My Own Worst Enemy
 

How serious is the warning "Disconneting your battery while the car is running will ruin your alternator" ?

Pretty serious.

Might of blown a diode(s) in the Alternator, but maybe not. Is the alternator warning light now on all the time? If it is, well, damage has been done. If not, and it's charging OK - and not overcharging - you're fine.
If you've damaged it, you'll need to get the alternator off and in to an auto-electrical place for repair, it's not a hard job.
Good luck....

About 2 years ago I got out to Pocono for a DE and found out that my voltage regulator was intermiittently overcharging. This was to the point that my battery was hot to the touch. Fortunately it didn't explode. I just had a lot of cleaning up to do from it boiling over.

The next day I drove to the track for the classroom instruction and then drove home (approx. 150 miles) by starting up the car and then disconnecting the positive cable. The car ran fine. (I subsequently installed a new voltage regulator.) The alternator checked out fine by a local rebuilding shop.

It worked for me and saved my a big tow/flatbed charge.

rbertels
87 Coupe

I agree with Saffs with the modern electrics you will sometimes get away with it(the shorter the duration of the event the better)
Back in the 60s when I raced a Triumph Bonneville I used to remove the battery entirely and bolt the +/- leads together(I was young and stupid unlike now when I am older and stupid). The old Zener diode got hot but no permanent damage resulted after 3 season of racing

Not a good idea. In my younger and foolish days I disconnected the leads from the battery of my 76 Triumph Spitfire while the motor was running and after about 5 seconds a mini-mushroom cloud rose from the alternator (Lucus-Prince of Darkness). Needless to say I replaced it with a Delco and have never disconnected leads since.

Chuck