Patent ID: 8473161

Claim:
An electric power steering apparatus driving a motor to generate a steering assist force according to a steering torque, comprising: a battery for supplying electric power to the motor; an auxiliary power supply charged by the battery and capable of supplying electric power to the motor; an output circuit for supplying the motor with electric power from at least one of the battery and the auxiliary power supply; a breakdown detector for detecting a breakdown of the battery; and a control circuit which, in the event of a battery breakdown detected by the breakdown detector, uses the stored energy of the auxiliary power supply for steering assist and performs output decreasing control to gradually decrease an upper limit of a power supplied for the steering assist according to a length of time elapsed, the control circuit calculating available energy presently stored in the auxiliary power supply after the battery breakdown and determining a length of time to the end of the steering assist based on the amount of energy stored in the auxiliary power supply to thereby perform the output decreasing control until the length of time has elapsed.