Capacity estimation method, degradation estimation method and degradation estimation apparatus for lithium-ion cells, and lithium-ion batteries

• US 20010022518A1
• Filed: 03/12/2001
• Published: 09/20/2001
• Est. Priority Date: 03/13/2000
• Status: Active Grant

• Current Assignee: Nippon Telegraph and Telephone Corporation
• Original Assignee: Nippon Telegraph and Telephone Corporation
• Inventors: Asakura, Kaoru, Hirai, Toshiro
• Granted Patent: US 6,586,940 B2
• Time in Patent Office: Days
• US Class Current: 324/426
• CPC Class Codes: G01R 31/3842 combining voltage and current measurements
• CPC Class Codes: G01R 31/392 Determining battery ageing or deterioration, e.g. state of health
• CPC Class Codes: G01R 31/396 Acquisition or processing of data for testing or for monitoring individual cells or groups of cells within a battery
• CPC Class Codes: H01M 10/48 Accumulators combined with arrangements for measuring, testing or indicating the condition of cells, e.g. the level or density of the electrolyte constructional details of current conducting connections for detecting conditions inside cells or batteries, e.g. details of voltage sensing terminals, H01M50/569
• CPC Class Codes: H02M 3/07 using capacitors charged and discharged alternately by semiconductor devices with control electrode , e.g. charge pumps
• CPC Class Codes: Y02E 60/10 Energy storage using batteries

Abstract

A capacity estimation method for a Li-ion cell is provided. In the method, according to a first aspect, an elapsed time from the time when charge voltage in constant current charge reaches a predetermined voltage to the time when charge condition is changed to a constant voltage mode is used for calculating an estimated capacity of said Li-ion cell. According to a second aspect, a charge current after a lapse of a predetermined time from the time when charge condition is changed to a constant voltage mode is used. According to a third aspect an elapsed time from the time when charge condition is changed to a constant voltage mode to the time when charge current becomes α (0<α<1) times is used.

Claims

1. A capacity estimation method for a Li-ion cell, comprising the steps of:

obtaining, when said Li-ion cell is charged by a constant current and constant voltage method, an elapsed time t from the time when charge voltage in constant current charge reaches a predetermined voltage Vs to the time when charge condition is changed from a constant current mode to a constant voltage mode; and

calculating an estimated capacity Ce of said Li-ion cell by using said elapsed time t.

2. The capacity estimation method for said Li-ion cell as claimed in

claim 1

, further comprising the steps of:

obtaining said elapsed time t on condition that charge current in said constant current charge is from Co/(30 hours) to Co/(1 hour) wherein Co is a nominal capacity of said Li-ion cell; and

calculating said estimated capacity Ce by using an equation Ce/Co=A×

t^B and said elapsed time t wherein A and B are positive constants.

3. The capacity estimation method for said Li-ion cell as claimed in

claim 2

, further comprising the steps of:

repeating a charge and discharge cycle at least equal to or more than two times by using a Li-ion cell which is a capacity estimation target or the same kind of Li-ion cell as said Li-ion cell, wherein said charge and discharge cycle includes a charge period from 3 hours to 10 days, a discharge period, a rest period which is provided as necessary between said charge period and said discharge period;

obtaining an elapsed time tn from the time when charge voltage in constant current charge reaches a predetermined voltage Vs to the time when charge condition is changed from a constant current mode to a constant voltage mode and discharge capacity Cn in each cycle; and

determining values of said constants A and B from said elapsed time tn and said discharge capacity Cn.

4. A degradation estimation method for a Li-ion cell, comprising the steps of:

obtaining, when said Li-ion cell is charged by a constant current and constant voltage method, an elapsed time t from the time when charge voltage in constant current charge reaches a predetermined voltage Vs to the time when charge condition is changed from a constant current mode to a constant voltage mode;

calculating an estimated capacity Ce of said Li-ion cell by using said elapsed time t; and

judging that said Li-ion cell is degraded when said estimated capacity Ce becomes smaller than a predetermined limit capacity Cmin.

5. The degradation estimation method for said Li-ion cell as claimed in

claim 4

, further comprising the step of:

calculating said estimated capacity Ce by using an equation Ce/Co=A×

t^B wherein A and B are positive constants.

14. The degradation estimation method for said Li-ion cell as claimed in

claim 4

, wherein said estimated capacity Ce is calculated by using an equation Ce/Co=At+B wherein A and B are positive constants.

