CN107607881A - A kind of evaluation method of lithium-ion-power cell self discharge uniformity - Google Patents

Abstract

A method for evaluating the self-discharge consistency of a lithium-ion power battery, comprising: step 1, among lithium iron phosphate batteries of the same production batch, the same specification model, and the same material system, select more batteries with better consistency in capacity and internal resistance At room temperature, use the constant current and constant voltage charging system and constant current discharge system to cycle charge and discharge multiple times; step 2, adjust the battery to an empty state at room temperature; step 3, screen the battery with a monthly self-discharge rate greater than 3%. Step 4, adjust the battery SOC to a state of charge of 10% SOC-30% SOC; Step 5, measure the open circuit voltage of the battery after shelving at high temperature, and calculate the voltage drop △V of the battery or the voltage drop K per unit time; Step 6. Calculate the average value X and standard deviation σ of the ΔV or K values of all batteries, and select the batteries within the confidence interval of [X‑2σ, X+2σ]. The screening method of the invention can improve the self-discharge rate of the battery, shorten the storage period and reduce the production cost.

Description

A method for evaluating the self-discharge consistency of lithium-ion power batteries

technical field

The present application relates to the field of power batteries, in particular to a method for evaluating the self-discharge consistency of lithium-ion power batteries.

Background technique

As the core energy component of electric vehicles, the development of power batteries has become a key factor in the development of electric vehicles, and lithium iron phosphate power batteries occupy the leading position in electric vehicles due to their superior safety performance, long cycle life and stable voltage platform. The main market for power batteries. However, under the current level of power battery technology, electric vehicles must use a battery pack composed of multiple single cells to meet the requirements of use, which puts forward high requirements on the consistency of lithium iron phosphate power batteries.

At present, lithium iron phosphate power batteries need to be screened for performance consistency of single batteries before being assembled into groups, and batteries with good consistency are combined into battery packs for use. The existing method for judging the consistency of batteries is mainly to combine static parameters (such as voltage, internal resistance, batch, etc.) and dynamic parameters (such as self-discharge, charging constant current ratio, etc.) for grouping. Among them, due to the structure of the lithium iron phosphate material and the battery manufacturing process, etc., the self-discharge rate of the lithium iron phosphate battery is relatively high. At present, the self-discharge rate screening method of lithium iron phosphate battery is mainly characterized by the voltage difference per unit time. This method needs to record the voltage and time before and after the self-discharge test to calculate the self-discharge rate of the single battery, and then compare the self-discharge Or the same single battery for matching group. This method requires more manpower and electric power, which not only increases the production cost, but also prolongs the production cycle. Moreover, most of this method adopts the self-discharge screening of lithium iron phosphate batteries in the discharge state. However, due to the polarization of the battery, the voltage rebounds after the battery is discharged, and the degree and rate of voltage rebound between different batteries are different, resulting in The voltage consistency of the battery discharge state before shelving is very poor, and the voltage after shelving is even greater than the voltage before shelving, that is, the voltage difference is negative, so this method of using the discharge state voltage difference to screen the consistency of the battery There are certain disadvantages; another method is the barcode management method, which achieves the consistency of self-discharge of battery packs through automated production lines. This method is difficult to realize in China within a short period of time.

Therefore, it is necessary to provide a method for evaluating the self-discharge consistency of lithium-ion power batteries for low-cost, easy-to-operate self-discharge consistency screening.

Contents of the invention

The purpose of the present invention is to change the existing deficiency of using the voltage difference in the discharge state to screen the self-discharge of the lithium iron phosphate battery in the prior art, and to provide a method for evaluating the self-discharge consistency of the lithium-ion power battery. In order to improve the self-discharge rate of the battery, effectively shorten the shelf life and reduce production costs.

