Jumping from a running vehicle : Exactly ! (#1) When either Vehicle is running, the rotating Alternator can be totally destroyed if anything goes wrong ! That is usually $100 (min.) to as much as $500 and that would be before $-Labor ! (#2) If either end is hooked up as reversed polarity, there is a monstrous current surge capability ! (#3) If there is a "dead-short" anywhere in your hook-up on/in either vehicle, the resulting Current-Surge will fall somewhere between the two extremes noted for #1 and #3. Proper : Throw Jumper Cables on ground stretching them from vehicle to vehicle to assure proper length ... with out either vehicle physically touching the other. Hook up first Positive cable clamp onto dead battery Positive Terminal first, keeping Negative totally isolated from everything (laying on Mother Earth Ground - not touching anything conductive); hook up second Positive Cable Clamp to Good-Battery Positive-Terminal second (leaving Negative cable lying on Mother Earth Ground - not touching anything conductive); next pick up First Negative Cable Clamp from Mother Earth Ground and connect it to Dead-Battery Negative Battery Terminal ; and finally , pick up last Negative Cable Clamp from Mother Earth Ground and slightly tap or slap the metal part to the bad battery Negative Battery Terminal: if any sparks are visible ( somewhat normal) , they should not cause any metal pitting or burning, and should only be an fraction or so of an inch long. Any result greater than this means that something is really wrong !
If huge sparks, then start by assuming a dead short somewhere on the faulty vehicle, and that could be :1) Battery; 2) Alternator; 3) Heavy Current Carrying Wiring (Alt, Bat, Fuse-Block, Relay-Panel, etc.). Go from there ! Good Luck !
