# Patent

## Title
Method for grouping lithium secondary battery packs

## Abstract

A method for grouping lithium secondary battery packs comprises the following steps: charging and discharging a battery for 1-3 cycles, recording the last discharge capacity C0, setting a capacity lower limit, and determining the battery with the C0 thereof not less than the capacity lower limit to be an eligible battery; discharging the battery to a discharge cut-off voltage Vd so the battery is discharged to a power empty state; charging the empty battery to a capacity C1; storing the battery for time t1 in an environment with a temperature ranging between 20-50° C., recording the battery voltage V1, storing the battery again for time t2 in the environment with the temperature ranging between 20-50° C., recording the battery voltage V2, calculating a voltage difference ΔV=V2−V1 and setting the range of ΔV; and grouping eligible batteries in a previous step according to a certain capacity grouping standard.

## Background
CROSS REFERENCE TO RELATED APPLICATIONS
This application is a Continuation of co-pending Application No. PCT/CN2012/070500 filed on Jan. 17, 2012, for which priority is claimed under 35 U.S.C. §120; and this application claims priority of Application No. 201110110667.3 filed in P.R. China on Apr. 29, 2011 under 35 U.S.C. §119, the entire contents of all of which are hereby incorporated by reference.


FIELD OF THE INVENTION
The present invention relates to a secondary battery technology, particularly to a method for grouping lithium secondary battery packs.
BACKGROUND OF THE INVENTION
The technology of lithium ion batteries is advancing fast. Meanwhile, lithium ion batteries have been widely applied in the fields of power source and power storage. Comparing with the batteries used in miniature portable electronic devices (such as mobile phones), the batteries used in the fields of power source and power storage are requested to provide features such as high output power and prolonged service life. Thus, the batteries for power source or power storage normally need to be connected in parallel or in series to form a battery pack, which is of high value and expensive. Therefore, batteries applied in fields of power source and power storage demand higher reliability and higher consistency. Since failure of any individual battery of a battery pack would induce the entire battery pack to malfunction or even cause a safety problem, the batteries to be used in battery packs should be tested and sifted to guarantee operational consistency of each battery. The factors to evaluate the consistency of batteries include capacity, voltage, discharge voltage platform, internal resistance, and self-discharge rate, wherein self-discharge rate is particularly important but hard to evaluate. In a system where batteries cannot be charged and discharged uniformly, the operational consistency of the batteries will have great changes after long time storage or long time usage because of different self-discharge rates. In an industrial standard test method for self-discharge, a fully-charged battery (SOC (State Of Charge) 100%) is left unused for 1 month, and then the charge retention rate thereof is measured. A test method for self-discharge commonly used by the manufacturers includes the following steps: charging a battery to a fully-charged state (SOC 90-100%) or a half-charged state (SOC 40-60%), storing the battery at a room temperature or a high temperature, and evaluating the self-discharge level of the battery via detecting the voltage difference between the pre-storage battery and post-storage battery. However, the conventional test methods for self-discharge have a disadvantage that a voltage-capacity relationship of the lithium ion battery is unclear. For example, there is no obvious voltage-capacity relationship in the fully-charged state and the half-charged state of a lithium iron phosphate battery. Therefore, the abovementioned methods cannot achieve the desired test effect unless the batteries are stored for a long time. Thus, the abovementioned methods are unsuitable for mass-production. Further, the higher the charged level of batteries, the greater the safety risk for storing or assembling the batteries. Furthermore, the self-discharge test of fully-charged or half-charged batteries spends a lot of time, normally 15-30 days. Thus, each cycle of batch production requires a long time, which causes high carrying costs, raises the total cost and customers are not satisfied with delivery date. If the test period is shortened, the batteries of high self-discharge are hard to be picked out. Then, the customers will complain about the unqualified products. Traditionally, the reliability of battery packs conflicts with carrying cost and fast delivery. A China Patent No. CN101764259A disclosed a self-discharge test method using a condition of SOC 50-90% and storage for 7-14 days, which uses higher SOC batteries and longer storage time. While picking those batteries out, there are both qualified batteries and unqualified batteries for self-discharge. With high SOC, a high self-discharge battery is easily to heat up and has risks of burning spontaneously, which may generate a temperature of over 130° C. High temperature may melt the internal separation membranes of neighboring batteries, which may further cause a chain reaction of internal short circuit of the batteries. Many fires occurred in battery manufacturers are attributed to spontaneous heating and burning of batteries. On the other hand, while the batteries are stored with SOC 50-90%, the voltage thereof is within the discharge platform section and relatively stable. In such a case, the voltage of the battery is hard to reveal the self-discharge level of the battery, and the batteries with high self-discharge cannot be all identified even though longer test time is used. Thus, there are always some unqualified batteries escaping from testing and mixed in battery packs. Dangers of malfunction, fire and explosion are hidden in the abovementioned unqualified batteries.
SUMMARY OF THE INVENTION
The objective of the present invention is to avoid the deficiency in the existing technology and provide a method for grouping lithium secondary packs, which spends less time in testing self-discharge (SD) of batteries and favors mass-production of battery packs, whereby batteries can be safely and efficiently assembled into highly-reliable battery packs.
Via studying the discharge curves of lithium cobalt oxide batteries, lithium manganese oxide batteries, lithium nickel manganese cobalt oxide batteries, lithium iron phosphate batteries and other lithium ion batteries, we found in the initial stage of discharge polarization makes the voltage drop quickly, then in the middle stage the voltage is relatively stable, and at last the voltage descends steeply and linearly in the final stage, where 0-10% capacity of a lithium ion battery is discharged. Via the characteristic of the horizontal discharge platform section and the linearly-descending final stage of the discharge curve, we can obtain the relationship of the ratio of the voltage to the residual capacity in the descending section of the discharge curve. Then, the SD rate can be worked out according to the variation of the storage voltage. In a power empty state or a low-charged state (SOC 0-10%), the voltage of a battery discharged spontaneously is very easy to change within 1-10 days of storage, and the SD rate within 1-10 days can be calculated from the voltage variation. On the other hand, in a power empty state or a low-charged state (SOC 0-10%), the energy released by the internal short circuit of a battery is neither sufficient to raise the temperature of the battery over the burning point nor able to melt the internal separation membranes of neighboring batteries. Therefore, a battery in a power empty state or a low-charged state would not cause a chain reaction of fires or explosions.
