The inconsistency between single batteries often causes problems such as too fast capacity decay and short life during the cycle of battery packs. Improving the consistency of batteries is of great significance to the popularization and application of lithium-ion batteries.

1 Inconsistency analysis
Definition of inconsistency

The inconsistency of the lithium-ion battery pack means that after the single battery of the same specification and model is composed of the battery pack, its voltage, capacity, internal resistance, life, temperature effect, self-discharge rate and other parameters are different. After the single battery is manufactured, the initial performance itself has certain differences. With the use of the battery, these performance differences continue to accumulate, and because the use environment of each single battery in the battery pack is not completely the same, it also leads to the gradual amplification of the inconsistency of the single battery, which accelerates the battery performance attenuation, and eventually causes the premature failure of the battery pack.

Inconsistent performance

The inconsistency of lithium-ion batteries is mainly manifested in two aspects: the difference of battery performance parameters (battery capacity, internal resistance and self-discharge rate, etc.) and the difference of battery state of charge (SOC).

2 Causes of inconsistency

There are many reasons for the inconsistency of lithium-ion batteries, mainly during the manufacturing process and use. Each part of the manufacturing process, such as the uniformity of the paste during batching, the control of the surface density and surface tension during coating, will cause the difference in the performance of a single battery.

3 Methods to improve battery consistency
Control of production process

The control of the production process is mainly carried out from the two aspects of raw materials and production technology. In terms of raw materials, try to select the same batch of raw materials to ensure the consistency of raw material particle size and performance. The production process should be strictly regulated for the entire production process, such as ensuring that the slurry is stirred evenly, not placed for a long time, controlling the speed of the coating machine to ensure the thickness and uniformity of the coating, the appearance inspection of the pole sheet, the weighing division, and the control of the amount of liquid injection and the formation, volume separation, storage conditions, etc.

Control of the assembly process

The control of the assembly process mainly refers to the sorting of the battery, the battery pack adopts a unified specification and model of the battery, and the battery voltage, capacity, internal resistance and so on are measured to ensure the consistency of the initial performance of the battery.


Control of use and maintenance processes

Real-time monitoring of the battery. The consistency screening of the battery during the assembly can ensure the consistency of the battery pack in the initial use. If you want to understand the consistency of the battery during use, you need to monitor the battery in real time. Timely adjustment or replacement of the extreme parameter battery through real-time monitoring can ensure the benign operation of the battery pack.

A balanced management system is introduced. The battery is intelligently managed by proper balancing strategy and balancing circuit. At present, common balancing policies include external voltage based balancing policies, SOC based balancing policies and capacity based balancing policies. The balance circuit can be divided into passive balance and active balance according to the energy consumption mode. Among them, active equalization can realize the lossless energy flow between batteries, which is the focus of research at home and abroad. The methods commonly used in active equalization include battery bypass method, switched capacitance method, switched inductance method, DC/DC conversion method, etc.

Thermal management of batteries. In addition to keeping the working temperature of the battery pack within the optimal range as far as possible, the thermal management of the battery should also try to ensure the consistency of the temperature conditions between the batteries, so as to effectively ensure the consistency of the performance between the batteries. Adopt reasonable control strategy. When the output power allows, minimize the depth of battery discharge, and avoid overcharging of the battery, which can extend the cycle life of the battery pack. Strengthen battery pack maintenance. At a certain interval, the battery pack is charged for small current maintenance, and pay attention to cleaning.

4. Lithium-ion battery assembly method
Voltage combination method

Voltage allocation method can be divided into static voltage allocation method and dynamic voltage allocation method. Static voltage grouping method is also called no-load grouping method, without load, only considering the battery itself, measuring the self-discharge rate of the screened single battery stored in the full charge state after standing for several decades and the open circuit voltage of the battery in different storage periods under the full charge state, this method is the simplest to operate, but it is not accurate. The dynamic voltage combination method is not accurate because it does not take into account the load change and other factors.

Static capacity allocation method

The battery is charged and discharged under the set conditions, the capacity is calculated by discharge current and discharge time, and the battery is grouped according to the capacity. This method is simple and feasible, but it can only reflect the same capacity of the battery under specific conditions, can not explain the complete working characteristics of the battery, and has certain limitations.

Internal resistance group method

The internal resistance of a single battery is mainly considered. This method can achieve rapid measurement, but because the internal resistance of the battery changes with the discharge process, it is difficult to accurately measure the internal resistance.

Multi-parameter grouping method

At the same time, considering the capacity, internal resistance, voltage, self-discharge rate and other external conditions, the battery can be sorted out with good consistency. However, the premise of this method is that the single parameter sorting should be accurate, and the time is too long.

Dynamic characteristic combination method

The dynamic characteristic grouping method uses the charge-discharge characteristic curve of the battery to sort the battery for grouping. The charge-discharge curve can reflect most of the characteristics of the battery, and the dynamic characteristic combination method can ensure the consistency of various performance indicators of the battery. The dynamic characteristic grouping method has a lot of data and is usually implemented by computer program. In addition, the combination utilization of the battery in this method is reduced, which is not conducive to the reduction of the cost of the battery pack. The determination of standard curve or reference curve is also a difficult point in its implementation.