Sorry for the long delay. You are extremely lucky the engine shut off when you disconnected the battery. Doing that while the engine is running was a test used decades ago by people who didn't understand how charging systems worked.
If your alternator is NOT defective, removing the battery cable removes the cushion that helps regulate system voltage. The voltage produced by a good alternator varies between around 8 to 16 volts. The battery smooths out those pulses and gives the voltage regulator a steady voltage to watch. Without the battery, the regulator sees the pulsing drops to 10 volts and tries to increase output voltage to 13.75 to 14.75. If you increase engine rpm while that is happening, the output voltage can very easily exceed 30 volts. Any guess what happens to the engine computer, body computer, remote keyless entry computer, air bag computer, anti-lock brake controller, and electronic radiator fan relay when system voltage goes too high? On older, more reliable cars, the worst that would happen was you'd burn out any light bulbs that were turned on at that time. On newer cars, you can easily do thousands of dollars worth of damage in a few seconds. Please don't do that test again.
The best way to tell if your charging system is working is to measure battery voltage while the engine is running. With the engine off, battery voltage should be near 12.6 volts if it's fully charged. With the engine running, it must stay between 13.75 and 14.75 volts. Lower than 13.75 volts and the battery will never fully charge. Over 14.75 volts and water will start to boil out. Don't panic if your voltage is a little out of that range. If the battery was run down, the system voltage will likely be less than 13.75 volts, but you will see it rise as the battery recharges.
Monday, June 1st, 2009 AT 3:51 PM