In summary, the present invention achieves the objective of safely and efficiently assembling batteries into highly-reliable battery packs via the following steps: using a small current to discharge the batteries to a power empty state or a low-charged state (SOC 0-10%) or charge batteries to a low-charged state (SOC 0-10%) with a small amount of electricity; testing the batteries after a short time storage to pick out the low-voltage batteries with high SD; and assembling the batteries in a power empty state or a low-charged state.
In a first embodiment, the method of the present invention comprises the following steps:
Step (1): charging and discharging batteries for 1-3 cycles, recording a last discharge capacity C0 of the batteries, setting a capacity lower limit, and determining the batteries with the C0 thereof not less than the capacity lower limit to be eligible batteries.
The capacity lower limit is a minimum eligible capacity defined by industry technology and normally equal to or slightly higher than a rated capacity C5 to guarantee that a capacity of each of the batteries delivered to the customers can reach the rated capacity C5, which is within the range of 5-500 Ah. In the abovementioned process, the batteries are charged with a constant current of 0.1-5 C5 to a charge cut-off voltage Vc of 3.6-4.2V, and the batteries are changed to be charged with a constant voltage to the charge cut-off current of 0.01-0.5 C5, and then the batteries are discharged with a constant current of 0.2-10 C5 to a discharge cut-off voltage Vd of 2-3V.
The charge cut-off voltage Vc of the battery is determined by a system of the battery. For example, the lithium cobalt oxide-graphite system has a charge cut-off voltage Vc of 4.2V, and the lithium iron phosphate-graphite system has a charge cut-off voltage Vc of 3.6V. The discharge cut-off voltage Vd of the battery is also determined by the system of the battery. For example, the lithium cobalt oxide-graphite system has a discharge cut-off voltage Vd of 3V, and the lithium iron phosphate-graphite system has a discharge cut-off voltage Vd of 2V. The rated capacity is a capacity labeled by the manufacturer for a battery or a battery pack and means the electricity discharged by a battery at an ambient temperature of 23±2° C. with a discharge rate of 5 h to the discharge cut-off voltage Vd. The rated capacity is denoted by C5, and a unit thereof is Ah or mAh.
Step (2): succeeding to the last discharge of Step (1), discharging continuously the batteries with 0.01-0.5 C5 to the discharge cut-off voltage Vd of 2.0-3.0V so as to discharge the batteries to a power empty state. Discharging at a low discharge rate is to reduce an influence of polarization, whereby the batteries can be discharged to a power empty state more uniformly, and whereby the discharged batteries have higher consistency in voltages after discharging.
Step (3): charging the batteries in the power empty state that were discharged in Step (2) with 0.01-0.5 C5 to a capacity C1, which is 0.1-10% of the rated capacity C5, preferably 2-5% of the rated capacity C5.
Step (4): storing the batteries processed in Step (3) for time t1 in an environment with a temperature ranging between 20-50° C., and measuring and recording a battery voltage V1 of each battery after being stored. Then storing continuously the batteries for time t2 in the environment with the temperature ranging between 20-50° C., and measuring a battery voltage V2 of each battery after being stored, calculating a voltage difference ΔV=V2−V1 and setting the range of ΔV to be from −20 mV to 10 mV, preferably from −10 mV to 5 mV. The time t1 ranges from 1 to 48 hours, and the time t2 ranges from 24-240 hours.
Step (5): grouping the eligible batteries in Step (4) into a plurality of battery packs each including specified number of series batteries according to a certain capacity grouping standard, and welding each of the battery packs to form a battery assembly. The capacity difference of the batteries in one of the battery packs is not greater than 5% of the rated capacity C5, preferably not greater than 2% of the rated capacity C5.
In a second embodiment, the method of the present invention comprises the following steps:
Step (1): charging and discharging the batteries for 1-3 cycles, recording the last discharge capacity C0 of each battery, setting a capacity lower limit, and determining the batteries with the C0 thereof not less than the capacity lower limit to be eligible batteries.
The capacity lower limit is the minimum eligible capacity defined by the industry technology and normally equal to or slightly higher than the rated capacity C5 to guarantee that the capacity of each of the batteries delivered to the customers can reach the rated capacity C5, which is within the range of 5-500 Ah. In the abovementioned process, the batteries are charged with a constant current of 0.1-5 C5 to the charge cut-off voltage Vc of 3.6-4.2V, and the batteries are changed to be charged with a constant voltage to the charge cut-off current of 0.01-0.5 C5, and then the batteries are discharged with a constant current of 0.2-10 C5 to the discharge cut-off voltage Vd of 2-3V.
The charge cut-off voltage Vc of the battery is determined by the system of the battery. For example, the lithium cobalt oxide-graphite system has a charge cut-off voltage Vc of 4.2V, and the lithium iron phosphate-graphite system has a charge cut-off voltage Vc of 3.6V. The discharge cut-off voltage Vd of the battery is also determined by the system of the battery. For example, the lithium cobalt oxide-graphite system has a discharge cut-off voltage Vd of 3V, and the lithium iron phosphate-graphite system has a discharge cut-off voltage Vd of 2V. The rated capacity is the capacity labeled by the manufacturer for a battery or a battery pack and means the electricity discharged by a battery at an ambient temperature of 23±2° C. with a discharge rate of 5 h to the discharge cut-off voltage Vd. The rated capacity is denoted by C5, and a unit thereof is Ah or mAh.
Step (2): succeeding to the last discharge of Step (1), discharging continuously the batteries with 0.01-0.5 C5 to the discharge cut-off voltage Vd of 2.0-3.0V so as to discharge the batteries to a power empty state. Discharging at a low discharge rate is to reduce the influence of polarization, whereby the batteries can be discharged to a power empty state more uniformly, and whereby the discharged batteries have higher consistency in voltages after discharging.
Step (3): storing the batteries processed in Step (2) for time t1 in the environment with the temperature ranging between 20-50° C., and measuring and recording the battery voltage V1 of each battery after being stored. Then storing continuously the batteries for time t2 in the environment with the temperature ranging between 20-50° C., and measuring the battery voltage V2 of each battery after being stored, calculating the voltage difference ΔV=V2−V1 and setting the range of ΔV to be from −20 mV to 10 mV, preferably from −10 mV to 5 mV. The time t1 ranges from 1 to 48 hours, and the time t2 ranges from 24-240 hours.
Step (4): grouping the eligible batteries in Step (3) into a plurality of battery packs each including a specified number of series batteries according to a capacity grouping standard, and welding each of the battery packs to form a battery assembly. The capacity difference of the batteries in one of the battery packs is not greater than 5% of the rated capacity C5, preferably not greater than 2% of the rated capacity C5.
In a third embodiment, the method of the present invention comprises the following steps:
Step (1): charging and discharging a battery for 2-3 cycles, recording a second last discharge capacity C0 of each of the batteries, setting a capacity lower limit, and determining the batteries with the C0 thereof not less than the capacity lower limit to be eligible batteries, wherein a last discharge capacity is controlled to be 90-99.9% of the rated capacity C5 so that a remaining capacity of the batteries is 0.1-10% of the rated capacity C5 to make the batteries in a low-charged state.
The capacity lower limit is the minimum eligible capacity defined by the industry technology and normally equal to or slightly higher than the rated capacity C5 to guarantee that the capacity of each of the batteries delivered to the customers can reach the rated capacity C5, which is within the range of 5-500 Ah. In the abovementioned process, the batteries are charged with a constant current of 0.1-5 C5 to a charge cut-off voltage Vc of 3.6-4.2V, and the batteries are changed to be charged with a constant voltage to the charge cut-off current of 0.01-0.5 C5, and then the batteries are discharged with a constant current of 0.2-10C5 to the discharge cut-off voltage Vd of 2-3V.
The charge cut-off voltage Vc of the battery is determined by the system of the battery. For example, the lithium cobalt oxide-graphite system has a charge cut-off voltage Vc of 4.2V, and the lithium iron phosphate-graphite system has a charge cut-off voltage Vc of 3.6V. The discharge cut-off voltage Vd of the battery is also determined by the system of the battery. For example, the lithium cobalt oxide-graphite system has a discharge cut-off voltage Vd of 3V, and the lithium iron phosphate-graphite system has a discharge cut-off voltage Vd of 2V. The rated capacity is the capacity labeled by the manufacturer for a battery or a battery pack and means the electricity discharged by a battery at an ambient temperature of 23±2° C. with a discharge rate of 5 h to the discharge cut-off voltage Vd. The rated capacity is denoted by C5, and a unit thereof is Ah or mAh.
Step (2): Storing the battery processed in Step (1) for time t1 in the environment with the temperature ranging between 20-50° C., and measuring and recording the battery voltage V1 of each battery after being stored. Then storing continuously the batteries for time t2 in the environment with the temperature ranging between 20-50° C., and measuring the battery voltage V2 of each battery after being stored, calculating the voltage difference ΔV=V2−V1 and setting the range of ΔV to be from −20 mV to 10 mV, preferably from −10 mV to 5 mV. The time t1 ranges from 1 to 48 hours, and the time t2 ranges from 24-240 hours.
Step (3): grouping the eligible batteries in Step (2) into a plurality of battery packs each including a specified number of series batteries according to a capacity grouping standard, and welding each battery pack to form a battery assembly. The capacity difference of the batteries in one of the battery packs is not greater than 5% of the rated capacity C5, preferably not greater than 2% of the rated capacity C5.
In the present invention, the discharge cut-off voltage Vd of the battery is determined by the battery system and ranges from 2.0-3.0V. For example, the lithium iron phosphate-graphite system has a discharge cut-off voltage Vd of 2V, and the lithium cobalt oxide-graphite system has a discharge cut-off voltage Vd of 3V.
Among factors influencing the reliability of the battery packs, the SD level of the batteries in one battery pack is a particularly important one. Therefore, picking out high-SD batteries is a critical factor to affect the reliability of the battery packs.
The capacity of the battery is proportional to the stable open-circuit voltage. Theoretically, the capacity of the battery can be obtained as long as the voltage thereof can be precisely measured. The relationship between the capacity and the stable open-circuit voltage of the battery is particularly obvious in a power empty state or a low-charged state (SOC 0-10%). A fully-charged battery is at a high risk of danger for storage, and is easily to form a short circuit when in operation. For the lithium iron phosphate battery, the relationship between the capacity and the stable open-circuit voltage is unobvious and likely to be interpreted incorrectly in the fully-charged state. For various types of lithium ion batteries, the relationship between the capacity and the stable open-circuit voltage is so clear in the power empty state or the low-charged state (SOC 0-10%) that even a slight SD can also be reflected by a drop of the stable open-circuit voltage. The lower the capacity of the battery, the more obvious the abovementioned phenomenon. Therefore, the present invention stores the batteries in the power empty state or the low-charged state (SOC 0-10%) and detects the variations of the open-circuit voltage of the batteries to effectively pick out high-SD batteries to guarantee the reliability of the battery packs.
A series of batteries features that an identical current flows through each of the series of batteries. In other words, the capacities of charge or discharge are identical for all batteries of the series Therefore, if the initial practical capacities (not the relative capacity (%)) of the batteries of the series are consistent, the discharge consistency of the battery pack can be ensured. The method of the present invention tests the batteries in the power empty state or the low-charged state to pick out high SD batteries, and then charges the batteries to 0-10% of the rated capacity C5 to guarantee the qualified batteries to discharge consistently. Besides, the batteries in the power empty state or the low-charged state have a low risk of safety problems caused by short circuit, drop or being pierced in the fabrication process. According to the standard battery-discharge regulation, the battery discharged with 0.2 C5 to the standard discharge cut-off voltage Vd is regarded as in SOC 0%. In present invention, the battery that is discharged with 0.01-0.5 C5 to a discharge cut-off voltage Vd of 2.0-3.0V is defined to be in a power empty state. If a piercing test is performed on the battery, which is in a power empty state defined by the present invention, to cause an internal short circuit in the battery, the released residual capacity thereof is only a very small amount of electricity that will not result in obvious heating-up, firing or explosion. As long as there is current leakage occurring in the battery having a very small residual capacity, the variation of voltage can obviously reflect the current leakage. If the piercing test is performed on the battery, which is discharged to the standard discharge cut-off voltage Vd according to the standard battery-discharge regulation and then charged to 0-10% of the rated capacity C5 (SOC 0-10%), to cause an internal short circuit in the battery, the released heat neither melts the separation membrane of the battery nor causes fire or explosion of the battery, not to mention to cause the chain reaction of the neighboring batteries. Therefore, the battery in the power empty state or the low-charged state is very safe for fabrication. Further, while the electricity of the battery decreases in the power empty state or the low-charged state, the voltage thereof drops linearly and steeply. Thus, the voltage variation can reflect the SD level of the battery obviously and can be used to effectively pick out batteries with high-SD level. Thereby is promoted the reliability of the battery packs.
The method for grouping lithium secondary battery packs of the present invention has the following advantages: each batteries in one battery pack is less likely to be affected or endangered by self-discharge; the batteries have good consistency in discharge; the battery packs are assembled safely; the performance of battery packs has high reliability; the method is easy to operate and able to accelerate the fabrication process; the method has high potential in the market.