Technical scheme of the present invention is:

A method for evaluating the self-discharge consistency of a lithium-ion power battery, comprising:

Step 1. Among the lithium iron phosphate batteries of the same production batch, the same specification and model, and the same material system, select multiple batteries with good consistency in capacity and internal resistance; at room temperature (20±2)°C, use constant current Constant voltage charging system and constant current discharging system, that is, discharge the battery under test at room temperature with a current of 0.02C to the minimum discharge voltage specified by the manufacturer, let it stand for 1 hour, and then charge it with a constant current of 0.2C to the minimum discharge voltage specified by the manufacturer. Switch to constant voltage charging at the highest charging voltage of the battery, and stop charging when the charging current drops to 0.02C, and cycle charging and discharging many times in this way;

Step 2. At room temperature (20±2)°C, adjust the SOC (state of charge) of the above-mentioned lithium iron phosphate battery to an empty state, that is, 0% SOC (state of charge);

Step 3. At room temperature (20±2)°C, put the above-mentioned lithium iron phosphate battery on hold for 7 days, measure the open circuit voltage of each single battery with a multimeter every 24 hours, and calculate the open circuit voltage difference △OCU_1 of adjacent 24 hours , and perform linear regression Y=A+BX on the 7-day open circuit voltage difference △OCU_1, where Y is the open circuit voltage difference △OCU_1, X is the number, that is, the number of open circuit voltage difference △OCU_1, and A is the slope , B is the intercept; select a single battery with B>0, at this time, the voltage of the lithium iron phosphate battery is recorded as OCU_t1; in this way, the battery with a monthly self-discharge rate greater than 3% is screened out;

Step 4, charging the above-mentioned lithium iron phosphate battery, that is, adjusting the SOC of the battery to a state of charge of 10% SOC-30% SOC;

Step 5. At a high temperature of 30°C-45°C, put the lithium iron phosphate battery with a state of charge of 10%SOC-30%SOC on hold for 7 days, measure the open circuit voltage OCV_t2 of the battery after being put on hold at high temperature; calculate the voltage drop of the battery △V=OCV_t1-OCV_t2 or the voltage drop K=(OCV_t1-OCV_t2)/t per unit time, where t is the unit time, which can be set as days or hours;

Step 6. Calculate the average value X and standard deviation σ of the ΔV or K values of all batteries, and screen out the batteries within the [X-2σ, X+2σ] confidence interval.

By adopting the present invention, it is possible to change the deficiency that the lithium iron phosphate battery uses the voltage difference in the discharge state to screen the self-discharge in the prior art, and initially screen the iron phosphate with a self-discharge rate greater than 3% by using the voltage difference in the lower state of charge. For lithium batteries, put them aside at high temperature to screen batteries with similar self-discharge rates. Using high-temperature shelving can improve the self-discharge rate of batteries, effectively shorten the shelf life, and reduce production costs, which has considerable practical significance.

detailed description

Experimental equipment: MACCOR battery tester (US, model 4200: 5V, 10A, 8 channels and 10V, 5A, 8 channels), multimeter (US fulke, 17B), CSZ high temperature explosion-proof box ( US CSZ ), Jufu high and low temperature explosion-proof Box (China Taiwan, ETH-1000-40-CP-AR) and data collector (Japan Yokogawa, MX100);

Experimental power battery samples: 1) nominal voltage 3.2V, nominal capacity 10Ah; 2) composition of the battery: the positive active material is LiFePO ₄ , the negative active material is artificial graphite, and the main component of the electrolyte is LiPF ₆ /EC+DMC +VC+PS, and celgard diaphragm; 3) Dimensions: 23mm (thickness) x68mm (width) x88mm (height).

To screen the consistency of self-discharge of the above batteries, the specific steps are as follows:

1. Select 10 batteries with good consistency of capacity (10Ah±5mAh) and internal resistance (<4mΩ) for experiment. At room temperature (20±2)°C, adopt the constant current and constant voltage charging system and constant current discharging system, that is, the battery to be tested is discharged at a current of 0.02C under the test temperature condition, until the battery voltage reaches 2.1V, stand for 1h, and then Charge with a constant current of 0.2C, turn to constant voltage charging when the battery voltage reaches 3.65V, stop charging when the charging current drops to 0.02C, and cycle charging and discharging 10 times in this way;

2. At room temperature (20±2)°C, adjust the SOC (state of charge) of the above-mentioned lithium iron phosphate battery to an empty state, that is, 0% SOC (state of charge);

3. At room temperature (20±2)°C, put the above-mentioned lithium iron phosphate battery on hold for 7 days, measure the open circuit voltage of each single battery with a multimeter every 24 hours, and calculate the open circuit voltage difference △OCU_1 of adjacent 24 hours, And perform linear regression Y=A+BX on the 7-day open circuit voltage difference △OCU_1, and select single batteries with B>0 (batteries 1, 2, 3, 5, 6, 7, 8), and the results are shown in the table , at this time, the voltage of the lithium iron phosphate battery is recorded as OCU_t1;

battery number B value after linear regression of voltage difference 1 B>0 2 B>0 3 B>0 4 B<0 5 B>0 6 B>0 7 B>0 8 B>0 9 B<0 10 B<0