6. A degradation estimation apparatus for a Li-ion cell, comprising:

means for obtaining, when said Li-ion cell is charged by a constant current and constant voltage method, an elapsed time t from the time when charge voltage in constant current charge reaches a predetermined voltage Vs to the time when charge condition is changed from a constant current mode to a constant voltage mode; and

a circuit calculating an estimated capacity Ce of said Li-ion cell by using said elapsed time t; and

means for outputting a signal representing that said Li-ion cell is degraded when said estimated capacity Ce becomes smaller than a predetermined limit capacity Cmin.

7. The degradation estimation apparatus for said Li-ion cell as claimed in

claim 6

, wherein said estimated capacity Ce is calculated by using an equation Ce/Co=A×

t^B wherein A and B are positive constants.

8. A Li-ion battery which includes a charge and discharge control part, comprising:

a memory storing numerical data used for calculating an estimated capacity Ce of Li-ion cells in said Li-ion battery;

means for obtaining, when said Li-ion cell is charged by a constant current and constant voltage method, an elapsed time t from the time when charge voltage in constant current charge reaches a predetermined voltage Vs to the time when charge condition is changed from a constant current mode to a constant voltage mode; and

a circuit for calculating said estimated capacity Ce of said Li-ion cells by using said elapsed time t and said numerical data.

9. The Li-ion battery as claimed in

claim 8

, wherein said estimated capacity Ce is calculated by using an equation Ce/Co=A×

t^B wherein A and B are positive constants.

10. The Li-ion battery as claimed in

claim 9

, further comprising:

means for outputting a warning representing that said Li-ion battery is degraded when said estimated capacity Ce becomes smaller than a predetermined limit capacity Cmin.

11. The Li-ion battery as claimed in

claim 10

, wherein said warning is any one of a warning by an electrical signal, a warning by displaying characters or an image and a warning by a beep or voice.

12. A capacity estimation method for a Li-ion cell, comprising the steps of:

obtaining, when said Li-ion cell is charged by a constant current and constant voltage method, an elapsed time t from the time when charge voltage in constant current charge reaches a predetermined voltage Vs to the time when the charge voltage reaches a charge limit voltage Vc on condition that charge current is equal to or smaller than Co/(20 hours) wherein Co is a nominal capacity of said Li-ion cell; and

calculating an estimated specific capacity Ce/Co of said Li-ion cell by using an equation Ce/Co=At+B, wherein Ce is an estimated capacity of said Li-ion cell, and A and B are positive constants.

13. The capacity estimation method for said Li-ion cell as claimed in

claim 12

, further comprising the steps of:

repeating a charge and discharge cycle at least equal to or more than three times by using the same kind of Li-ion cell as a Li-ion cell which is a target of capacity estimation, wherein said charge and discharge cycle includes a charge period from 3 days to 10 days by said constant current and constant voltage method, a discharge period, a rest period which is provided as necessary between said charge period and said discharge period, wherein charge current in said charge period is equal to or smaller than Co/(20 hours), discharge current in said discharge period is from Cot(5 hours) to Co/(0.5 hour) and a predetermined discharge cut-off voltage Vd is used in said discharge period;

obtaining an elapsed time tn from the time when charge voltage in constant current charge reaches a predetermined voltage Vs to the time when the charge voltage reaches a charge limit voltage Vc and discharge capacity Cn in each cycle; and

determining values of said constants A and B from said elapsed time tn and said discharge capacity Cn.

15. A capacity estimation apparatus for a Li-ion cell, comprising:

means for obtaining, when said Li-ion cell is charged by a constant current and constant voltage method, an elapsed time t from the time when charge voltage in constant current charge reaches a predetermined voltage Vs to the time when the charge voltage reaches a charge limit voltage Vc; and

a circuit calculating an estimated specific capacity Ce/Co of said Li-ion cell by using an equation Ce/Co=At+B, wherein Ce is an estimated capacity of said Li-ion cell, Co is a nominal capacity of said Li-ion cell, and A and B are positive constants;

a circuit estimating degradation state of said Li-ion cell on the basis of said estimated specific capacity Ce/Co; and

means for displaying estimation result obtained by said circuit for estimating, or, generating an alarm which warns of degradation of said Li-ion cell.