BRIEF DESCRIPTION OF THE DRAWINGS
The solo FIGURE shows a discharge curve of a lithium iron phosphate battery.



DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The present invention will be further demonstrated with embodiments below.
Please refer to the sole FIGURE showing a discharge curve of a Type 11585135Fe lithium iron phosphate battery.
Embodiment I
The Type 11585135Fe lithium iron phosphate battery is a product of our company and has a nominal voltage of 3.2V and a rated capacity of 10 Ah. The Type 11585135Fe lithium iron phosphate batteries are to be assembled into a plurality of 1P-12S battery packs. According to the method of the present invention, the batteries are processed with the following steps:
Step (1): charging and discharging the batteries for two cycles with a charge/discharge current of 0.5 C5, recording the last discharge capacity C0, and determining the batteries with C0≧10 Ah to be eligible batteries;
Step (2): discharging continuously the eligible batteries of Step (1) with a small discharge current of 0.2 C5 to a discharge cut-off voltage of 2.0V to make the batteries in a power empty state;
Step (3): storing the batteries processed in Step (2) at a room temperature for 24 hours, recording the battery voltage V1 of each of the batteries after being stored, then storing the batteries at a room temperature for 120 hours, recording the battery voltage V2 of each of the batteries after being stored, calculating a voltage difference ΔV=V2−V1, and determining the batteries with the voltage difference ΔV of −5 mV ̃5 mV to be eligible batteries; (Table.1 lists the data obtained from batteries processed the preceding steps.)













