[dandoc]

To test for a bad alternator:
-Start the vehicle's engine as normal and allow it to idle.
-Remove the negative battery cable from the battery.
If the engine stops upon removing the negative cable, you've got a bad alternator.

The battery is used to start the engine and to support the alternator's current to the vehicle's electrical accessories. A good alternator can support the engine and some accessories (up to it's rated amperage output) without a battery in the circuit. It'd be a good idea to check all ground wires for broken/cracked insulation, broken wires, and tight connections that make contact with bare metal. It may be teadious, but could likely save you money that you'd otherwise spend at the repair shop.