16. A Li-ion battery which includes a charge and discharge control part, comprising:

a memory storing numerical data used for calculating an estimated specific capacity Ce/Co of Li-ion cells in said Li-ion battery, wherein Ce is an estimated capacity of said Li-ion cells and Co is a nominal capacity of said Li-ion cells;

means for obtaining an elapsed time t from the time when charge voltage in constant current charge reaches a predetermined voltage Vs to the time when the charge voltage reaches a charge limit voltage Vc when said Li-ion cell is charged by a constant current and constant voltage method; and

a circuit calculating said estimated specific capacity Ce/Co of said Li-ion cell by using an equation Ce/Co=At+B and said numerical data, wherein A and B are positive constants;

means for outputting said estimated specific capacity Ce/Co.

17. A capacity estimation method for a Li-ion cell, comprising the steps of:

obtaining, when said Li-ion cell is charged by a constant current and constant voltage method, a charge current after a lapse of a predetermined time from the time when charge condition is changed from a constant current mode to a constant voltage mode; and

calculating an estimated capacity of said Li-ion cell by using said charge current.

18. A capacity estimation method for a Li-ion cell, comprising the steps of:

obtaining, when said Li-ion cell is charged by a constant current and constant voltage method, an elapsed time t from the time when charge condition is changed from a constant current mode to a constant voltage mode to the time when charge current becomes α

(0<

α

<

1) times of charge current Ic in said constant current mode; and

calculating an estimated capacity of said Li-ion cell by using said elapsed time t.

19. A capacity estimation method for a Li-ion cell, comprising the steps of:

repeating a charge and discharge cycle a plurality of times, wherein said charge and discharge cycle includes a charge period in which said Li-ion cell is charge by a constant current and constant voltage method and a discharge period in which said charged Li-ion cell is discharged;

obtaining, when said Li-ion cell is charged by a constant current and constant voltage method, an elapsed time t_α

from the time when charge condition is changed from a constant current mode to a constant voltage mode to the time when charge current becomes α

(0<

α

<

1) times of charge current Ic0 in said constant current mode;

obtaining an estimated specific capacity Ce/Co by using a first equation Ce/Co=−

At_α

+B when said elapsed time t_α

increases or remains unchanged as the number of repetition of said cycle increases, wherein Ce is a specific capacity, Co is a nominal capacity and A and B are positive constants;

obtaining said estimated specific capacity Ce/Co by using a second equation Ce/Co=+At_α

+B when said elapsed time t_α

decreases as the number of repetition of said cycle increases, wherein B′

is a positive constant; and

obtaining said estimated capacity Ce by using said estimated specific capacity Ce/Co and said nominal capacity Co.

20. The capacity estimation method for said Li-ion cell as claimed in

claim 19

, wherein α

is ½

.

21. The capacity estimation method for said Li-ion cell as claimed in

claim 19

, comprising the steps of:

repeating a charge and discharge cycle equal to or more than 5 times or 6 times which is more favorable, wherein said charge and discharge cycle includes a charge period in which the same kind of Li-ion cell- as a Li-ion cell which is a target of capacity estimation is charged by said constant current and constant voltage method and a discharge period in which said Li-ion cell is discharged until voltage of said Li-ion cell reaches a discharge cut-off voltage Vd, wherein charge period per one cycle is from 3 hours to 10 days or from 3 days to 10 days which is more favorable, discharge current is from 0.2 CmA to 2.0 CmA in which 1 CmA is a value obtained by dividing Co by one hour, ambient temperature is from 0°

C. to 45°

C. or from 15°

C. to 30°

C. which is more favorable;

obtaining said elapsed time t_α

and capacity of said Li-ion cell C by integrating said discharge current with respect to time in each cycle;

calculating a specific capacity C/Co in each cycle;

recording t_α

and C/Co obtained in nth cycle (n is a positive integer) as functions of n t_α

(n) and Cr(n);

determining said constants A, B and B′

such that said first equation and said second equation approximately represent correlation between t_α

(n) and Cr(n) when assuming that t_α

=t_α

(n) and Ce/Co=Cr(n); and

estimating capacity of said Li-ion cell which is a target of capacity estimation by using equations obtained by substituting determined values of said constants A, B and B′

into said first equation and said second equation.

