A method for monitoring the operation of a vehicle battery circuit with parallel batteries and a battery circuit

A battery circuit (1) for a vehicle (300), said battery circuit (1) comprises a primary battery circuit (101) comprising a primary battery (102); a secondary battery circuit (104) comprising a secondary battery (105), and a generator configured to supply a first demanded voltage to a positive pole of the primary battery, in response of a first voltage demand signal applied to the generator. The battery circuit (1) further comprises a connection circuit (100) comprising an input (107) connected to a positive pole of the primary battery (102), and an output (108) connected to a positive pole of the secondary battery (105), the connection circuit further comprises a first branch (109) which connects the input (107) to the output (108), wherein the first branch (109) comprises a diode (D1). A control circuit (110) is provided, configured to generate a first voltage demand signal and to determine that the generator supplies the first demanded voltage to the positive pole of the primary battery (102), and then to measure the voltage of the input (107), Vin0, and the voltage of the output (108), Vout0, and to generate a second voltage demand signal to increase the generator output, from the first demanded voltage to a second demanded voltage, and to measure the voltage of the input (107), Vin1, and the voltage of the output (108), Vout1, and then to determine the operation status of the battery circuit (1) based on the measured values.