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?
Asked
Active
Viewed 8.7k times
2 Answers
20
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.
Nick T
- 103
- 4
HandyHowie
- 25,151
- 3
- 38
- 75
-
And fires where quite a possibility in old lead/acid batteries if the user overcharged them. – Hennes Nov 04 '15 at 12:14
-
Random link from the first page of Google results, among many, many more. Positive first, and negative last. – JPhi1618 Nov 04 '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 04 '15 at 13:32
-
Ah yes, I got them the wrong way round in my question... Should say "positive lead first" dammit. – user2800708 Nov 04 '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.
Hennes
- 589
- 4
- 15
-
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 04 '15 at 12:36
-
Besides accidentally hitting the car body when tightening the bolt on the positive lead? – Hennes Nov 04 '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 04 '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 04 '15 at 21:23
-
@Paulster2: I don't say that H² isn't produced. Using the data from the publication, a car battery (6 cells) would generate 2.7l per hour when being overcharged with 1A. This can be an issue when charging many batteries in a room. If a battery is not being charged, the rate should be much much lower. For a single car battery, the generated H² should dilute fast enough in air and not be an issue. The air inside the battery is highly explosive,(2 parts H² and 1 part O²),but it still needs to be ignited. So, you need sparks at the vent. I never heard about about exploding car batteries due to H². – sweber Nov 04 '15 at 22:09
-
@sweber - Read this, this, & this for starters (many more articles out there). The LA Times article states, "...a study by the National Highway Traffic Safety Administration found that battery explosions in 1993 alone injured 2,280 people severely enough to have required hospital treatment." It is a real and present danger (even if the article is a few years old). – Pᴀᴜʟsᴛᴇʀ2 Nov 04 '15 at 22:18
-
@sweber - The hydrogen gas emissions is why you should put the negative ground of a recipient vehicle being jumped away from the battery and it should be the last connection made. This helps protect the both vehicles and the person(s) doing the jumping. Keeping the last connection away from the battery so that the spark which will happen has less chance of igniting the hydrogen gas if it is possibly there. I personally don't want anyone to get burned by sulfuric acid due to a batter exploding in their face. – Pᴀᴜʟsᴛᴇʀ2 Nov 04 '15 at 22:22
-
1