CN108508365B

CN108508365B - Lithium ion battery self-discharge screening method

Info

Publication number: CN108508365B
Application number: CN201710246912.0A
Authority: CN
Inventors: 单海鹏; 李明; 李凡群
Original assignee: Wanxiang Group Corp; Wanxiang A123 Systems Asia Co Ltd
Current assignee: Wanxiang A123 Systems Asia Co Ltd
Priority date: 2017-04-16
Filing date: 2017-04-16
Publication date: 2021-02-26
Anticipated expiration: 2037-04-16
Also published as: CN108508365A

Abstract

The invention relates to the technical field of lithium ion batteries, in particular to a lithium ion battery self-discharge screening method which comprises the steps of charging to a full-charge state, shelving, drawing a voltage and time square root curve, fitting the curve, calculating the slope of the fitting curve, judging and the like. The self-discharge screening method of the invention has the advantages that the self-discharge screening time period can greatly shorten the process time; the self-discharge screening method has high screening recognition rate and precision, and can effectively screen the lithium ion battery with abnormal self-discharge.

Description

Lithium ion battery self-discharge screening method

Technical Field

The invention relates to the technical field of lithium ion batteries, in particular to a lithium ion battery self-discharge screening method.

Background

With the development of science and technology and the improvement of environmental awareness of people, it is a necessary trend to develop an electric vehicle using electricity as a power source to replace the existing fuel vehicle, and the importance of the electric vehicle as a core component, namely a power battery, is particularly prominent.

Lithium ion power batteries are important objects for research and development of power batteries for electric vehicles due to the characteristics of high energy density, small self-discharge, long service life and the like.

The power battery for the electric automobile needs to combine a plurality of single batteries with small capacity and low voltage into a battery pack with high capacity and high voltage in a series/parallel connection mode, and the consistency difference of the single batteries directly determines the performance of the battery pack. The self-discharge is one of the biggest factors influencing the service life of the battery combination bag body, if the self-discharge is inconsistent, when the number of the batteries connected in series is large, the difference of the SOC state of each battery string among the battery bag bodies is large, the probability of overcharge and overdischarge is high, the bag body is difficult to boil for 100 cycles, and otherwise, a balancing system of the BMS with high price is used. At present, the parameters for judging the consistency of the battery mainly include internal resistance, voltage difference, capacity difference, self-discharge rate of the battery and the like, wherein the first three parameters are relatively easy to obtain, and the self-discharge rate of the battery is mainly represented by the voltage difference in unit time.

The self-discharge of the battery refers to the capacity loss of the battery caused by spontaneous reaction in the battery when an external circuit is disconnected, and for the lithium ion battery, the loss is divided into two types, namely reversible self-discharge caused by internal micro short circuit, the self-discharge rate is smaller under the normal condition, and the capacity loss caused by the oxidation reduction of the electrolyte on the positive electrode and the negative electrode is irreversible loss. However, no matter what kind of self-discharge, the self-discharge speed between batteries can be different, which causes the inconsistency of the batteries, and if the sorting is not good, the service life of the battery pack body is extremely short. The battery exhibits a significant micro-short circuit due to cell expansion in the charged state, and the micro-short circuit is reduced or eliminated in the discharged state due to the shrinkage of the cell.

In the prior art, the battery is charged to full charge (100% SOC) in the formation process of the lithium ion battery, the shelf time is prolonged, the voltage of the battery at each time point during the shelf is recorded, and a voltage value corresponding to a specific time point or a critical value of voltage change in unit time is selected as a basis for screening self-discharge; however, the screening method has obvious defects that the standing time is long, the time consumption is long, and the screening method can better screen the battery with larger self-discharge, but the identification rate and the accuracy of the battery with smaller self-discharge are poor, so that the screening is difficult.

Disclosure of Invention

In order to solve the problems, the invention provides the lithium ion battery self-discharge screening method which is short in screening time consumption and high in screening recognition rate and accuracy.

In order to achieve the purpose, the technical scheme adopted by the invention is as follows:

a lithium ion battery self-discharge screening method comprises the following steps:

1) charging the formed lithium ion battery to a full-charge state, and then placing the lithium ion battery;

2) recording voltage values corresponding to all time points in the laying process;

3) drawing a voltage and time square root scatter diagram (shown in figure 1) according to the recorded voltage values and corresponding time points, and performing linear fitting to obtain a fitting straight line;

4) calculating the slope of the fitted curve (as shown in fig. 1);

5) and determining the slope range of the fitting curve of the self-discharge normal battery which is placed at the same period, and judging the lithium ion battery with the slope of the fitting curve smaller than the slope range as the abnormal battery.

A voltage and time square root scatter diagram obtained by self-discharging of a normal battery/electric core can be subjected to linear fitting to obtain a straight line with a constant slope; after the abnormal battery is subjected to self-discharge shelving, the obtained voltage and time square root scatter diagram is fitted into a curve with a continuously changing slope (usually continuously decreasing) or a curve with a slope exceeding a normal range; therefore, the consistency of the tested battery/cell can be rapidly distinguished and judged. The self-discharge screening method of the invention is used for screening the batteries with poor consistency, and because the tested batteries have differences, the standard slope which is used as the basis for judgment has corresponding differences, and the specific testing method (such as the shelf processing time, the charge cut-off current and the like) also needs to be adjusted to a certain extent.

