A vehicle power supply system comprises a battery module that includes a plurality of battery cells, battery cell control devices and a signal transmission path through which signals are transmitted. The battery cell control device comprises a voltage measurement circuit that measures terminal voltages at the battery cells, an abnormality diagnosis circuit that diagnoses an abnormality in the battery cell control device, a timing control circuit that outputs a signal for instructing measurement phase and a signal for instructing diagnosis phase, and a communication circuit that outputs a signal indicating the terminal voltage and a signal based upon a diagnosis result by the abnormality diagnosis circuit.