While it might be possible to check the alternator for function that way, it's extremely hazardous to all the elecrtonic components in the circuit. The battery acts as a shock absorber in the curcuit, dampening out voltage spikes as components are turned on and off. As the connection to the battery is broken, the charging system, either generator or alternator, will spike the voltage up to extremely high levels, trying to maintain the current flow. It takes a moment for the voltage regulator to regain control of the circuit with the battery missing. I've monitored primary voltage on an oscilloscope at times, and seen spikes exceeding 100 volts occur just as the battery cable leaves the post.
If you've got NO solid state electronic components, such as diodes, transistors, computers, etc. on the vehicle, you'll probably get away with that test. Alternators have six high-current diodes, and most voltage regulators have very complex solid-state circuitry. Got a modern radio? There's all sorts of solid state electronics there. Radar detector? GPS unit? Cell phone charger? All those things are at severe risk if they're turned on, or even plugged in.
There are much better ways to check charging system output. A $10.00 digital multimeter from the FLAPS will tell you if the generator/alternator is producing a vlotage higher than the battery, without putting all your electronics in danger!
Jerry