22. A capacity estimation method for a Li-ion cell, comprising the steps of:

repeating a charge and discharge cycle a plurality of times, wherein said charge and discharge cycle includes a charge period in which said Li-ion cell is charged by a constant current and constant voltage method and a discharge period in which said charged Li-ion cell is discharged;

obtaining a charge current I_tm after a lapse of a predetermined time tm from the time when charge condition is changed from a constant current mode to a constant voltage mode, wherein 0<

tm≦

0.8 Co/Ic0 is satisfied, Co is a nominal capacity of said Li-ion cell and Ic0 is charge current in said constant current mode;

obtaining an estimated specific capacity Ce/Co by using a first equation Ce/Co=−

MI_tm+N when said charge current I_tm increases or remains unchanged as the number of repetition of said cycle increases, wherein Ce is a specific capacity of said Li-ion cell and M and N are positive constants;

obtaining said estimated specific capacity Ce/Co by using a second equation Ce/Co=+MI_tm+N′

when said charge current I_tm decreases as the number of repetition of said cycle increases, wherein N′

is a positive constant; and

obtaining said estimated capacity Ce by using said estimated specific capacity Ce/Co and said nominal capacity Co.

23. The capacity estimation method for said Li-ion cell as claimed in

claim 22

, said predetermined time tm is from 1 minute to 15 minutes.

24. The capacity estimation method for said Li-ion cell as claimed in

claim 22

, comprising the steps of:

repeating a charge and discharge cycle equal to or more than 5 times, wherein said charge and discharge cycle includes a charge period in which the same kind of Li-ion cell as said Li-ion cell which is a target of capacity estimation is charged by said constant current and constant voltage method and a discharge period in which said charged Li-ion cell is discharged until voltage of said Li-ion cell reaches a discharge cut-off voltage Vd, wherein charge period per one cycle is from 3 hours to 10 days, discharge current is from 0.2 CmA to 2.0 CmA in which 1 CmA is a value obtained by dividing Co by one hour, ambient temperature is from 0°

C. to 45°

C. or from 15°

C. to 30°

C. which is more favorable;

obtaining said charge current I_tm and capacity of said Li-ion cell C by integrating said discharge current with respect to time in each cycle;

calculating a specific capacity C/Co in each cycle;

recording I_tm and C/Co obtained in nth cycle (n is a positive integer) as functions of n which are I_tm(n) and Cr(n);

determining said constants M, N and N′

such that said first equation and said second equation approximately represent correlation between I_tm(n) and Cr(n) when assuming that I_tm=I_tm(n) and Ce/Co=Cr(n); and

estimating capacity of said Li-ion cell which is a target of capacity estimation by using equations obtained by substituting determined values of said constants M, N and N′

into said first equation and said second equation.

25. A degradation estimation method for a Li-ion cell, comprising the steps of:

repeating a charge and discharge cycle a plurality of times, wherein said charge and discharge cycle includes a charge period in which said Li-ion cell is charge by a constant current and constant voltage method and a discharge period in which said charged Li-ion cell is discharged;

obtaining, when said Li-ion cell is charged by a constant current and constant voltage method, an elapsed time t_α

from the time when charge condition is changed from a constant current mode to a constant voltage mode to the time when charge current becomes α

(0<

α

<

1) times of charge current Ic0 in said constant current mode;

obtaining an estimated specific capacity Ce/Co by using a first equation Ce/Co=−

At_α

+B when said elapsed time t_α

increases or remains unchanged as the number of repetition of said cycle increases, wherein Ce is a specific capacity, Co is a nominal capacity and A and B are positive constants;

obtaining said estimated specific capacity Ce/Co by using a second equation Ce/Co=+At_α

+B when said elapsed time t_α

decreases as the number of repetition of said cycle increases, wherein B′

is a positive constant; and

obtaining said estimated capacity Ce by using said estimated specific capacity Ce/Co and said nominal capacity Co; and

judging that said Li-ion cell is degraded when said estimated capacity Ce becomes smaller than a predetermined limit capacity.

26. A degradation estimation method for a Li-ion cell, comprising the steps of:

repeating a charge and discharge cycle a plurality of times, wherein said charge and discharge cycle includes a charge period in which said Li-ion cell is charged by a constant current and constant voltage method and a discharge period in which said charged Li-ion cell is discharged;

obtaining a charge current I_tm after a lapse of a predetermined time tm from the time when charge condition is changed from a constant current mode to a constant voltage mode, wherein 0<

tm≦

0.8Co/Ic0 is satisfied, Co is a nominal capacity of said Li-ion cell and Ic0 is charge current in said constant current mode;

obtaining an estimated specific capacity Ce/Co by using a first equation Ce/Co=-MI_tm+N when said charge current I_tm increases or remains unchanged as the number of repetition of said cycle increases, wherein Ce is a specific capacity of said Li-ion cell and M and N are positive constants;

obtaining said estimated specific capacity Ce/Co by using a second equation Ce/Co=+MI_tm+N′

when said charge current I_tm decreases as the number of repetition of said cycle increases, wherein N′

is a positive constant; and

obtaining said estimated capacity Ce by using said estimated specific capacity Ce/Co and said nominal capacity Co.

judging that said Li-ion cell is degraded when said estimated capacity Ce becomes smaller than a predetermined limit capacity.

