4

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?

19

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.

  • And fires where quite a possibility in old lead/acid batteries if the user overcharged them. – Hennes Nov 4 '15 at 12:14
  • Random link from the first page of Google results, among many, many more. Positive first, and negative last. – JPhi1618 Nov 4 '15 at 13:24
  • Yes, nothing new. It is a shame the instructions in the link didn't say why they should be done in that order. – HandyHowie Nov 4 '15 at 13:32
  • Ah yes, I got them the wrong way round in my question... Should say "positive lead first" dammit. – user2800708 Nov 4 '15 at 14:07
5

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.

Recap:

  • 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.

  • It does matter which you connect last which @HandyHowie points out in his answer. This is true of any negatively grounded (earthed) vehicle, which is the vast majority of vehicles on the road today. – Pᴀᴜʟsᴛᴇʀ2 Nov 4 '15 at 12:36
  • Besides accidentally hitting the car body when tightening the bolt on the positive lead? – Hennes Nov 4 '15 at 12:47
  • Everything you wrote is correct. The rule is just for safety, as you wrote in your last point. This is very important, because a car battery is strong enough to melt a wrench when touching bith poles. So, a short circuit is a little more than just a few sparks. Hydrogen isn't such an issue. May be, you should have emphaised this more. – sweber Nov 4 '15 at 13:13
  • Unfortunately, @sweber, hydrogen is still an issue with flooded lead-acid (FLA) batteries. Please read this publication about FLAB and water decomposition. It states hydrogen and oxygen are produced from FLA batteries under normal conditions without the battery being charged or discharged. It also states the gasses are produced in higher quantities when the heat is turned up. The page has to do with FLA batteries when used in power backups, but still applies to vehicles in general. – Pᴀᴜʟsᴛᴇʀ2 Nov 4 '15 at 21:23
  • 1
    @Paulster2: OK, you have convinced me. – sweber Nov 4 '15 at 22:52

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