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Originally Posted by Duster
The easist way to figure out, if it's the battery or alternator is to have the car running and disconnect the negative cable from the battery.
If the car stops running after a very short time, it's the battery. If the car stops running while the negative cable is connected to the battery, it's the alternator.
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Sorry, but
NO !!!
NEVER disconnect the battery cable while the car is running!
The battery acts like a voltage stabilizer or filter to the pulsating DC produced by the alternator.
Disconnecting a battery while the engine is running
could (read: Good possibility) destroy the sensitive electronic components connected to the electrical system such as the DME, radio/ audio system, alarm system, etc., or the charging system itself, especially with internal voltage regulators, such as found in the Boxster.
This is because the peak voltage can rise to 40 volts or more.
In older cars, with no electronic equipment, and only electro-mechanical voltage regulators and such, removing a battery terminal was an accepted practice to test charging systems. That is no longer the case.
Also, static electricity and spikes from connecting and disconnecting batteries could also damage sensitive electronic components.
DON"T DO IT !! It could result in much more damage than the cost of a new battery.