Thank You for writing into AskPatty! To address your question, the sole function of a battery is for start-up, the alternator is what allows your electrical components to function when the engine is on. If you had a reason to "jump" your vehicle, the alternator can charge your battery if there is not a "dead cell" which the only fix is a replacement.
Hope this helps!
A. J. Valle
〉 Answered on Jun 11th, 2012 by Amanda J. Pierce, MBA "AJ", Certified Mechanic, Project Coordinator; Primavera Scheduler at VIGOR Alaska Industrial Shipyard & BizzM3ch Solutions