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 . therefore , the hazard caused by aging of the electric circuit can be prevented by monitoring the equivalent resistance , which infers the change of the assembly quality .