2001 ford focus, started then died-battery tested & is good alternator?

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i heardi could test to see if it is the alternator, by fully charging the battery starting the car then take off the pos + cable (off the car w/problems ford focus) leaving the neg - cable on the battery, & if the car still runs (with the neg cable still connected) then it is the alternator. if it dies then it is something other then the alternator. is this true? help