[jdmccright]

Maybe I'm reading it wrong, but in a car with alternator/battery, if the alternator can't provide all the power needed to run the vehicle's systems then eventually the battery would become depleted because the extra power needed is supplied by the stored battery power. The alternator must be able to provide all the power needed plus enough excess to keep the battery fully charged.

If the alternator died, the car would continue to run for a short time on battery power alone until voltage would be too low to run the lights, fire the plugs and the engine would quit.

If the battery died, the car would have to either be jump started or started by popping the clutch (manual transmission). Once started, the engine should run because the alternator can supply the power, but as stated, the battery would not be there to help filter any noise or power fluctuations as the engine speed changes. This is evidenced in most race cars that have no battery or starter. They are started externally and run solely off alternator power.