This turned into a hot topic while I was away from my computer before hitting the post button on my last post.
When the car is running the alternator provides the current needed for running the engine, headlights, radio and seat heaters etc. At the same time it charges the battery if needed.
The alternator indeed must be grounded in order to work (like the battery, it is a complete circuit).
Depending on how your ignition and alternator are wired, If you disconnect the battery (positive lead
or negative lead), the car may very well keep running on the alternators output. This is very bad for electronic items in the car, as the battery is needed to help regulate the voltage, and to some extent filter out the AC ripple that the diode bridge in the alternator doesn't completely remove.
The reason you need a very high current switch for your battery disconnect, regardless of which lead you switch is because the battery alone turns the starter over, which is where all the heavy lifting is done in the electrical system.
I was just outside taking some measurements with a clamp on DC Ammeter (a Fluke i1010 used with a Fluke 83 multimeter).
- A word of warning here. Most multi-meters are only rated to measure 10 amps or so, and measuring high current with them generally requires the use of a clamp on ammeter attatchment where the meter is actually measuring voltage output from the clamp. Measuring DC current requires a specialized clamp on meter which has it's own power source. The i1010 I used is rated to measure up to 1000 amps DC.
On startup in 50 degree weather, using the min/max function on the meter, my Cobra drew a maximum of 368 amps measured on the negative lead of the battery, and was within 5 amps of that (363 amps max) when measuring at startup on the positive lead.
The reason the current changed was as much because the
oil was stirred up as anything, plus the alternator had run for 30 seconds or so between measurements which raised the battery voltage slightly. If you understand ohms law, when your dealing with 360 amps, a very small rise in voltage will make a measurable difference in current measured.
On a colder day the current draw at startup would have been much higher.
But you can see from these measurements why you need a very high current rated disconnect no matter which lead you disconnect with it.