A vehicle including an internal combustion engine, a DC power source and a controller are described. The internal combustion engine includes an engine starting system and an electrical charging system. A method for monitoring the DC power source includes determining a State of Charge (SOC) for the DC power source. Upon detecting that the SOC is less than a threshold SOC, routines are executed in the controller to evaluate a plurality of potential root causes associated with the low SOC. At least one of the potential root causes associated with the low SOC may be identified as a candidate root cause, and a fault probability for each of the candidate root causes is determined. One of the candidate root causes is determined to be a final root cause based upon the fault probabilities associated with the candidate root causes.