27. A degradation estimation apparatus for a Li-ion cell, comprising:

a cell control part comprising;

means for repeating a charge and discharge cycle a plurality of times, wherein said charge and discharge cycle includes a charge period in which said Li-ion cell is charge by a constant current and constant voltage method and a discharge period in which said charged Li-ion cell is discharged;

means for obtaining, when said Li-ion cell is charged by a constant current and constant voltage method, an elapsed time t_α

from the time when charge condition is changed from a constant current mode to a constant voltage mode to the time when charge current becomes α

(0<

α

<

1) times of charge current Ic0 in said constant current mode;

a memory storing said elapsed time t_α

as a function t_α

(n) of n in the nth cycle for each cycle;

a logic circuit comprising;

means for obtaining an estimated specific capacity Ce/Co by using a first equation Ce/Co=-At_α

+B when said elapsed time t_α

increases or remains unchanged as the number of repetition of said cycle increases, wherein Ce is a specific capacity, Co is a nominal capacity and A and B are positive constants;

means for obtaining said estimated specific capacity Ce/Co by using a second equation Ce/Co=+At_α

+B when said elapsed time t_α

decreases as the number of repetition of said cycle increases, wherein B′

is a positive constant; and

means for obtaining said estimated capacity Ce by using said estimated specific capacity Ce/Co and said nominal capacity Co; and

means for judging that said Li-ion cell is degraded when said estimated capacity Ce becomes smaller than a predetermined limit capacity.

28. A degradation estimation apparatus for a Li-ion cell, comprising:

a cell control part comprising;

means for repeating a charge and discharge cycle a plurality of times, wherein said charge and discharge cycle includes a charge period in which said Li-ion cell is charged by a constant current and constant voltage method and a discharge period in which said charged Li-ion cell is discharged;

means for obtaining a charge current I_tm after a lapse of a predetermined time tm from the time when charge condition is changed from a constant current mode to a constant voltage mode, wherein 0<

tm≦

0.8 Co/Ic0 is satisfied, Co is a nominal capacity of said Li-ion cell and Ic0 is charge current in said constant current mode;

a memory storing said charge current I_tm as a function I_tm(n) of n in the nth cycle for each cycle;

a logic circuit comprising;

means for obtaining an estimated specific capacity Ce/Co by using a first equation Ce/Co=−

MI_tm+N when said charge current I_tm increases or remains unchanged as the number of repetition of said cycle increases, wherein Ce is a specific capacity of said Li-ion cell and M and N are positive constants;

means for obtaining said estimated specific capacity Ce/Co by using a second equation Ce/Co=+MI_tm+N′

when said charge current I_tm decreases as the number of repetition of said cycle increases, wherein N′

is a positive constant; and

means for obtaining said estimated capacity Ce by using said estimated specific capacity Ce/Co and said nominal capacity Co.

means for judging that said Li-ion cell is degraded when said estimated capacity Ce becomes smaller than a predetermined limit capacity.

29. A Li-ion battery which includes a degradation estimation function, comprising:

a measuring part, when a charge and discharge cycle is repeated a plurality of times, obtaining an elapsed time t_α

from the time when charge condition is changed from a constant current mode to a constant voltage mode to the time when charge current becomes α

(0<

α

<

1) times of charge current Ic0 in said constant current mode, wherein said charge and discharge cycle includes a charge period in which said Li-ion cell is charged by a constant current and constant voltage method and a discharge period in which said charged Li-ion cell is discharged;

a memory storing said elapsed time t_α

as a function t_α

(n) of n in the nth cycle for each cycle;

a logic circuit comprising;

means for obtaining an estimated specific capacity Ce/Co by using a first equation Ce/Co=-At_α

+B when said elapsed time t_α

increases or remains unchanged as the number of repetition of said cycle increases, wherein Ce is a specific capacity, Co is a nominal capacity and A and B are positive constants;

means for obtaining said estimated specific capacity Ce/Co by using a second equation Ce/Co=+At_α

+B′

when said elapsed time t_α

decreases as the number of repetition of said cycle increases, wherein B′

is a positive constant; and

means for obtaining said estimated capacity Ce by using said estimated specific capacity Ce/Co and said nominal capacity Co; and

means for judging that said Li-ion cell is degraded when said estimated capacity Ce becomes smaller than a predetermined limit capacity.

