Abstract: A detecting device and a detecting method for monitoring a battery module are provided. The battery module is electrically connected to a load and includes a first battery unit, a second battery unit and a connecting device. The connecting device connects the positive electrode of the first battery unit to the negative electrode of the second battery unit. The detecting device includes a shunt, a voltage detecting module and a control module. The shunt is serial connected between the battery module and the load to measures the load current. The voltage detecting module measures the voltage difference across the connecting device between the positive electrode and the negative electrode. The control module computes the equivalent resistance across the connecting device between the positive electrode and the negative electrode according to the load current and the voltage difference.