Determinants of battery pack consistency

At present, evaluating the consistency of lithium-ion power batteries The main standards are capacity, internal resistance and discharge voltage.A common screening method is to collect lithium-ion single cells apacity, average discharge voltage and battery life at full charge Internal resistance is used as the basic data to screen the battery, Binning and grouping. But the battery goes through this small current After the technical capacity screening, the assembled batteries are in the process of use There will still be a lot of low open circuit voltage, electrical properties performance differences are becoming more and more pronounced, leading to blocks or even entire battery packs due to the Abnormal failure.The performance difference of the single battery should be It definitely exists in use, this problem can only be reduced and cannot be completely eliminated. Consistency for battery cells The problem is mainly developed from two aspects when applying in groups researched: one is the sorting of single cells; the other isMitigate inconsistencies with sound battery management The main methods include battery balancing, battery Thermal Management and Modeling and Condition Considering Cell Differences state estimation, etc. This article mainly starts with the sorting of single cells, and concludes According to the practical application, the following eight consistency criteria are summarized Precisely carry out monomer sorting and grouping to achieve the elimination of no The purpose of a good battery.

1 When the power battery is formed into a group, the monomers are the same
Sexual Assurance

1.1 Consistent capacity
Capacity is divided into rated capacity and actual capacity.
The rated capacity of the battery refers to the design and manufacture of the battery
When specifying or guaranteeing the battery under certain discharge conditions should discharge a minimum amount of power; the battery's The actual capacity refers to the battery under certain discharge conditions The actual amount of electricity released is mainly affected by the discharge rate and temperature effects. Rated capacity refers to the standard capacity of the battery called capacity. When the capacity is divided, the ambient temperature of the battery is Under the condition of 25±5℃, discharge the current at a rate of 3h to Multiplication of termination voltage, current value and discharge time data Product to calculate the rated capacity. In the same battery pack, Control the rated capacity difference of the single battery within a certain range within. For example, a single battery pack requires a single capacity
The difference is controlled within the range of 50mAh.

1.2 Consistent internal resistance
Internal resistance refers to the current flowing through the battery when it is working
The internal resistance of the battery is generally divided into AC internal resistance.
resistance and DC internal resistance, the daily measurement is the AC internal resistance,The internal resistance is mainly determined by the electrode material, electrolyte, and diaphragm electricity.
resistance and the contact resistance of each part of the parts, and the electrical
The size, structure, assembly, etc. of the pool are related. Normal Under the circumstance, the large current discharge capacity of the battery with small internal resistance Strong, the battery with large internal resistance has weak discharge capacity. Inside the resistor If the resistance is large, a large amount of Joule heat will be generated, which will cause The internal temperature of the battery rises, causing the battery to discharge to work The voltage drops, the discharge time shortens, and the battery Energy, life, etc. have a serious impact, the unit of internal resistance
is Ω. The battery is at 25±5℃, with AC internal resistance The tester measures its AC impedance, AC internal resistance test The test frequency of the instrument is 1kHz. In the same battery pack, Control the internal resistance of the battery within a certain range. E.g, A battery is 32650-5Ah, and the internal resistance is 7~9mΩ
between.
1.3 Consistent constant current ratio
The constant current ratio refers to the constant current charging when the monomer is charged. The amount of electricity and the total amount of electricity charged by constant current and constant voltage The higher the constant current ratio, the better the battery performance. divide capacity When the battery is at 25±5℃ at 3h rate current Constant current discharge to the specified voltage, and then current at a rate of 3h

Constant current charging to a specified voltage, constant voltage charging at this voltage, until charging stops. It can calculate the ratio of the amount of electricity charged by constant current to the total amount of electricity charged by constant current and constant voltage. In the same battery pack, the single battery adopts a uniform constant current ratio grade.

1.4 Consistent discharge platform
The discharge plateau is a feature of the battery discharge curve, that is, after the battery is fully charged, the current will flow at a rate of 3h.
Time to discharge to specified voltage. The higher the discharge platform, the better the battery performance. When the capacity is divided, the battery is charged at 25±5℃ with a constant current at a rate of 3h to a constant voltage charging voltage, and then transferred to a constant voltage charging until the charging stops. Discharge at a rate of 3h until the discharge is terminated. The time for the voltage to drop to the specified voltage was recorded. In the same battery pack, the single battery needs to use the same discharge level
table grade.