TABLE 1





Battery
C0
C0
V1
V2
ΔV
ΔV


No.
(mAh)
Evaluation
(V)
(V)
(mV)
Evaluation





















F1
10850
Eligible
2.831
2.829
−2
Eligible


F2
10910
Eligible
2.841
2.840
−1
Eligible


F3
11020
Eligible
2.797
2.799
2
Eligible


F4
10870
Eligible
2.819
2.821
2
Eligible


F5
10730
Eligible
2.815
2.816
1
Eligible


F6
11030
Eligible
2.823
2.822
−1
Eligible


F7
10660
Eligible
2.816
2.814
−2
Eligible


F8
11030
Eligible
2.836
2.830
−6
Ineligible


F9
10970
Eligible
2.836
2.836
0
Eligible


F10
10920
Eligible
28.16
2.843
27
Ineligible


F11
11080
Eligible
2.815
2.816
1
Eligible


F12
10990
Eligible
2.822
2.820
−2
Eligible


F13
10850
Eligible
2.812
2.810
−2
Eligible


F14
11060
Eligible
2.797
2.799
2
Eligible


F15
10850
Eligible
2.815
2.817
2
Eligible


F16
11100
Eligible
2.800
2.802
2
Eligible


F17
10850
Eligible
2.823
2.825
2
Eligible


F18
10980
Eligible
2.793
2.793
0
Eligible


F19
10930
Eligible
2.818
2.820
2
Eligible


F20
10940
Eligible
2.811
2.801
−10
Ineligible


F21
10980
Eligible
2.811
2.813
2
Eligible


F22
11040
Eligible
2.807
2.810
3
Eligible


F23
11010
Eligible
2.807
2.808
1
Eligible


F24
10950
Eligible
2.802
2.804
2
Eligible


F25
10990
Eligible
2.797
2.799
2
Eligible


F26
10720
Eligible
2.812
2.809
−3
Eligible


F27
10680
Eligible
2.816
2.814
−2
Eligible


F28
10580
Eligible
2.818
2.816
−2
Eligible









Step (4): grouping the batteries whose capacity difference are within 3% to assemble into the battery packs. Table.1 shows there are two groups of batteries allowed to be assembled into the 1P-12S battery packs, including a group containing F1, F2, F4, F5, F7, F13, F15, F17, F19, F24, F26 and F27, and another group containing F3, F6, F9, F11, F12, F14, F16, F18, F21, F22, F23 and F25;
Step (5): welding and assembling each of the battery packs after the eligible batteries have been grouped in Step (4), adding 12S protection boards to each of the battery packs to form a battery assembly. The 12S protection board has no balancing function, wherein the over-charge protection voltage thereof is 3.9V/section and the over-discharge protection voltage thereof is 2V/section.
Verification
A first battery pack assembled according to the abovementioned steps is charged with a constant current of 5 A to a charge cut-off voltage of 43.8V, and is changed to be charged with a constant voltage of 43.8V to a charge cut-off current of 500 mA. Then, the first battery pack is discharged with a constant current of 5 A to a discharge cut-off voltage of 24V, totally releasing a capacity of 10672 mAh. The instantaneous discharge cut-off voltages of the batteries in the first battery pack have high consistency, as shown in Table.2.










TABLE 2








Discharge Cut-Off



Battery No.
Component Voltage (V)









F1
2.092



F2
2.098



F4
2.056



F5
2.085



F7
2.074



F13
2.065



F15
2.058



F17
2.036



F19
2.038



F24
2.072



F26
2.009



F27
2.039










Then, the first battery pack is charged and discharged with a current of 0.5 C5 for at least one cycle. The discharged capacities and the total discharge cut-off voltages are recorded, and the total discharge cut-off voltages are used to calculate the average discharge cut-off component voltages, as shown in Table.3.










TABLE 3






Discharge
Total discharge
Average Discharge


Number of
Capacity
cut-off
Cut-Off Component


Cycles
(mAh)
voltage (V)
Voltage (V)


















1
10672
24.72
2.060


2
10682
24.73
2.061


3
10676
24.73
2.061


4
10675
24.74
2.062


5
10685
24.76
2.063


6
10682
24.73
2.061


7
10670
24.75
2.063


8
10668
24.74
2.062


9
10676
24.73
2.061


10
10685
24.75
2.063


11
10668
24.77
2.064


12
10665
24.73
2.061


13
10662
24.75
2.063


14
10665
24.74
2.062


15
10668
24.73
2.061


16
10660
24.72
2.060


17
10658
24.76
2.063


18
10655
24.75
2.063


19
10658
24.74
2.062


20
10662
24.73
2.061









With the number of cycles increasing, the released capacities of the first battery pack remain stable, and the total discharge cut-off voltages as well as the average discharge cut-off component voltages have a good consistency.
Safety Test
A nail is used to pierce the battery F28, which is not used to assemble the battery packs, to cause the internal short circuit of the battery at a room temperature of 28° C. The short-circuit point is detected to have a maximum temperature of 33° C.-5° C. higher than the room temperature, and the battery neither burns nor explodes.
Embodiment II
The Type 130125155Fe lithium iron phosphate battery is a product of our company and has a nominal voltage of 3.2V and a rated capacity of 20 Ah. The Type 130125155Fe lithium iron phosphate batteries are to be assembled into a plurality of 1P-8S battery packs. According to the method of the present invention, the batteries are processed with the following steps:
Step (1): charging and discharging the batteries for one cycle with a charge/discharge current of 0.5 C5, recording the last discharge capacity C0, and determining the batteries with C0≧20 Ah to be eligible batteries;
Step (2): discharging continuously the eligible batteries of Step (1) with a small discharge current of 0.2 C5 to a discharge cut-off voltage of 2.0V;
Step (3): charging the batteries with a charge current of 0.2 C5 for 12 minutes to a low-charged state (SOC 4%);
Step (4): storing the batteries processed in Step (3) at a room temperature for 12 hours, recording the battery voltage V1 of each of the batteries after being stored, then storing the batteries at a room temperature for 144 hours, recording the battery voltage V2 of each of the batteries after being stored, calculating a voltage difference ΔV=V2−V1, and determining the batteries with the voltage difference ΔV of −10 mV ̃0 mV to be eligible batteries; (Table.4 lists the data obtained from batteries processed the preceding steps.)













