A voltage supply system having a first battery with a first anode and a first cathode, and a contactor coupled in series between the first anode and an electrical node is provided. The system includes a first voltage sensor generating a signal indicative of a first voltage level of the first battery, and a second voltage sensor generating a signal indicative of a second voltage level between the electrical node and the first cathode. The system includes a DC-DC voltage converter coupled between the electrical node and the first cathode; and a second battery, and a microprocessor. The microprocessor disables operation of the DC-DC voltage converter such that the converter does not apply a voltage to the second battery, if both the first voltage level is substantially equal to the second voltage level, and the first voltage level is less than a first threshold voltage level.