Charging method for non-aqueous electrolyte secondary battery, and battery pack

1. A charging method for one or more non-aqueous electrolyte secondary batteries, the method comprising n steps of constant-current charging processes, said n steps comprising:

(1) charging the one or more secondary batteries at a current Ic(k) until a charge voltage per one battery reaches a voltage Ec(k), where k is an integer equal to or greater than 1 and equal to or smaller than n−1 where n≧2; and

(2) when the charge voltage per one battery reaches the voltage Ec(k), charging the one or more secondary batteries at a current Ic(k+1) smaller than the current Ic(k) until the charge voltage per one battery reaches a voltage Ec(k+1) higher than the voltage Ec(k), wherein

the current Ic(k) and the current Ic(k+1) are determined according to frequency of use or number of charge and discharge cycles of the one or more secondary batteries.

2. The charging method for one or more non-aqueous electrolyte secondary batteries according to claim 1 , the method further comprising:

after the charge voltage per one battery reaches the voltage Ec(k), charging the one or more secondary batteries at the voltage Ec(k) until a charge current decreases to the current Ic(k+1); and

after the charge voltage per one battery reaches the voltage Ec(k+1), charging the one or more secondary batteries at the voltage Ec(k+1) until the charge current decreases to a current Ic(k+2).

3. The charging method for one or more non-aqueous electrolyte secondary batteries according to claim 1 , wherein the current Ic(k) and the current Ic(k+1) are decreased every time when the frequency of use or the number of charge and discharge cycles is increased by a certain number.

4. The charging method for one or more non-aqueous electrolyte secondary batteries according to claim 3 , wherein the current Ic(k) and the current Ic(k+1) are decreased at a common rate every time when the frequency of use or the number of charge and discharge cycles is increased by a certain number, and the rate is lowered gradually as the frequency of use or the number of charge and discharge cycles is increased.

5. The charging method for one or more non-aqueous electrolyte secondary batteries according to claim 1 , wherein when the frequency of use or the number of charge and discharge cycles reaches a predetermined number, the current Ic(k) and the current Ic(k+1) are each decreased to be smaller than initial values of the current Ic(k) and the current Ic(k+1).

6. The charging method for one or more non-aqueous electrolyte secondary batteries according to claim 1 , wherein a ratio Ic(k+1)/Ic(k) of the current Ic(k+1) to the current Ic(k) is equal to or greater than 0.1 and equal to or smaller than 0.75.

7. The charging method for one or more non-aqueous electrolyte secondary batteries according to claim 1 , wherein:

the one or more secondary batteries comprise a positive electrode, a negative electrode, and a non-aqueous electrolyte; and

the positive electrode includes a material represented by the general formula: LiNi x Co y M 1-x-y O 2 , where M is at least one element selected from the group consisting of group 2 elements, group 3 elements, group 4 elements, group 7 elements, and group 13 elements in the long form of the periodic table, 0.3≦x≦1, and 0<y<0.4.

8. The charging method for one or more non-aqueous electrolyte secondary batteries according to claim 7 , wherein:

when the number of total steps of the constant-current charging processes is “f”, the current Ic(f) is equal to or above 0.3 It and equal to or below 0.7 It, and the voltage Ec(f) is above 4 V and equal to or below 4.4 V, where It=a rated capacity/X, X being a length of time in which a quantity of electricity corresponding to the rated capacity is charged or discharged.

9. The charging method for one or more non-aqueous electrolyte secondary batteries according to claim 1 , wherein:

when k=1, the current Ic(k) is equal to or above 0.7 It and equal to or below 2 It, and the voltage Ec(k) is equal to or above 3.8 V and equal to or below 4 V, where It=a rated capacity/X, X being a length of time in which a quantity of electricity corresponding to the rated capacity is charged or discharged.

10. A battery pack comprising:

one or more non-aqueous electrolyte secondary batteries;

a charge circuit configured to charge the one or more secondary batteries with electric power from an outer power source; and

a controller configured to control the charge circuit, wherein:

the controller controls the charge circuit such that the one or more secondary batteries are charged by n steps of constant-current charging processes, the n steps comprising:

(1) charging the one or more secondary batteries at a current Ic(k) until a charge voltage per one battery reaches a voltage Ec(k), where k is an integer equal to or greater than 1 and equal to or smaller than n−1 where n≧2; and

(2) when the charge voltage per one battery reaches the voltage Ec(k), charging the one or more secondary batteries at a current Ic(k+1) smaller than the current Ic(k) until the charge voltage per one battery reaches a voltage Ec(k+1) higher than the voltage Ec(k), and

the controller determines the current Ic(k) and the current Ic(k+1) according to frequency of use or number of charge and discharge cycles of the one or more secondary batteries.

11. The battery pack of claim 10 , wherein the controller is further controller configured to control the charge circuit to:

after the charge voltage per one battery reaches the voltage Ec(k), charge the one or more secondary batteries at the voltage Ec(k) until a charge current decreases to the current Ic(k+1); and

after the charge voltage per one battery reaches the voltage Ec(k+1), charge the one or more secondary batteries at the voltage Ec(k+1) until the charge current decreases to a current Ic(k+2).

12. The battery pack of claim 10 , wherein the controller decreases the current Ic(k) and the current Ic(k+1) every time when the frequency of use or the number of charge and discharge cycles is increased by a certain number.

13. The battery pack of claim 12 , wherein the controller decreases the current Ic(k) and the current Ic(k+1) at a common rate every time when the frequency of use or the number of charge and discharge cycles is increased by a certain number, and lowers the rate gradually as the frequency of use or the number of charge and discharge cycles is increased.

14. The battery pack of claim 10 , wherein when the frequency of use or the number of charge and discharge cycles reaches a predetermined number, the controller decreases the current Ic(k) and the current Ic(k+1) decreased to be smaller than initial values of the current Ic(k) and the current Ic(k+1), respectively.

15. The battery pack of claim 10 , wherein a ratio Ic(k+1)/Ic(k) of the current Ic(k+1) to the current Ic(k) is equal to or greater than 0.1 and equal to or smaller than 0.75.

16. The battery pack of claim 10 , wherein:

when k=1, the current Ic(k) is equal to or above 0.7 It and equal to or below 2 It, and the voltage Ec(k) is equal to or above 3.8 V and equal to or below 4 V, where It=a rated capacity/X, X being a length of time in which a quantity of electricity corresponding to the rated capacity is charged or discharged.

17. The battery pack of claim 10 , wherein:

when the number of total steps of the constant-current charging processes is “f”, the current Ic(f) is equal to or above 0.3 It and equal to or below 0.7 It, and the voltage Ec(f) is above 4 V and equal to or below 4.4 V, where It=a rated capacity/X, X being a length of time in which a quantity of electricity corresponding to the rated capacity is charged or discharged.

18. The battery pack of claim 10 , further comprising:

a charge/discharge cycle counter that counts the number of charge and discharge cycles.