30. A Li-ion battery which includes a degradation estimation function, comprising:

a measuring part, when a charge and discharge cycle is repeated a plurality of times, obtaining a charge current I_tm after a lapse of a predetermined time tm from the time when charge condition is changed from a constant current mode to a constant voltage mode, wherein 0<

tm≦

0.8 Co/Ic0 is satisfied, Co is a nominal capacity of said Li-ion cell and Ic0 is charge current in said constant current mode, wherein said charge and discharge cycle includes a charge period in which said Li-ion cell is charged by a constant current and constant voltage method and a discharge period in which said charged Li-ion cell is discharged;

a memory storing said charge current I_tm as a function I_tm(n) of n in the nth cycle for each cycle;

a logic circuit comprising;

means for obtaining an estimated specific capacity Ce/Co by using a first equation Ce/Co=−

MI_tm+N when said charge current I_tm increases or remains unchanged as the number of repetition of said cycle increases, wherein Ce is a specific capacity of said Li-ion cell and M and N are positive constants;

means for obtaining said estimated specific capacity Ce/Co by using a second equation Ce/Co=+MI_tm+N′

when said charge current I_tm decreases as the number of repetition of said cycle increases, wherein N′

is a positive constant; and

means for obtaining said estimated capacity Ce by using said estimated specific capacity Ce/Co and said nominal capacity Co.

means for judging that said Li-ion cell is degraded when said estimated capacity Ce becomes smaller than a predetermined limit capacity.

31. A capacity estimation method for a Li-ion cell, comprising the steps of:

obtaining, when said Li-ion cell is charged by a constant current and constant voltage method, a charge current It after a lapse of a time t which is between 30 seconds and 30 minutes from the time when charge condition is changed from a constant current mode to a constant voltage mode; and

calculating an estimated capacity Ce of said Li-ion cell by using said charge current It and a first equation Ce/Co=−

a×

It+b, wherein Co is a nominal capacity of said Li-ion cell, a and b are positive constants.

32. The capacity estimation method for said Li-ion cell as claimed in

claim 31

, wherein said constants a and b are calculated by a second equation a=d×

t^e and a third equation b=f×

t^h, wherein d, e, f, and h are constants and 0<

d, e<

0, 0<

f, and h<

0 are satisfied.

33. The capacity estimation method for said Li-ion cell as claimed in

claim 32

, further comprising the steps of:

repeating a charge and discharge cycle at least equal to or more than 5 times by using a Li-ion cell which is a capacity estimation target or the same kind of Li-ion cell as said Li-ion cell, wherein said charge and discharge cycle includes a charge period from 3 hours to one month in which charge is performed by a constant current and constant voltage method, a discharge period, a rest period which is provided as necessary between said charge period and said discharge period;

obtaining, in each cycle, charge currents It_ni after a lapse of times t_ni which are between 30 seconds and 30 minutes from the time when charge condition is changed from a constant current mode to a constant voltage mode, wherein n is a number which is assigned to each cycle and i is a number assigned to each time t_ni in each cycle;

obtaining discharge capacity Cn in each cycle;

determining said constants a and b in said first equation and d, e, f, and h in said first and second equations by using said time t_ni, said charge current It_ni and said discharge capacity Cn.

34. A Li-ion battery comprising:

a memory storing numeric data used for calculating an estimated capacity of said Li-ion battery; and

a circuit for calculating said estimated capacity by using said numeric data and charge current while charging said Li-ion battery.

35. The Li-ion battery as claimed in

claim 34

, further comprising:

means for outputting a warning representing that said Li-ion cell is degraded when said estimated capacity becomes smaller than a predetermined limit capacity.

36. The Li-ion battery as claimed in

claim 35

, wherein said warning is any one of a warning by an electrical signal, a warning by displaying characters or an image, and a warning by a beep or voice.

