Talking about the inconsistency of lithium ion battery pack

Due to differences in the parameter values of voltage, internal resistance, capacity, etc. of battery cells of the same type and specification, the performance indicators of the battery pack during use often do not reach the original level of the single battery cell, which seriously affects the lithium-ion battery pack applications.

This article conducts an in-depth analysis of the causes of inconsistencies in the battery pack, and summarizes the processes of production, assembly, use, and maintenance, and proposes measures to compensate for inconsistencies.

The embodiment of battery inconsistency

Lithium-ion battery consistency refers to the consistency of the initial performance indicators of single cells used in battery pack, including:

1.Capacity;

2.Impedance;

3.The electrical characteristics of the electrode;

4.Electrical connections;

5.Temperature characteristics;

6.Decay speed, etc.

The difference of the above factors will directly lead to the difference of battery discharge parameters during operation.

The inconsistency of the lithium-ion battery pack or the discrete phenomenon of the battery pack refers to the inconsistency of the following parameters after the single cells of the same specification and model form the battery pack:

1.Voltage;

2.Charge amount

3.Capacity;

4.Decline rate

5.Internal resistance and its rate of change over time;

6.Life;

7.Temperature effect

8.Self-discharge rate and its rate of change over time;

9.After the single battery is manufactured, there are certain performance differences.

The initial inconsistency accumulates with the continuous charge and discharge cycles of the battery during use, resulting in greater differences in the status of each single battery (SOC, voltage, etc.); the use environment in the battery pack is also different for each single battery.

This leads to the gradual enlargement of the inconsistency of the single cells during use, which in some cases accelerates the degradation of the performance of some single cells, and ultimately leads to premature failure of the battery pack.

Reasons for battery inconsistency

Divided by time sequence, the inconsistency of single cells in the battery pack is mainly reflected in two aspects:

1.During the manufacturing process of the battery cell, the process problems and the unevenness of the material make the activation degree and thickness of the active material of the battery plate, the microporosity, the interconnection, the separator, etc. have very small differences. The internal structure and material are different, it is not completely consistent.

2. In use, the difference in electrolyte density, temperature and ventilation conditions, self-discharge degree and charge-discharge process of each battery cell in the battery pack will be affected.

In view of the causes of these inconsistencies, can some measures be taken to completely eliminate the inconsistencies in the battery pack?

Many people think that battery inconsistency is a problem in the production process, and some people think it is a problem in the assembly process. The inconsistency of the battery can be completely eliminated through process control measures such as SPC.

However, practice has proved that even if the process of batching, slurry, coating, cutting, and rolling is strictly controlled, it can only reduce the standard deviation between batches of products, but cannot eliminate inconsistencies.

If there are many random factors affecting a random variable, and the influence of each factor alone cannot play a decisive role, and the influence of these factors can be superimposed, the random variable obeys a normal distribution, and the characteristic parameters include standard deviation and mean.

The voltage value of the battery charge and discharge process is a comprehensive reflection of the battery's thermodynamics and kinetics. It is not only affected by the process conditions of each process in the battery production process, but also by the current, temperature, time and accidental factors during the battery charge and discharge process. Therefore, the voltage value of each battery in the battery pack cannot be exactly the same.

Improvement measures for battery inconsistency

1.Measures for the production process of battery cells

1.1Control the consistency of various raw materials;

1.2Monitor the rheology of the slurry, do not leave the slurry for a long time to ensure that the rheology of the slurry is the same during coating;

1.3Monitoring of coating parameters, especially lithium iron phosphate slurry, due to the finer lithium iron phosphate particles and poor processing performance of the slurry, attention should be paid to slow down the coating machine during coating;

1.4Reasonable detection of slurry viscosity;

1.5Visual inspection of pole piece;

1.6Eliminate defective pole pieces;

1.7Pole piece weighing;

1.8Comparison of poor battery quality before and after injection;

1.9Formation temperature;

1.10Humidity control;

1.11Formulate standards for various raw materials, inspect and store raw materials in strict accordance with the standards;

1.12Consistency control of production process;

1.13Fine control the consistency of the production process;

1.14Strict statistical process control (SPC) in the process;

1.15Ensure that each process is within the specified tolerance range;

1.16Ensure that the process capability index follows the normal distribution law of production parameters.

