Lithium battery parameters and professional terms such as energy density, self-discharge rate, cut-off voltage, battery internal resistance, etc., are very confusing for many people who are new to lithium batteries.
So, what exactly do these common terms mean? The following examples will explain the meaning of the term in simple words.
1. Battery voltage
● Open circuit voltage
Open circuit voltage refers to the voltage value between the two terminal posts of the battery directly measured by the meter when the battery is not in working condition.
The open circuit voltage has a certain relationship with the residual energy of the battery, and the power display uses this principle. The open circuit voltage cannot be used as a standard to measure the battery voltage.
Through the open circuit voltage of the battery, the state of charge of the battery can be determined. Take the 12v battery as an example, mainly used in 12 v battery is mainly used in boats, mobile lighting, RV and other fields.
● Working voltage
Working voltage refers to the potential difference between the anode and cathode poles of the battery when the battery is in working condition, that is, when current flows through the circuit.
In the working state of the battery discharge, when the current flows through the battery, the resistance of the internal resistance must be overcome, so the working voltage is always lower than the open circuit voltage.
● Discharge cut-off voltage
Discharge cut-off voltage refers to the voltage reached when the battery is fully charged and discharged (if it continues to discharge, it is excessive discharge, which damages the battery's life and performance).
● Charge limiting voltage
Charge limiting voltage The voltage that changes from constant current to constant voltage charging during charging.
2. Battery capacity
Battery capacity refers to how much electricity the battery can store, and capacity is an important indicator of battery electrical performance, which is determined by the active material of the electrode.
Capacity is expressed in C and units in Ah (ampere-hours) or mAh (milliampere-hours). A battery with a capacity of 10 amps can be discharged for 2 hours at 5 amps and 1 hour at 10 amperes. Battery capacity can be divided into theoretical capacity, rated capacity, and actual capacity.
3. Battery internal resistance
The internal resistance of the battery refers to the resistance of current flowing through the inside of the battery when the battery is working. The size of the internal resistance is mainly affected by the material of the battery, the manufacturing process, the structure of the battery, and other factors.
The internal resistance of the battery includes ohmic internal resistance and polarized internal resistance.
● Ohmic internal resistance
Ohmic internal resistance is composed of electrode material, electrolyte, diaphragm resistance, and contact resistance of various parts. Ohmic resistors obey Ohm's law. Polarization resistance increases with the increase of current density, but it is not linear, and often increases linearly with the logarithm of current density.
● Internal polarization resistance
Internal polarization resistance includes resistance due to electrochemical polarization and concentration polarization.
Due to the existence of internal resistance, when the battery is discharged, the current passes through the internal resistance to generate heat and consume energy.
The greater the current, the more energy is consumed, so the smaller the internal resistance, the better the performance of the battery. Not only is the actual operating voltage of the battery high, but the energy consumed in the internal resistance is also small.
The presence of internal resistance causes the terminal voltage during charging to be higher than the electromotive force and the open circuit voltage.
The internal resistance of the battery is not constant and changes over time during discharge. Because the composition of the active material, electrolyte concentration, and temperature are constantly changing.
4. Charging and discharging of the battery
● Charge rate
C is the first letter of capacity, which is used to indicate the magnitude of the current when the battery is charged and discharged. For example, when the rated capacity of a rechargeable battery is 1100 mAh, it means that the discharge time at 1100 mAh (1C) can last for 1 hour. And if the discharge time at 200 mAh (0.2C) can last for 5 hours, the charge can also be calculated according to this control.
● Battery discharge rate
The discharge rate refers to the current value required to release its rated capacity within a specified time, which is numerically equal to a multiple of the battery's rated capacity.
With 48 A discharge, its discharge rate is 2 C. Conversely, 2 C discharge, discharge current is 48 A, 0.5 hours discharge; Charging with 12A, its charging rate is 0.5C, conversely, 0.5C charging, charging current is 12A, 2 hours charging;
The charge and discharge rate of the battery determines how fast we can store a certain amount of energy in the battery, or how fast we can release energy into the battery.