TABLE 4





Battery
C0
C0
V1
V2
ΔV
ΔV


No.
(mAh)
Evaluation
(V)
(V)
(mV)
Evaluation





















J1
20880
Eligible
3.155
3.152
−3
Eligible


J2
20750
Eligible
3.165
3.161
−4
Eligible


J3
20520
Eligible
3.158
3.152
−6
Eligible


J4
20810
Eligible
3.159
3.155
−4
Eligible


J5
20750
Eligible
3.161
3.156
−5
Eligible


J6
21030
Eligible
3.153
3.149
−4
Eligible


J7
20960
Eligible
3.152
3.131
−21
Ineligible


J8
21060
Eligible
3.157
3.153
−4
Eligible


J9
20940
Eligible
3.159
3.154
−5
Eligible


J10
20910
Eligible
3.164
3.160
−4
Eligible


J11
21060
Eligible
3.163
3.158
−5
Eligible


J12
20690
Eligible
3.161
3.158
−3
Eligible


J13
20840
Eligible
3.153
3.149
−4
Eligible


J14
21010
Eligible
3.155
3.151
−4
Eligible


J15
20550
Eligible
3.158
3.153
−5
Eligible


J16
20560
Eligible
3.160
3.155
−5
Eligible


J17
20650
Eligible
3.162
3.155
−7
Eligible


J18
20880
Eligible
3.157
3.151
−6
Eligible


J19
20730
Eligible
3.158
3.153
−5
Eligible


J20
20640
Eligible
3.156
3.142
−14
Ineligible









Step (5): grouping the batteries whose capacity differences are within 3% to assemble into the battery packs. Table.4 shows there are two groups of batteries allowed to be assembled into the 1P-8S battery packs, including a group containing J1, J4, J6, J8, J10, J12, J13 and J14 whose voltage differences are minimum and the SD levels have good consistency, and another group containing J2, J5, J9, J11, J15, J16, J18 and J19 whose voltage differences are close and the SD levels are closed;
Step (6): welding and assembling each of the battery packs after the eligible batteries have been grouped in Step (5), adding 8S protection boards to each of the battery packs to form a battery assembly. The 8S protection board has no balancing function, wherein the over-charge protection voltage thereof is 3.9V/section, and the over-discharge protection voltage thereof is 2V/section.
Verification
A first battery pack assembled according to the abovementioned steps is charged with a constant current of 10 A to a charge cut-off voltage of 29.2V, and is changed to be charged with a constant voltage of 29.2V to a charge cut-off current of 1000 mA. Then, the first battery pack is discharged with a constant current of 10 A to a discharge cut-off voltage of 16V, totally releasing a capacity of 20680 mAh. The instantaneous discharge cut-off voltages of the batteries in the first battery pack have high consistency, as shown in Table.5.










TABLE 5








Discharge Cut-Off



Battery No.
Component Voltage (V)









J1
2.088



J4
2.086



J6
2.065



J8
2.079



J10
2.035



J12
2.011



J13
2.045



J14
2.068










Then, the first battery pack is charged and discharged with a current of 0.5 C5 for at least one cycle. The discharged capacities and the total discharge cut-off voltages are recorded, and the total discharge cut-off voltages are used to calculate the average discharge cut-off component voltages, as shown in Table.6.










TABLE 6






Discharge
Total discharge
Average Discharge


Number of
Capacity
cut-off
Cut-Off Component


Cycles
(mAh)
voltage (V)
Voltage (V)


















1
20680
16.48
2.060


2
20674
16.48
2.060


3
20688
16.49
2.061


4
20662
16.47
2.059


5
20673
16.50
2.063


6
20656
16.52
2.065


7
20668
16.48
2.060


8
20671
16.49
2.061


9
20654
16.52
2.065


10
20648
16.51
2.064


11
20652
16.53
2.066


12
20642
16.50
2.063


13
20665
16.54
2.068


14
20638
16.52
2.065


15
20658
16.53
2.066


16
20652
16.55
2.069


17
20645
16.52
2.065


18
20633
16.54
2.068


19
20649
16.56
2.070


20
20652
16.52
2.065









With the number of cycles increasing, the released capacities of the first battery pack remain stable, and the total discharge cut-off voltages as well as the average discharge cut-off component voltages have a good consistency.
Safety Test
Nails are used to pierce the batteries J3 and J17, which are not used to assemble the battery packs, to cause the internal short circuit of the batteries at a room temperature of 28° C. The short-circuit points are detected to respectively have maximum temperatures of 43° C. and 45° C.—separately 15° C. and 17° C. higher than the room temperature, and the batteries neither burn nor explode.
Embodiment III
The Type 11065145Fe lithium iron phosphate battery is a product of our company and has a stander voltage of 3.2V and a rated capacity of 9 Ah. The Type 11065145Fe lithium iron phosphate batteries are to be assembled into a plurality of 1P-8S battery packs. According to the method of the present invention, the batteries are processed with the following steps:
Step (1): charging and discharging the batteries for two cycles with a charge/discharge current of 0.5 C5, recording the discharge capacity C0 of the first cycle, and determining the batteries with C0≧9 Ah to be eligible batteries; discharging the batteries with a constant current of 0.5 C5 for 115.2 minutes in the second cycle to release 96% electricity (SOC 4%) and make the batteries in a low-charged state;
Step (2): storing the batteries of SOC 4% processed in Step (1) at a room temperature for 12 hours, recording the battery voltage V1 of each of the batteries after being stored, then storing the batteries at room temperature for 168 hours, recording the battery voltage V2 of each of the batteries after being stored, calculating a voltage difference ΔV=V2−V1, and determining the batteries with the voltage difference ΔV of −10 mV ̃5 mV to be eligible batteries; (Table.7 lists the data obtained from batteries processed the preceding steps.)













