Sophisticated order to connect the poles for a jump start of the car battery
- Positives first (any order.)
- Negative pole to battery negative pole (only on one battery.)
- Negative pole to chassis of the car for the remaining battery.
- Disconnect the negative pole from the chassis first when you are finished.
- You connect the positives first because if you connect the negatives first (the chassis on most vehicles,) then you run a risk of shorting out the batteries if you bump the chassis with the positive cable. If the chassis aren't connected to each other, then it is much safer to connect the positive cables.
- You connect the final negative to the chassis because it will spark when you connect it - that closes the circuit. A spark can set off hydrogen gas that collects around batteries when they charge or discharge.
- You disconnect the negative cable from the chassis for the same reason you connected to the chassis last - it will spark, and you don't want to set off any hydrogen gas that may have accumulated around the batteries during the jump start.
The electrons don't care in the slightest in which order you make the connections, or if one connection is to the chassis.
It is "only" a matter of safety. Do it right and everyone is happy. Do it wrong, and you might slag or blow up a battery.
The above applies to the usual negative ground to chassis electrical systems used in modern cars. Older vehicles might use a positive ground, in which case you need to follow a different sequence.
Check the owner's manual before you try to jump start a car - some may say to not use a jump start at all.
The order of connecting two cars does matter for safety reasons. One of the most important considerations is to keep sparks away from the dead battery. Under some fault conditions a lead-acid battery will produce hydrogen gas, which is highly explosive in the presence of sufficient oxygen.
First, the two positive cables are connected while the cars are not touching each other. With no connections between the chassis of the two cars there is no current path hence no possibility of sparks.
Then the ground connection is made to the chassis of the car with the dead battery. Still no chance for sparks because no complete circuit was formed.
Finally, the ground connection to the chassis of the car with the good battery is made. Some sparking is likely, but this will be well removed from the dead battery.
Here a link to what many believe is the most authoritative source for such advice: https://www.cartalk.com/content/although-its-too-late-me-ive-already-done
In the case of lead-acid batteries, the exact order does not matter at all, the only important thing is the connection you make last and that should be the connection between battery - and the chassis.
Electronically it is irrelevant where the circuit is finally closed. But at the moment you make the last connection, there might be a spark. As lead-acid batteries can release flamable hydrogen gas, you generally don't want to cause sparks close to them. Respectively, it's better to make this final connection somewhere far away from the battery. The obvious solution is to use the chassis for that.
Also, it doesn't matter which car you connect first.