When reconnecting a battery, why is it recommended to connect the positive lead first, and the negative lead last? What difference does it make if they are connected in the opposite order?
The reason I would connect the positive cable first (in a negatively-earthed car) is that while tightening the positive connector with a spanner (wrench), if I were to touch the body of the car with the spanner at the same time, nothing would happen and I would be OK.
However if the negative was already connected to the battery and I shorted the positive to the body with the spanner, there would now be a short circuit with a big spark and a possible fire. For the same reason I would disconnect the negative connector first too.
For technical functioning it does not matter in which order you connect them.
However for safety reasons it does matter.
The whole body of the car is usually connected to the minus pole. And you can accidentally hit that body with the positive lead. So connecting the lead first is safer.
- Negative pole first:
Whole car (except a few parts like the positive pole) are connected. Any mistake with the other lead will lead to a short. And car batteries are very good* at shorts. You really do not want this.
Positive pole first:
Only a small part of the car is now connected to power. If you mess up by touching the car with the other lead nothing will happen.
Lastly: This might also be a habits for people who deal with electronics. Ground goes first. Always.
*: Or very bad. Depending on your point of view and your desire to prevent harm.