Patent ID: 8610405

Claim:
A charge and discharge circuit of a secondary battery, the charge and discharge circuit being capable of charging the secondary battery by a power source voltage and of supplying power from the secondary battery to an external device, the charge and discharge circuit comprising: a bidirectional regulator capable of adjusting a current or a voltage in both of a charge direction of feeding the current to a secondary battery side and a discharge direction of feeding the current from the secondary battery to an external device side; a charge and discharge detecting circuit for detecting a direction of the current flowing in the secondary battery; a switching control circuit for switching an operation direction of the bidirectional regulator into the charge direction or the discharge direction based on a detection result of the charge and discharge detecting circuit; and a battery voltage detection circuit for detecting a battery voltage of the secondary battery; wherein the charge and discharge detecting circuit includes: one or a plurality of field-effect transistors serially connected on a current pathway on which a charge current and a discharge current of the secondary battery flow; a section for applying bias voltages to gate terminals so that both end voltages of the field-effect transistors become predetermined voltages; and a comparison circuit for mutually comparing both the end voltages of the field-effect transistors; wherein the section for applying bias voltages makes resistances between sources and drains of the field-effect transistors small when current flowing through the field-effect transistors is large, and makes the resistances large when the current is small; wherein when the battery voltage detection circuit detects that the battery voltage of the second battery is lower than a first threshold voltage which is higher than a lowest operation voltage of the external device during discharge from the secondary battery to the external device, the bidirectional regulator starts a voltage raising operation to perform power supply to the external device; and wherein when the battery voltage direction circuit detects that the battery voltage of the secondary battery exceeds a second threshold voltage which is higher than the first threshold voltage during discharge from the secondary battery to the external device, the bidirectional regulator stops the voltage raising operation.