37. A capacity estimation method for a Li-ion cell, comprising the steps of:

obtaining, when said Li-ion cell is charged by a constant current and constant voltage method, an elapsed time t from the time when charge condition is changed from a constant current mode to a constant voltage mode to the time when charge current becomes α

(0<

α

<

1) times of charge current in said constant current mode;

obtaining an estimated capacity Ce of said Li-ion cell by using an equation Ce/Co=At+B and said elapsed time t, wherein Co is a nominal capacity of said Li-ion cell and A and B are positive constants.

38. The capacity estimation method for said Li-ion cell as claimed in

claim 37

, wherein α

is ½

.

39. The capacity estimation method for said Li-ion cell as claimed in

claim 37

, further comprising steps of:

repeating a charge and discharge cycle at least equal to or more than 5 times by using a Li-ion cell which is a capacity estimation target or the same kind of Li-ion cell as said Li-ion cell, wherein said charge and discharge cycle includes a charge period from 3 hours to 10 days in which charge is performed by said constant current and constant voltage method and charge current in said constant current mode is from Co/5 hours to Co/0.5 hour, a discharge period, a rest period which is provided as necessary between said charge period and said discharge period;

obtaining an elapsed time tn from the time when charge condition is changed from a constant current mode to a constant voltage mode to the time when charge current becomes α

(0<

α

<

1) times of charge current in said constant current mode and discharge capacity Cn in each cycle; and

determining values of said constants A and B from said elapsed time tn and said discharge capacity Cn.

40. The capacity estimation method for said Li-ion cell as claimed in

claim 39

, wherein α

is ½

.

41. A degradation estimation method for a Li-ion cell, comprising the steps of:

obtaining, when said Li-ion cell is charged by a constant current and constant voltage method, an elapsed time t from the time when charge condition is changed from a constant current mode to a constant voltage mode to the time when charge current becomes α

(0<

α

<

1) times of charge current in said constant current mode;

obtaining an estimated capacity Ce of said Li-ion cell by using an equation Ce/Co=At+B and said elapsed time t, wherein Co is a nominal capacity of said Li-ion cell and A and B are positive constants; and

judging that said Li-ion cell is degraded when said estimated capacity Ce becomes smaller than a predetermined limit capacity Cmin.

42. The capacity estimation method for said Li-ion cell as claimed in

claim 41

, wherein α

is ½

.

43. A degradation estimation apparatus for a Li-ion cell, comprising:

means for obtaining, when said Li-ion cell is charged by a constant current and constant voltage method, an elapsed time t from the time when charge condition is changed from a constant current mode to a constant voltage mode to the time when charge current becomes α

(0<

α

<

1) times of charge current in said constant current mode;

a circuit obtaining an estimated capacity Ce of said Li-ion cell by using said elapsed time t; and

means for outputting a signal representing that said Li-ion cell is degraded when said estimated capacity Ce becomes smaller than a predetermined limit capacity Cmin.

44. The capacity estimation apparatus for said Li-ion cell as claimed in

claim 43

, wherein α

is ½

, and said circuit obtains said estimated capacity Ce of said Li-ion cell by using an equation Ce/Co=At+B and said elapsed time t, wherein Co is a nominal capacity of said Li-ion cell and A and B are positive constants.

45. A Li-ion battery which includes a charge and discharge control part, comprising:

a memory storing numerical data used for calculating an estimated capacity Ce of Li-ion cells in said Li-ion battery;

means for obtaining, when said Li-ion cell is charged by a constant current and constant voltage method, an elapsed time t from the time when charge condition is changed from a constant current mode to a constant voltage mode to the time when charge current becomes α

(0<

α

<

1) times of charge current in said constant current mode;

a circuit obtaining an estimated capacity Ce of said Li-ion cell by using said elapsed time t.

46. The Li-ion battery for said Li-ion cell as claimed in

claim 45

, wherein α

is ½

, and said circuit obtains said estimated capacity Ce of said Li-ion cell by using an equation Ce/Co=At+B and said elapsed time t, wherein Co is a nominal capacity of said Li-ion cell and A and B are positive constants.

47. The Li-ion battery as claimed in

claim 45

, further comprising:

means for outputting a warning representing that said Li-ion cell is degraded when said estimated capacity Ce becomes smaller than a predetermined limit capacity Cmin.

48. The Li-ion battery as claimed in

claim 47

, wherein said warning is any one of a warning by an electrical signal, a warning by displaying characters or an image, and a warning by a beep or voice.