When the battery is charging, if it continues to charge after reaching its full state, it may cause the internal voltage of the battery to rise, battery deformation, night leakage, etc., and the performance of the battery will be significantly reduced and damaged.
● Over discharge
If the battery exceeds the battery discharge termination voltage value during the discharge process, and continues to discharge, it may cause the internal battery voltage to rise, and the reversibility of anode and cathode active materials will be damaged, so that the battery capacity will be significantly reduced.
● Depth of charge and discharge
A representation of the battery's holding capacity value. The depth of charge and discharge is expressed as a percentage.
SOC : State of charge, represents the ratio of the remaining capacity of the battery, after a period of use or long-term storage to the capacity of its fully charged state, commonly expressed as a percentage. Its value range is 0~1, when SOC=0 means that the battery is completely discharged, and SOC=1 means that the battery is completely full.
DOD: The depth of discharge of the battery is 0 when fully charged, and its DOD is 1 when completely discharged. Therefore, under normal circumstances, the DOD of the battery has a value between 0 and 1, and the relationship between DOD and SOC is: DOD+SOC=1.
For example, a battery with a capacity of 10 Ah becomes 2 Ah after discharging, which can be called 80% DOD. For example, a battery with a capacity of 10 Ah has a capacity of 8 Ah after charging, or 80% SOC. Described as complete and complete, it is often referred to as "100% DOD."
● Self-discharge rate
Lithium battery self-consumption, also known as charge retention capacity, refers to the battery's ability to retain stored power under certain conditions in the battery's open circuit state.
Mainly affected by the battery manufacturing process, materials, storage conditions, and other factors. It is an important parameter for measuring battery performance. Due to the instability of the electrodes in the electrolyte, a chemical reaction occurs between the two electrodes of the battery, the active substance is consumed.
The ambient temperature has a great influence on it, and too high a temperature will accelerate the battery self discharge rate. Battery self-discharge will directly reduce battery capacity. The self-discharge rate directly affects the battery's storage performance. The lower the self-discharge rate, the better the storage performance.
5. The life of the battery
It is divided into two parameters: cycle life and calendar life.
Cycle life refers to the number of times a battery can be cycled to charge and discharge. That is, at the ideal temperature and humidity, the rated charge and discharge current is used to charge and discharge, and the number of cycles experienced when the battery capacity decays to 80% is calculated.
Calendar life refers to the time taken by the battery to reach the end-of-life condition (capacity decay to 80%) under specific operating conditions under operating environment conditions. If the calendar life is closely combined with the specific use requirements, it is usually necessary to specify the specific use conditions, environmental conditions, storage intervals, etc.
6. Battery energy
Refers to the amount of energy stored by the battery, expressed in Wh. Battery energy is an important indicator of the work done by the battery to drive the equipment, and capacity cannot determine the amount of work done.
● Energy density
Refers to the energy released per unit volume or mass, usually expressed as volumetric energy density (Wh/L) or mass energy density (Wh/kg). At present, the energy density of lifepo4 batteries is between 3 and 1.5 times that of nickel-cadmium and nickel-metal hydride batteries, and the energy density is determined by the density of the material, not the structure.
● Power density
Divide the energy by time to get the power in W or kW. Specific power is an important indicator to assess whether the battery meets the acceleration requirements of electric vehicles. The power battery with high specific power is fast like a rabbit and can provide high instantaneous current to ensure good acceleration performance of the car.
● Memory effect
The "memory effect" of a battery is the percentage of a battery that is not fully discharged and can be charged on the next charge. The substances in the battery are crystallized, such as in nickel-cadmium batteries, which reduces the activity of the cathode.
In order to eliminate the memory effect of the battery, it must be completely discharged and then charged. Lithium-ion batteries have no memory effect. With a brief introduction to terminology and examples to illustrate, you can have a deeper understanding. Voltage, capacity, internal resistance, charge-discharge ratio, etc. are all standards for measuring a battery.