Discharge controller

1. A discharge controller comprising:

a secondary battery;

a switch section for opening/closing a discharge path from said secondary battery to a load;

a voltage detector for detecting a terminal voltage of said secondary battery; and

a cut-off voltage control section for controlling the switch section to be opened when the terminal voltage as detected by the voltage detector is not more than a predetermined cut-off voltage of discharge V E , while measuring an amount of decrease per unit time of the terminal voltage as detected by said voltage detector to set the cut-off voltage of discharge V E to a lower voltage, for a larger amount of decrease per unit time of the terminal voltage as detected by said voltage detector.

2. The discharge controller according to claim 1 , wherein:

said cut-off voltage control section measures the amount of decrease in the terminal voltage as an amount of change in voltage Vx in a predetermined measurement time t, and uses a value obtained by dividing the amount of change in voltage Vx by the measurement time t as the amount of decrease per unit time of the terminal voltage.

3. The discharge controller according to claim 2 , wherein:

said secondary battery is a nonaqueous-electrolyte secondary battery.

4. The discharge controller according to claim 3 , wherein:

said cut-off voltage control section sets the cut-off voltage of discharge V E from the following expression (1):

V E =β−α×( V x /t ) 2   (1)

in the expression (1), α falls in a range of :5≦α≦25, and β falls in a range of 1.5≦β≦3.0.

5. The discharge controller according to claim 3 , wherein:

said secondary battery is made up of assembled cells wherein plurality of cells are connected in series; and

said cut-off voltage control section sets the cut-off voltage of discharge V E from the following expression (2):

V E ={β−α×( V x /t ) 2 }×X   (2)

in the expression (2), X indicates a number of cells connected in series, α falls in a range of :5≦α≦25, and β falls in a range of 1.5≦β≦3.0.

6. The discharge controller according to claim 1 , wherein:

said cut-off voltage control section measures the amount of decrease per unit time of the terminal voltage when the terminal voltage as detected by said voltage detector falls in a range not higher than 3.0 V and not less than 2.0 V per unit cell of the secondary battery, and not more than a predetermined threshold voltage.

7. The discharge controller according to claim 1 , wherein:

said secondary battery is regulated with a positive electrode capacity such that the discharge ends by the decrease in potential of the positive electrode at the end of discharge.