Patent ID: 8248023

Claim:
A method of externally charging a powertrain comprising: monitoring a voltage level of a first battery; comparing the monitored voltage level to first and second voltage thresholds, the first voltage threshold having a higher voltage than the second voltage threshold, wherein the first voltage threshold comprises an overall voltage of the first battery at which the engine may not start due to low overall battery voltage and the second voltage threshold comprises a voltage of the first battery at which it is undesirable to initiate an engine start procedure; when the monitored voltage is at least the second voltage threshold, initiating an engine starting procedure, and terminating the engine start procedure when both the monitored voltage level is below the first voltage threshold and an engine does not start during the engine starting procedure; and when the monitored voltage level is below the second voltage threshold, charging the first battery by supplying power from an external power source and increasing voltage of the power supplied by the external power source within the powertrain.