A vehicle power supply device comprises a lithium battery module that includes a plurality of lithium battery cells, first control devices, voltage detection harnesses via which terminal voltages at individual lithium battery cells are input to the first control devices, a second control device and a signal transmission path through which signals are transmitted. The first control device comprises a selection circuit that selects terminal voltages at individual lithium battery cells, a voltage measurement circuit that measures the selected terminal voltages, balancing switches used to discharge individual lithium battery cells, a balancing switch control circuit that controls open/close of the balancing switches, and a diagnosis circuit for detecting an electrically abnormal connection in the detection harnesses.