AAGGGGGGGGGGGGGHHHHHHHHHHHHHHHHHH! I can't believe people are still doing this stupid trick! DO NOT take the battery cables off while the engine is running. By their very nature, AC generators put out high voltage "ripple" which is a variation of a couple of volts. That voltage goes right back to run the generator and will increase its voltage output. System voltage can not be controlled by the voltage regulator when the battery isn't in the circuit to smooth out that ripple. The only thing that saved you from destroying every computer module on the car is that you probably didn't increase engine speed. Had you done that, you could easily have rendered the car "not worth repair" due to the high cost of all those computers.
Volksawagens have another "Got'cha" that the dealers don't like you to know about. Disconnecting the battery or letting it run dead can lock up many of the computers and the radio. On the newer models that have the insanely complicated and unreliable "throttle-by-wire", (remember Toyota's throttle trouble?), Once the battery is disconnected, even if the engine starts, it won't increase in speed when you press the gas pedal and it won't come out of park. It must be dragged to the dealership to have "minimum throttle" relearned and other computers unlocked. If you don't know the code for the radio, you must buy a new radio because there is no record kept for that code.
Sorry for sounding so irate, but it really frustrates me when manufacturers do things like this to their loyal customers. None of these tricks benefits you, the owner.
Removing a battery cable to see if the generator was working was done many years ago, (before computers on cars), by mechanics who didn't understand how these simple systems work. A much better test that YOU can do is to measure battery voltage while the engine is running. It must be between 13.75 and 14.75 volts. If it is too high, suspect the voltage regulator. That will tend to boil the water out of the battery acid and eventually ruin the battery. If it is low, the battery will never fully recharge while driving.
It sounds like you might have a bad battery based on your observation of not holding a charge. Measure the battery voltage again when the engine is not running. Fully charged, a good battery will measure very near 12.6 volts. If you find closer to 11.0 volts or less, suspect a dead cell and the battery must be replaced. This is where you want to do something to prevent the computers from being without power. They make "memory savers" that use a 9 volt transistor battery and plug into the cigarette lighter, but those only work on cars where the lighter can be used without turning on the ignition switch. If you have to turn the ignition switch on, that means that little battery will try to run the fuel pump, ignition system, dash board, and everything else. It can't handle that so it's purpose will be defeated.
I used a small battery charger whenever I replaced someone's battery. I put the negative clip on the engine, but then had to put the positive on the positive battery cable and be very careful that it didn't fall off or touch anything metal. Most newer cars have a convenient terminal in the fuse box under the hood that you can connect the positive to but you still have to watch that the positive battery cable doesn't touch anything metal.
Again, please forgive my enthusiasm. I just get real frustrated when I learn about all the problems people caused themselves since the engineers have removed all shred of common sense from today's cars. Just because the tv ads say it, it doesn't mean they have our best interest at heart.
By the way, all you need to measure battery voltage is an inexpensive digital voltmeter. You can find REAL nice ones at Radio Shack for 40 bucks, but you can get all the same features for 10 bucks from Harbor Freight Tools if you have one of those stores nearby, and they often have a perfectly fine model on sale for $2.99. I can walk you through how to use it if you never have used one before.
Sorry for jumping in here rasmatz. This subject is very dear to me so I had to stick my nose in.
Friday, April 8th, 2011 AT 8:58 PM