Measures of the battery pack assembly process

Ensure that the battery pack adopts battery cells of uniform specifications and models, and ensure the quality of the battery cells, especially the consistency of the initial voltage. The selection conditions:

1.Battery cell voltage;

2.Internal resistance of battery cells;

3.The formation data of battery cells, such as battery cell capacity.

At present, automatic battery cell sorting machine has been widely used, whether in battery cell manufacturers or battery pack assembly manufacturers.

The general matching conditions for many battery manufacturers to match lithium-ion batteries (the battery performance is not consistent, the advantages are simple and convenient) are:

1.Poor battery capacity: <3%;

2.Internal resistance difference: <5%;

3.Self-discharge rate difference: <5%;

4.Average discharge voltage difference: <5%.

Use the shape, distance, and area of the battery charge-discharge curve formed by each detection point within the set time interval to perform scientific classification to determine the consistency of the battery.

On the basis of capacity or voltage, calculate the shape of the charge and discharge curve, the distance between the curves, the area enclosed by the curve, etc., and select the parameters that can reflect the consistency of the curve for judgment.

In the process of matching charging and discharging, the curves are closer, the relative distance is small, the area enclosed by the curve is small, and the cells with small differences between groups are matched to achieve the optimal consistent grouping.

Battery balance management

From the perspective of the battery management system, detect the cell parameters during the use of the battery pack, especially the voltage distribution during use, grasp the inconsistency development law of the cells in the battery pack, and adjust or replace the extreme parameter batteries in time to ensure the battery The inconsistency of group parameters does not increase with time.

Avoid overcharging the battery and try to prevent deep discharge of the battery.

Ensure a good use environment for the battery pack, try to ensure a constant temperature, reduce vibration, and ensure that the battery poles are polluted by water and dust.

At the same time, from the perspective of energy management and strategy, it introduces a practical battery pack energy management and balance system, formulates a reasonable battery balance strategy, and actively intervenes and reduces battery inconsistencies.

The battery pack and BMS test systems can do these tasks well.

Battery thermal management

In the process of battery use, the difference in factors such as internal resistance and battery layout will cause differences in its own temperature and ambient temperature during charging and discharging, which will directly lead to differences in its output performance. The role of battery thermal management is:

1.Keep the operating temperature of the battery pack within the optimal operating temperature range of the battery.

2. Ensure the consistency of the temperature conditions between the batteries, thereby ensuring the consistency of the battery parameters. (The battery life is different in different temperature states, and the degradation rate doubles for every 10°C increase in temperature)

Control Strategy

In terms of energy management, when the output power allows, minimize the depth of battery discharge. The consistency of lithium-ion batteries under deep discharge conditions deteriorates, and the life of the battery pack will also be reduced.

Try to prevent the battery from deep discharge and avoid overcharging of the battery. Equipped with a balance circuit in the system, it can prevent individual batteries from overcharging, appropriately reduce the charge termination voltage, and extend the cycle life of the battery pack.

Other use process

In the routine maintenance process, the battery with low capacity in the measurement is separately maintenance charged to restore its performance.

Perform low-current maintenance charging on the battery pack at certain intervals to promote the balance and performance recovery of the battery pack itself.

Regarding the use environment, ensure that the battery pack has a good use environment, reduce vibration, and avoid pollution of battery poles by water and dust.

In summary, improving the consistency of lithium-ion batteries is a systematic project that requires the joint consideration of battery design, production, quality control, application, and maintenance.

Here, the definition of battery consistency and the reasons for inconsistency are analyzed and researched, and the inconsistency distribution of the battery is analyzed. It is proposed that the inconsistency distribution follows the normal distribution theory, and it is proposed to prevent inconsistencies in the processes of manufacturing, matching, use, and maintenance. Several measures for sexual expansion.