Various concepts of power lithium-ion batteries

?1. Voltage (V)

①Open circuit voltage: refers to the voltage of the battery when it is not connected to an external circuit or external load. There is a certain relationship between the open circuit voltage and the remaining energy of the battery, and the power display uses this principle.

②Working voltage: refers to the potential difference between the positive and negative electrodes of the battery when the battery is in working condition, that is, when current flows in the circuit, also known as the load voltage. In the battery discharge working state, when current flows through the interior of the battery, it must overcome the resistance of the internal resistance, so the operating voltage is always lower than the open circuit voltage.

③ Discharge cut-off voltage: refers to the voltage reached when the battery is fully charged and discharged (if it continues to discharge, it is over-discharge, which will damage the life and performance of the battery).

④Charging limit voltage: the voltage at which constant current changes to constant voltage during charging.

2. Battery capacity (Ah)

①Meaning: Battery capacity refers to the amount of electricity that the battery can store. Capacity is an important indicator of battery electrical performance, and it is determined by the active material of the electrode.

②Unit: The capacity is expressed in C, and the unit is expressed in Ah (ampere-hour) or mAh (milliamp-hour).

③Formula: C=It, that is, battery capacity (Ah) = current (A) x discharge time (h).

④For example: A battery with a capacity of 10 Amp can be discharged for 2 hours at 5 Amp, and can be discharged for 1 hour at 10 Amp.

⑤Influencing factors: The actual capacity of the battery mainly depends on the following factors: the quantity and quality of the active material, and the utilization rate of the active material.

⑥Rated capacity: The battery capacity measured under specified conditions and given by the manufacturer.

⑦Usable capacity: The amount of electricity released from a fully charged battery under specified conditions.

⑧Theoretical capacity: assuming that the active material is fully utilized, the capacity that the battery can release.

3. Battery energy (Wh)

①Meaning: Refers to the amount of energy stored in the battery, expressed in Wh

②Formula: Energy (Wh) = rated voltage (V) & times; working current (A) & times; working time (h).

③Example: The energy of a 3.2V15Ah single cell is 48Wh, and the energy of a 3.2V100Ah battery pack is 320Wh.

Battery energy is an important indicator for measuring the work done by a battery-driven device. Capacity does not determine the amount of work done.

4. Energy density (Wh/Kg)

① Meaning: refers to the energy released per unit volume or unit mass, usually expressed by volume energy density (Wh/L) or mass energy density (Wh/kg).

②Example: For example, if a lithium-ion battery weighs 325g, has a rated voltage of 3.7V, and a capacity of 10Ah, its energy density is 113Wh/kg. The following table is a theoretical value. In actual applications, the casing in the battery structure must be considered , parts and other factors.

At present, the energy density of lithium-ion batteries is 3 and 1.5 times that of nickel-cadmium and nickel-hydrogen batteries. The energy density is determined by the material density and structure.

5. Power and power density

①Power refers to the energy output by the battery per unit time under a certain discharge system, and the unit is W or kW.

②Power density, also called specific power, is the power output by the battery per unit mass or unit volume, and the unit is W/kg or W/L.

Specific power is an important indicator to evaluate whether the battery and battery pack meet the acceleration and climbing capabilities of electric vehicles.

6. Discharge rate (A)

①Meaning: The discharge rate refers to the current value required to discharge its rated capacity (C) within a specified time. It is numerically equal to a multiple of the battery's rated capacity.

②Example: Take a 10Ah battery as an example. If the battery is discharged at 2A, the discharge rate is 0.2C. If the battery is discharged at 20A, the discharge rate is 2C.

7. Charging method

①CC/CV: CC means constant current, charging the battery with a fixed current; CV means constant voltage, charging the battery with a fixed voltage, and the charging current will gradually decrease as the battery is full.

②Trickle charging: refers to charging the battery with a current of less than 0.1C. It is generally used when supplementary charging is performed when the battery is nearly fully charged. If the battery does not have strict requirements on charging time, it is recommended to use trickle charging.

③Float charging: Charge the battery with constant voltage at any time to maintain a certain state of charge.

8. Depth of charge and discharge (SOCDOD): a method of expressing the battery’s retained capacity value.

①State-of-charge (SOC): The percentage of the remaining capacity of the battery after discharge to the full charge capacity.