4. Charge the above-mentioned lithium iron phosphate battery, that is, adjust the battery SOC to 10% SOC (state of charge);

5. At a high temperature of 30°C, the above-mentioned lithium iron phosphate battery (10% SOC) was left for 7 days, and the open circuit voltage OCV_t2 of the battery after being left at high temperature was measured; the voltage drop of the battery was calculated △V=OCV_t1-OCV_t2, as shown in the table;

battery number ΔV 1 -0.019 2 -0.01 3 -0.01 5 -0.009 6 -0.01 7 -0.007 8 -0.006

6. Calculate the average value of △V of all batteries X=-0.01 and standard deviation σ=0.004, select the battery within the [-0.018, -0.002] confidence interval, that is, select batteries 2, 3, 5, 6, 7, 8 , these batteries have better self-discharge rate consistency.

The above-mentioned embodiment is a preferred embodiment of the present invention, but the embodiment of the present invention is not limited by the above-mentioned embodiment, and any other changes, modifications, substitutions, combinations, Simplifications should be equivalent replacement methods, and all are included in the protection scope of the present invention.

Claims (3)

1. a kind of evaluation method of lithium-ion-power cell self discharge uniformity, including：

Step 1, same production batch, same specifications and models, identical material system ferric phosphate lithium cell in, select capacity and The preferably multiple batteries of internal resistance uniformity；At room temperature (20 ± 2) DEG C, using constant-current constant-voltage charging system and constant-current discharge system Degree, i.e., mesuring battary, to the minimum discharge voltage of battery as defined in manufacturer, is stood into 1h at room temperature with 0.02C current discharges, Then with 0.2C constant-current charges, to constant-voltage charge is turned during battery maximum charging voltage as defined in manufacturer, it is down to charging current Stop charging during 0.02C, such cycle charge-discharge is multiple；

Step 2, under the conditions of room temperature (20 ± 2) DEG C, adjust above-mentioned ferric phosphate lithium cell SOC (state-of-charge) to empty electric state, i.e., 0%SOC (state-of-charge)；

Under the conditions of step 3, room temperature (20 ± 2) DEG C, above-mentioned ferric phosphate lithium cell is shelved 7 days, surveyed every 24 hours with universal meter The open-circuit voltage of each cell is measured, calculates the open-circuit voltage difference △ OCU_1 of adjacent 24 hours, and to the open-circuit voltage of 7 days Difference △ OCU_1 carry out linear regression Y=A+BX, and wherein Y is that open-circuit voltage difference △ OCU_1, X are number, i.e. open-circuit voltage Difference △ OCU_1 number, A are slope, and B is intercept；Choose B>0 cell, now ferric phosphate lithium cell voltage be designated as OCU_t1；With this, moon battery of the self-discharge rate more than 3% is screened out；

Step 4, charging process is carried out to above-mentioned ferric phosphate lithium cell, i.e., adjusted battery SOC to 10%SOC-30%SOC lotus Electricity condition；

Step 5, at a high temperature of 30 DEG C -45 DEG C, by the above-mentioned ferric phosphate lithium cell with 10%SOC-30%SOC state-of-charges Shelve 7 days, measure the open-circuit voltage OCV_t2 after battery high-temperature is shelved；Calculate the voltage drop △ V=OCV_t1-OCV_t2 of battery Or voltage drop K=(OCV_t1-OCV_t2)/t in the unit interval, wherein t are the unit time, can be set to day or hour；

The average value X and standard deviation sigma of step 6, △ V or the K values of all batteries of calculating, are filtered out in [X-2 σ, X+2 σ] confidence area Between battery.

2. the evaluation method of lithium-ion-power cell self discharge uniformity according to claim 1, it is characterised in that circulation Discharge and recharge number is 10 times.

3. the evaluation method of lithium-ion-power cell self discharge uniformity according to claim 1, it is characterised in that select Capacity and the preferably multiple batteries of internal resistance uniformity, refer to selecting capacity 10Ah ± 5mAh, and internal resistance<4m Ω 10 electricity Pond.