Ah)
Cycle life: Under certain conditions, the rechargeable battery is repeatedly charged and discharged, and the number of charge and discharge times that can occur when the battery performance such as capacity falls below the specified requirements. Lithium-ion battery GB stipulates that the capacity retention rate of the battery after 500 cycles under 1C conditions is above 60%.
Capacity density: the amount of electricity that can be released per unit mass or unit volume, generally expressed in mA·h/L or mA·h/kg.
Energy density: The energy that can be released per unit mass or unit volume, generally expressed in W·h/L or W·h/kg.
Coulombic efficiency: Under certain charging and discharging conditions, the percentage of the charge released during discharge to the charge charged during charging is also called charge and discharge efficiency.
Charging characteristics: The characteristics displayed when the battery is charging, such as charging curve, charging capacity, charging rate, charging depth, charging time, etc.
Charging curve: When the battery is charging, its voltage changes with time.
Overcharge: The process of continuing to charge beyond the specified charge termination voltage, at which time the service life of the battery is affected.
Discharge characteristics: The characteristics displayed when the battery is discharged, such as discharge curve, discharge capacity, discharge rate, discharge depth, discharge time, etc.
Discharge capacity: The amount of charge released when the battery is discharged. It is generally expressed as the product of time and current. For example A.h, mA.h (1A.h=3600C)
Discharge rate: A measure of the speed of discharge. All the capacity is discharged in 1 hour, which is called 1C discharge; in 5 hours, it is called C/5 discharge. C (magnification) is used to indicate the ratio of the current magnitude when the battery is charging and discharging, that is, the magnification.
Depth of discharge: A measure of the degree of discharge. It is the percentage of the discharge capacity to the total discharge capacity, referred to as DOD
Overdischarge: Refers to exceeding the specified cut-off voltage and continuing to discharge when it is lower than the cut-off voltage. At this time, liquid leakage may easily occur or the service life of the battery may be affected.
Self-discharge: The process in which the battery loses capacity when it is not connected to an external load during the storage process.
Internal short circuit: The state when the positive and negative electrodes inside the battery form an electrical path, mainly due to the destruction of the diaphragm, the mixing of conductive impurities, and the formation of dendrites.