Preferably, in step 1, during charging, the lithium ion battery is charged with a constant current to a charging termination voltage, and then charged with a constant voltage until the current is reduced to 0.1-0.05C.

Preferably, in step 1, the leaving treatment is performed at normal temperature.

Preferably, the shelf life is 1 to 30 days.

Preferably, the corresponding voltage values are voltage values recorded every hour after the start of the rest.

Preferably, the lithium ion battery is a nickel cobalt lithium manganate ternary lithium ion battery or a lithium iron phosphate lithium ion battery.

Therefore, the invention has the following beneficial effects:

(1) the self-discharge screening method of the invention has the advantages that the self-discharge screening time period can greatly shorten the process time;

(2) the self-discharge screening method has high screening recognition rate and precision, and can effectively screen the lithium ion battery with abnormal self-discharge.

Drawings

FIG. 1 is a graph of voltage versus square root of time data for a normally discharged lithium ion battery;

FIG. 2 is a graph of discharge-time square root data for a test lithium ion battery;

FIG. 3 is a graph of the derivation of the discharge-time square root data for a test lithium ion battery.

Detailed Description

The technical solution of the present invention will be further described with reference to the following embodiments.

It is to be understood that the described embodiments are merely a few embodiments of the invention, and not all embodiments. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention.

Examples

In the embodiment, the nickel cobalt lithium manganate ternary lithium ion battery is used as a screening object.

And the standard battery and the corresponding self-discharge curve thereof are used as judgment standards to judge whether the battery cell 1 and the battery cell 2 of the lithium ion battery are normal or not.

1) charging the formed lithium ion battery cell 1, the formed lithium ion battery cell 2 and a standard battery, firstly charging the lithium ion battery with a constant current until the charging termination voltage of the lithium ion battery is 4.2V, then charging the lithium ion battery with a constant voltage charging mode until the current is reduced to 0.1C, and then carrying out shelving treatment at the normal temperature of 25 ℃, wherein the shelving treatment time is 10 days;

2) recording corresponding voltage values every other hour after the laying-aside is started;

3) drawing a voltage and time square root scatter diagram of the recorded voltage values and corresponding time points, and fitting the curve to obtain a fitted curve; simultaneously drawing a fitting curve of the self-discharge normal battery, wherein the self-discharge curve of the standard battery is a linear straight line, and determining the slope range of the normal battery discharge-time square root curve, as shown in figure 1; then drawing the battery discharge-time square root curves of the battery core 1 and the battery core 2

4) Calculating the slope of the fitting curve; since the embodiment shows two abnormal cells with excessive self-discharge, the obtained curve of voltage and time square root dispersion point is not a linear curve, so the slope is not constant but continuously variable; as shown in fig. 2, the slope of the fitting curve made by the parameters of the two cells is continuously decreased, and then the curve shown in fig. 2 is derived to make a derivative curve of the discharge-time square root data of the lithium ion battery shown in fig. 3;

5) according to fig. 3, it can be seen that the slope of the corresponding curve of the abnormal battery decreases rapidly along with the increase of the shelf time, which is different from the normal battery, so far, it is determined that the battery core 1 and the battery core 2 are abnormal batteries.

It will be understood that modifications and variations can be made by persons skilled in the art in light of the above teachings and all such modifications and variations are intended to be included within the scope of the invention as defined in the appended claims.

Claims

1. A lithium ion battery self-discharge screening method is characterized by comprising the following steps:

2) recording voltage values corresponding to all time points in the laying process; the corresponding voltage value is recorded every other hour after the beginning of the shelf;

3) drawing a voltage and time square root scatter diagram according to the recorded voltage values and corresponding time points, and performing linear fitting to obtain a fitting straight line;

4) calculating the slope of the fitting straight line;

5) and determining the slope range of the fitting curve of the self-discharge normal battery which is placed at the same time, and judging the lithium ion battery with the slope of the fitting curve smaller than the slope range or the lithium ion battery with the slope changing continuously as the abnormal battery.

2. The lithium ion battery self-discharge screening method according to claim 1, characterized in that:

in the step 1), during charging, constant-current charging is firstly carried out until the charging termination voltage of the lithium ion battery is reached, and then charging is carried out in a constant-voltage charging mode until the current is reduced to 0.1-0.05C.

3. The lithium ion battery self-discharge screening method according to claim 1, characterized in that:

in the step 1), the standing treatment is carried out at normal temperature.

4. The lithium ion battery self-discharge screening method according to claim 1, characterized in that:

the standing time is 1-30 days.

5. The lithium ion battery self-discharge screening method according to claim 1, characterized in that:

the lithium ion battery is a nickel cobalt lithium manganate ternary lithium ion battery or a lithium iron phosphate lithium ion battery.