A power supply device, comprising: a plurality of battery packs connected in parallel to a load; and a plurality of battery controllers (3A,3B, 3C) corresponding to the plurality of battery packs in a one-to-one correspondence, wherein each of the battery packs includes a relay (7A, 7B) configured to control an output from a corresponding one of the battery packs, and wherein the plurality of battery controllers (3A, 3B, 3C) has a master-slave configuration, and each of the battery controllers (3A, 3B, 3C) is configured to execute a self-diagnosis and a mechanical operation diagnosis of the relay (7A, 7B) of the corresponding one of the battery packs before starting a battery control, characterized in that: each of the plurality of battery controllers (3A, 3B, 3C) includes a communication part (23A, 23B, 23C) configured to perform communication using: a first communication scheme in which the communication is performed in synchronization with an edge timing of a first communication period that is consecutively repeated; and a second communication scheme in which the communication is performed in synchronization with an edge timing of a second communication period that is consecutively repeated and shorter than the first communication period, during the self-diagnosis, communication for exchanging information on the self-diagnosis between a master controller and a slave controller (3B, 3C) is performed by the first communication scheme, and during the mechanical operation diagnosis, communication for exchanging information on the mechanical operation diagnosis between the master controller and the slave controller (3B, 3C) is performed by the second communication scheme. a plurality of battery packs connected in parallel to a load; and a plurality of battery controllers (3A,3B, 3C) corresponding to the plurality of battery packs in a one-to-one correspondence, wherein each of the battery packs includes a relay (7A, 7B) configured to control an output from a corresponding one of the battery packs, and wherein the plurality of battery controllers (3A, 3B, 3C) has a master-slave configuration, and each of the battery controllers (3A, 3B, 3C) is configured to execute a self-diagnosis and a mechanical operation diagnosis of the relay (7A, 7B) of the corresponding one of the battery packs before starting a battery control, characterized in that: each of the plurality of battery controllers (3A, 3B, 3C) includes a communication part (23A, 23B, 23C) configured to perform communication using: a first communication scheme in which the communication is performed in synchronization with an edge timing of a first communication period that is consecutively repeated; and a second communication scheme in which the communication is performed in synchronization with an edge timing of a second communication period that is consecutively repeated and shorter than the first communication period, during the self-diagnosis, communication for exchanging information on the self-diagnosis between a master controller and a slave controller (3B, 3C) is performed by the first communication scheme, and during the mechanical operation diagnosis, communication for exchanging information on the mechanical operation diagnosis between the master controller and the slave controller (3B, 3C) is performed by the second communication scheme. each of the plurality of battery controllers (3A, 3B, 3C) includes a communication part (23A, 23B, 23C) configured to perform communication using: a first communication scheme in which the communication is performed in synchronization with an edge timing of a first communication period that is consecutively repeated; and a second communication scheme in which the communication is performed in synchronization with an edge timing of a second communication period that is consecutively repeated and shorter than the first communication period, a first communication scheme in which the communication is performed in synchronization with an edge timing of a first communication period that is consecutively repeated; and a second communication scheme in which the communication is performed in synchronization with an edge timing of a second communication period that is consecutively repeated and shorter than the first communication period, during the self-diagnosis, communication for exchanging information on the self-diagnosis between a master controller and a slave controller (3B, 3C) is performed by the first communication scheme, and during the mechanical operation diagnosis, communication for exchanging information on the mechanical operation diagnosis between the master controller and the slave controller (3B, 3C) is performed by the second communication scheme.