Back in 1994, the R/R (and eventually the alternator) went bad in my 1989 GS500E shortly after jumping it from my buddy's running car. Since then, if I need to jump it from a car battery, the car is off.
+1 12v is 12v no matter the size of the source, but when the car is running you are no longer dealing with 12v. With the car running you have 12-16v, there in lies the problem.