1.5 Self-discharge consistent
Self-discharge refers to the phenomenon that the battery loses its capacity due to the chemical reaction of its own internal substances after a period of time without external work. Self-discharge is mainly affected by factors such as battery manufacturing process, materials, and storage conditions. The magnitude of self-discharge is expressed by self-discharge rate or charge retention rate. Self-discharge rate refers to the ratio of the capacity reduction to the initial capacity after the battery is fully charged and stored for a certain period of time. The unit of self-discharge rate is "%";
The charge retention rate refers to the ratio of the remaining capacity to the initial capacity after the battery is fully charged and stored at a certain temperature for a certain period of time. The unit of charge retention is "%". The relationship between the two is: 1- self-discharge rate = charge retention rate. The smaller the self-discharge rate of the battery, the better, the greater the charge retention, the better. The battery with larger self-discharge often shows that the voltage drops rapidly after a period of storage. In the same battery pack, the self-discharge of the single cells is consistent.

1.6 Consistent voltage
The battery voltage refers to the potential difference between the positive and negative electrodes of the battery, the voltage separates the circuit voltage and the working voltage, and the open circuit voltage refers to the potential difference between the two poles of the battery when the battery is not charged and discharged. If the materials of the positive and negative electrodes of the battery are exactly the same, then no matter how big the battery is and how the geometry changes, its open circuit voltage will be the same. The working voltage refers to the potential difference between the two poles of the battery when the battery is charging and discharging. The charging voltage is higher than the open-circuit voltage, and the discharging voltage is lower than the open-circuit voltage. Under the same conditions, the higher the discharge voltage, the better the battery performance. The voltage is affected by the battery charge, temperature and other conditions, and the voltage unit is "volts (V)". In the same battery pack, the voltage of the single cell is controlled within a certain pressure difference range.

1.7 Charged power
The amount of charge refers to the amount of charge (different from the capacity) of the battery, that is, the amount of power that can be provided when the current state is discharged to the final voltage at a 3h rate current value. Generally, the amount of charge in the battery pack can be expressed as a percentage of the amount of charge and the actual capacity, that is SOC, the unit of charge is percentage (%). Under the same conditions, the charge is related to the open circuit voltage. The more current, the higher the open circuit voltage. The charge amount between the series modules in the battery pack should be the same. Generally, active balancing is used, and batteries connected in parallel in the same module generally use self-balancing.

1.8 Battery Batches
Batch consistency means that the electrical properties of the same type of batteries produced using the same batch of materials, the same process, the same equipment, and at the same time are basically the same. The same battery pack is selected from the same batch of batteries. If the number of a batch is not enough, adjacent batches of batteries from the same process are selected.

2 Single Unit Consistency Guarantee Measures
When making ingredients, attention should be paid to the selection of positive and negative materials, as well as the inspection of positive and negative materials, diaphragm paper, electrolyte, etc. Coating should control the compaction density of the pole piece, which is consistent with the thickness of the pole piece. Winding or lamination should control dust, micro-shorts, black core, uneven winding or lamination. The liquid injection should ensure that the sulfur salt concentration and moisture content of the incoming electrolyte are qualified, and the injection volume should be controlled to be consistent. The pre-charging should control the pre-charging process setting parameters to be consistent. The capacity distribution should control the parameters (current value, cut-off voltage, cut-off time) in the capacity distribution process to be consistent; ensure that the battery surface is clean and the battery is in good contact with the fixture; control the equipment error of the capacity distribution detection equipment itself, and regularly calibrate the current accuracy and voltage Accuracy; ensure the reliability of the battery clamping contacts; ensure that the battery is fully charged (discharged) before being placed in the cabinet; pay attention to ensuring the uniformity of the ambient temperature in the capacity-sharing workshop.

3 Conclusion
The reasons for the inconsistency of battery cells include two aspects: the difference in the initial performance of the cells during the manufacturing process and the inconsistent changes in the internal performance of the battery due to inconsistent use conditions during the use process. By grading the cells through the above items, the grouped power battery pack can be used to alleviate the short battery life, electrical performance degradation and safety caused by the inconsistency of the battery cells during application.
Congruence issues.