TABLE 7





Battery
C0
C0
V1
V2
ΔV
ΔV


No.
(mAh)
Evaluation
(V)
(V)
(mV)
Evaluation





















K1
9360
Eligible
3.157
3.151
−6
Eligible


K2
9550
Eligible
3.168
3.162
−6
Eligible


K3
9510
Eligible
3.151
3.153
2
Eligible


K4
9630
Eligible
3.158
3.159
1
Eligible


K5
9350
Eligible
3.168
3.155
−13
Ineligible


K6
9530
Eligible
3.156
3.152
−4
Eligible


K7
9460
Eligible
3.162
3.163
1
Eligible


K8
9540
Eligible
3.167
3.162
−5
Eligible


K9
9520
Eligible
3.157
3.153
−4
Eligible


K10
9610
Eligible
3.166
3.160
−6
Eligible


K11
9480
Eligible
3.153
3.156
3
Eligible


K12
9390
Eligible
3.165
3.159
−6
Eligible


K13
9740
Eligible
3.153
3.151
−2
Eligible


K14
9620
Eligible
3.168
3.161
−7
Eligible


K15
9590
Eligible
3.159
3.143
−16
Ineligible


K16
9470
Eligible
3.165
3.159
−6
Eligible


K17
9640
Eligible
3.161
3.157
−4
Eligible


K18
9380
Eligible
3.167
3.162
−5
Eligible


K19
9660
Eligible
3.168
3.161
−7
Eligible


K20
9540
Eligible
3.159
3.156
−3
Eligible









Step (3): grouping the batteries whose capacity differences are within 4% to assemble into the battery packs. Table.7 shows there are two groups of batteries allowed to be assembled into the 1P-8S battery packs, including a group containing K3, K4, K7, K9, K11, K13, K17 and K20 whose voltage differences are minimum and the SD levels have good consistency, and another group containing K1, K2, K6, K8, K10, K12, K16 and K18 whose voltage differences are close and the SD levels are closed;
Step (4): welding and assembling each of the battery packs after the eligible batteries have been grouped in Step (3), adding 8S protection boards to each of the battery packs to form a battery assembly. The 8S protection board has no balancing function, wherein the over-charge protection voltage thereof is 3.9V/section, and the over-discharge protection voltage thereof is 2V/section.
Verification
A first battery pack assembled according to the abovementioned steps is charged with a constant current of 4.5 A to a charge cut-off voltage of 29.2V, and is changed to be charged with a constant voltage of 29.2V to a charge cut-off current of 450 mA. Then, the first battery pack is discharged with a constant current of 4.5 A to a discharge cut-off voltage of 16V, totally releasing a capacity of 9360 mAh. The instantaneous discharge cut-off voltages of the batteries in the first battery pack have high consistency, as shown in Table.8.










TABLE 8








Discharge Cut-Off



Battery No.
Component Voltage (V)









K3
2.098



K4
2.089



K7
2.005



K9
2.069



K11
2.039



K13
2.071



K17
2.055



K20
2.048










Then, the first battery pack is charged and discharged with a current of 0.5 C5 for at least one cycle. The discharged capacities and the total discharge cut-off voltages are recorded, and the total discharge cut-off voltages are used to calculate the average discharge cut-off component voltages, as shown in Table.9.










TABLE 9






Discharge
Total discharge
Average Discharge


Number of
Capacity
cut-off
Cut-Off Component


Cycles
(mAh)
voltage (V)
Voltage (V)


















1
9360
16.47
2.059


2
9382
16.56
2.070


3
9375
16.57
2.071


4
9370
16.67
2.084


5
9368
16.66
2.083


6
9359
16.68
2.085


7
9350
16.51
2.064


8
9366
16.59
2.074


9
9352
16.57
2.071


10
9348
16.59
2.074


11
9356
16.63
2.079


12
9358
16.59
2.074


13
9344
16.65
2.081


14
9349
16.64
2.080


15
9357
16.71
2.089


16
9351
16.62
2.078


17
9346
16.78
2.098


18
9339
16.81
2.101


19
9350
16.75
2.094


20
9345
16.72
2.090









With the number of cycles increasing, the released capacities of the first battery pack remain stable, and the total discharge cut-off voltages as well as the average discharge cut-off component voltages have a good consistency.
Safety Test
Nails are used to pierce the batteries K14 and K19, which are not used to assemble the battery packs, to cause the internal short circuit of the batteries at a room temperature of 27° C. The short-circuit points are detected to respectively have maximum temperatures of 40° C. and 42° C.—separately 13° C. and 15° C. higher than the room temperature, and the batteries neither burn nor explode.