②Depth of discharge (DOD): a parameter indicating the discharge state of the battery, which is equal to the percentage of actual discharge capacity and rated capacity.

③ Deep discharge deepdischarge: Indicates the degree to which 50% or more of the stored capacity is released.

④Example: The depth of charge and discharge is expressed as a percentage. For example, a battery with a capacity of 10Ah has a capacity of 2Ah after discharge, which can be called 80%DOD; a battery with a capacity of 10Ah has a capacity of 8Ah after charging, and 80%SOC. Describes full discharge, usually called 100% DOD.

9. Internal resistance (mΩ)

①Meaning: The internal resistance of the battery refers to the resistance to the current flowing through the interior of the battery when the battery is working. The internal resistance is mainly affected by factors such as battery materials, manufacturing processes, and battery structure.

②Classification: Battery internal resistance includes ohmic internal resistance and polarization internal resistance. Ohmic internal resistance is composed of electrode materials, electrolytes, diaphragm resistance and contact resistance of various parts. Polarization internal resistance includes electrochemical polarization and concentration difference. Resistance due to polarization.

③Influencing factors: Battery internal resistance is a very complex and very important characteristic. Factors that affect internal resistance include materials, structures, etc.

④ Result: Due to the existence of internal resistance, when the battery is discharged, heat will occur when the current passes through the internal resistance, consuming energy. The larger the current, the more energy consumed, so the smaller the internal resistance, the better the performance of the battery. Not only does the battery The actual working voltage is high and the energy consumed in the internal resistance is also small.

10. Self-discharge rate (%/month)

① Meaning: During the storage process of the battery, the capacity will gradually decrease. The ratio of the reduced capacity to the battery capacity is called the self-discharge rate.

②Cause: Due to the instability of the electrodes in the electrolyte, a chemical reaction occurs between the two electrodes of the battery, the active material is consumed, the chemical energy converted into electrical energy is reduced, and the battery capacity decreases.

③Influencing factors: The ambient temperature has a greater impact on it. Excessive temperature will accelerate the self-discharge of the battery.

④ Expression: The expression method and unit of battery capacity decay (self-discharge rate) is: %/month.

⑤ Result: Battery self-discharge will directly reduce battery capacity, and self-discharge rate directly affects battery storage performance. The lower the self-discharge rate, the better the storage performance.

11. Cycle life (times)

①Meaning: A secondary battery undergoes a charge and discharge called a cycle or a cycle. After repeated charge and discharge, the capacity of the battery will gradually decrease. Under certain discharge conditions, when the battery capacity drops to 80%, the cycle the battery undergoes The number of times is the cycle life.

②Influencing factors: Improper use of batteries, battery materials, electrolyte composition and concentration, charge and discharge rate, depth of discharge (DOD%), temperature, manufacturing process, etc. all have an impact on the cycle life of the battery.

12. Memory effect

①Meaning: The memory effect of the battery refers to the percentage of the battery that is not fully discharged that can be charged the next time.

②Reason: Crystallization of materials in the battery occurs. For example, in nickel-cadmium batteries, Cd continuously aggregates into clusters to form large pieces of metallic cadmium, which reduces the activity of the negative electrode.

③Prevention: In order to eliminate the memory effect of the battery, it must be completely discharged before charging, and then charged again.

Lithium-ion batteries have no memory effect.

13. Discharge platform

Refers to the part of the discharge curve where the voltage remains basically level. The higher, longer and more stable the discharge platform is, the better the discharge performance of the battery is.

14. Battery pack consistency

A battery pack is composed of multiple single cells connected in series or parallel. The overall performance and life of the battery pack depends on one of the cells with poorer performance, which requires high consistency in the performance of each cell in the battery pack. In addition to the performance error of the single cell itself and the quality of the raw materials, the most important reason is the manufacturing process. Improvement of the process is very important to improve the quality of the battery.

15. Formation

After the battery is made, the internal positive and negative active materials are activated through a certain charge and discharge method, and the process of improving the battery's charge and discharge performance, self-discharge, storage and other comprehensive properties is called formation. Only after the battery is formed can its true performance be reflected. At the same time, the sorting process during the formation process can improve the consistency of the battery pack and improve the performance of the final battery pack.