Posted by Ben on February 18, 01 at 20:24:45:
Have received several email requests on how2 jump start, so here are my thoughts on the subject.
First, why it can/may damage your alternator. When an engine's starter is first energized, there are huge inrush currents needed to build the magnetic fields necessary. Rule of thumb is 10 times locked rotor current, an approximate number. So a start motor can pull several hundred amps of current (500-1,000 amps). The current will come from the vehicle that has the juice (your's that's jumping the dead one).
To avoid damaging your alternator, here's the sequence and what/why's. Good vehicle with the good battery is vehicle A, dead battery is vehicle B.
A's engine off, connect pos battery to B's pos battery (make sure vehicles are not touching, else there's a neg connection). Start A's engine. Connect B's neg battery term to A's alternator bracket (to avoid sparks at the battery, there's hydrogen & oxygen gases around batteries). Careful, there's rotating stuff.
Let A charge B for a while. Depending on how dead B is, up to 15 minutes with A's engine RPM held above 2Krpm. This will charge B's battery enough to allow it to help A's battery in a second.
TURN "OFF" A's engine and allow it to stop rotating for a few seconds.
Now start B's engine with A's battery connected, but A's engine OFF.
This will have A's alternator not supplying any current and the current draw from B's starter motor will only get current from A's & B's battery. Once started, leave it running for a bit and then carefully remove the connections. Safest is to stop both engines, but sometimes B will stall. A's alternator diodes will not be exposed to hundreds of amps and will save an alternator replacement down the road.
Alternator diodes are only rated for a fraction of the rated current of the alternator and will fry if exposed to high current draw. That is why your own vehicles starting circuit disconnects the alternator while starting and reconnects after the engine starts. The battery is to supply this spike current, that is why batteries are rated in "cold cranking current". The better ones are rated 800-900-1000-plus amps.
Also, why the connections and wiring between the battery & starter must be in tight and low resistance.