## Claims

1. A method for grouping lithium secondary battery packs comprising the steps of:
Step (1): charging and discharging batteries for 1-3 cycles, recording a last discharge capacity C0 of each of the batteries, setting a capacity lower limit, and determining the batteries with the C0 thereof not less than the capacity lower limit to be eligible batteries, wherein the capacity lower limit is a minimum eligible capacity defined by industry technology and normally equal to or higher than a rated capacity C5;
Step (2): discharging continuously the batteries with 0.01-0.5C5 to a discharge cut-off voltage Vd of 2.0-3.0V so as to discharge the batteries to a power empty state;
Step (3): charging the batteries, which have been discharged to the power empty state in Step (2), with 0.01-0.5C5 to a capacity C1, which is 0.1-10% of the rated capacity C5;
Step (4): storing the batteries processed in Step (3) for time t1 in an environment with a temperature ranging between 20-50° C., and measuring and recording a battery voltage V1 of each of the batteries after being stored; storing continuously the batteries for time t2 in the environment with the temperature ranging between 20-50° C., and measuring a battery voltage V2 of each of the batteries after being stored, calculating a voltage difference ΔV=V2-V1, and setting a range of ΔV to be from −20mV to 10mV, wherein t1 ranges from 1 to 48 hours and t2 ranges from 24-240 hours;
Step (5): grouping the eligible batteries in Step (4) into a plurality of battery packs including a specified number of series batteries according to a certain capacity grouping standard, and welding each of the battery packs to form a battery assembly, wherein a capacity difference of the batteries in one of the battery packs is not greater than 5% of the rated capacity C5.

2. The method of claim 1, wherein the batteries are charged with a constant current of 0.1-5 C5 to a charge cut-off voltage of 3.6-4.2V, and the batteries are changed to be charged with a constant voltage to a charge cut-off current of 0.01-0.5C5, and then the batteries are discharged with a constant current of 0.2-10 C5 to the discharge cut-off voltage Vd of 2-3V, wherein the capacity C1 is 2-5% of the rated capacity C5, and wherein the ΔV is measured at a room temperature and has an eligible range from −10mV to 5mV, and wherein the capacity difference of the batteries in one of the battery packs is not greater than 2% of the rated capacity C5.

3. A method for grouping lithium secondary battery packs, comprising the steps of:
Step (1): charging and discharging batteries for 1-3 cycles, recording a last discharge capacity C0 of each of the batteries, setting a capacity lower limit, and determining the batteries with the C0 thereof not less than the capacity lower limit to be eligible batteries, wherein the capacity lower limit is a minimum eligible capacity defined by industry technology and normally equal to or higher than a rated capacity C5;
Step (2): discharging continuously the batteries with 0.01-0.5 C5 to a discharge cut-off voltage Vd of 2.0-3.0V so as to discharge the batteries to a power empty state;
Step (3): storing the batteries processed in Step (2) for time t1 in an environment with a temperature ranging between 20-50° C., and measuring and recording a battery voltage V1 of each of the batteries after being stored; storing continuously the batteries for time t2 in the environment with the temperature ranging between 20-50° C., and measuring a battery voltage V2 of each of the batteries after being stored, calculating a voltage difference ΔV=V2-V1, and setting a range of ΔV to be from −20mV to 10mV, wherein t1 ranges from 1 to 48 hours and t2 ranges from 24-240 hours;
Step (4): grouping the eligible batteries in Step (3) into a plurality of battery packs each including a specified number of series batteries according to a certain capacity grouping standard, and welding each of the battery packs to form a battery assembly, wherein a capacity difference of the batteries in one of the battery packs is not greater than 5% of the rated capacity C5.

4. The method of claim 3, wherein the batteries are charged with a constant current of 0.1-5 C5 to a charge cut-off voltage of 3.6-4.2V, and the batteries are changed to be charged with a constant voltage to a charge cut-off current of 0.01-0.5 C5, and then the batteries are discharged with a constant current of 0.2-10 C5 to the discharge cut-off voltage Vd of 2-3V, wherein no batteries is charged after Step (2), and wherein the ΔV is measured at a room temperature and has an eligible range from -10mV to 5mV, and wherein the capacity difference of the batteries in one of the battery packs is not greater than 2% of the rated capacity C5.

5. A method for grouping lithium secondary battery packs, comprising the steps of:
Step (1): charging and discharging batteries for 2-3 cycles, recording a second last discharge capacity C0 of each of the batteries, setting a capacity lower limit, and determining the batteries with the C0 thereof not less than the capacity lower limit to be eligible batteries, wherein the capacity lower limit is a minimum eligible capacity defined by industry technology and normally equal to or higher than a rated capacity C5, and wherein a last discharge capacity is controlled to be 90-99.9% of the rated capacity C5 so that a remaining capacity of the batteries is 0.1-10% of the rated capacity C5 to make the batteries in a low-charged state;
Step (2): storing the batteries processed in Step (1) for time t1 in an environment with a temperature ranging between 20-50° C., and measuring and recording a battery voltage V1 of each of the batteries after being stored; storing continuously the batteries for time t2 in the environment with the temperature ranging between 20-50° C., and measuring a battery voltage V2 of each of the batteries after being stored, calculating a voltage difference AV=V2-V1, and setting a range of AV to be from −20mV to 10mV, wherein t1 ranges from 1 to 48 hours and t2 ranges from 24-240 hours;
Step (3): grouping the eligible batteries in Step (2) into a plurality of battery packs each including a specified number of series batteries according to a certain capacity grouping standard, and welding each of the battery packs to form a battery assembly, wherein a capacity difference of the batteries in one of the battery packs is not greater than 5% of the rated capacity C5.

6. The method of claim 5, wherein the last discharge releases 95-98% of the rated capacity C5 with only 2-5% of the rated capacity C5 remaining in the batteries, and wherein ΔV is measured at a room temperature and is eligible with a range from −10mV to 5mV, and wherein the capacity deviation of each battery of the battery packs is not greater than 2% of the rated capacity C5.