The battery simply starts the vehicle. After it is started, the alternator takes over and generates energy to keep the vehicle running and recharges the battery. If the alternator is not generating any power, the vehicle runs off the battery - for a short time. The battery does not have enough stored energy to keep the vehicle going for very long. If the battery is worn down enough because of a failing alternator, it will need to be replaced.
〉 Answered on Apr 15th, 2008 by Suzanne Grego, Technician at City of Philadelphia Fleet Management