Don't do this. I used to do this myself.
Originally Posted by inuvik
Sparks are to be avoided when connecting & disconnecting the cables, especially near the battery. Any lead-acid battery, the type most commonly used in cars for ages, is a source of explosive hydrogen.
Basically, the idea is to connect one car's electrical system in parallel to the other. The practice I found involves connecting the positive battery terminals first - battery to battery. Next, connect one car's chassis to the other. This will complete the circuit and, if all is in order, electrical current will begin to flow through it. Any sparks made during the final connection will thus be away from the battery.
When disconnecting, remove the chassis connections first. The circuit will be broken and any sparks formed should be away from the batteries as much as possible. Then remove the rest of the connections.
Here's a few things to think about:
1. The Mirage battery is not maintenance-free, meaning it is not sealed. It has the old-timey caps on it that allow periodic checking of the electrolyte levels. Bonus trivia: does anyone here know how to do this?
2. Sparks can damage electronic equipment, and you are smart to avoid that. Today's cars have more electronics than ever. Manufacturers should design to accommodate some electrical spikes, but don't rely on it. For what was described as a light show makes me wonder if the impaired car had electrical problems.
3. Using the vehicle chassis as a negative connection has been a common practice in the automotive industry for a long time, but some cars use chassis positive. Check this before connecting! On any car, verify which terminal is attached to the chassis.
4. Using the vehicle chassis as a negative connection is only as good as the grounding wiring and the battery. If there are loose, corroded, or otherwise poor connections, the jump start will likely fail. Also, paint is an insulator. It can be difficult to find a chassis ground connection that will conduct electricity.
5. Personally, I think it is best if both drivers make their own connections, for several reasons.
a. Four hands help keep the cable clamps from touching each other. No arc welding, please.
b. Each driver is responsible for their own vehicle. If one is not comfortable with the process, simply pull the clamps from their car.
c. Witness the process by watching and listening. When the good car is loaded, the engine usually slows down a bit (Yes, I jump start with the vehicle running) and that is to be expected. Again, any unusual noises, and either driver can pull the plug on the experiment.
6. Learn this before having to do it in inclement weather.
Let me know if there is anything I missed.