Very basic car battery question
July 16, 2014 10:25 AM   Subscribe

The internet says positive should be reconnected first, but that's not as easy as it should be. What's the best option?

Borrowing my dad's car for the summer. Last week, I left it parked in the driveway while going out of town, but two nights later, had a call from a neighbor to say that the alarm was going off repeatedly (and had been all night.) We got another neighbor to disconnect the battery-- alarm problem solved, as far as neighborhood peace is concerned.

Then I get home and see that only the positive terminal has been disconnected, and when I try to disconnect the negative, the bolt won't budge. I'm really not that comfortable with electricity... or cars... and so far, I can't find much help elsewhere that says connecting them this way is a good or safe idea. Can you convince me otherwise and walk me through any special handling, or else confirm that I should pay someone with a torquier wrench to come help me out?

I was planning to take the car in for maintenance this week anyway, so everything is getting checked out, but I'd like to be able to get the thing there.
posted by notquitemaryann to Travel & Transportation (13 answers total) 3 users marked this as a favorite
Car batteries make me nervous too but I've worked on them many times in my life and never had an issue. Just don't create a connection - ie connect a metal tool to the positive terminal and some other metal part or negative terminal - while you are reconnecting the terminal. You should just be able to drop the connector onto the terminal and tighten, no problem.

I am not an electrician or car mechanic, just a dude who's had dead batteries periodically.
posted by natteringnabob at 10:33 AM on July 16, 2014

Technically, yes, connect the neg. first, but I understand your problem.

If you can get the pos. cable on without touching anything, you can probably just put it on just fine. After all, the neighbor got it off just fine without disconnecting the neg. first, right?

I wouldn't worry about it all that much--again, just don't let it touch anything on the way back to the battery.

Please note, though, that I am just an Internet stranger and not responsible if there is something in the way that causes an arc once it gets closer to the battery that you didn't know to look for, for example...
posted by TinWhistle at 10:34 AM on July 16, 2014

The rule to connect the positive first and disconnect the negative first is to minimize the danger of short circuits and electrocution.

In automotive electrical systems, negative serves as ground. So the whole chassis is electrically connected to the negative battery terminal. If the battery is completely disconnected, or if just the positive is connected, you have to avoid touching the positive terminal and the negative terminal at the same time. But when the negative is connected, you could complete a circuit by connecting the positive terminal to any other metal in the car.

Really the correct solution is to figure out how to disconnect the negative terminal. However, if you do decide to go ahead and just re-connect the positive, first remove any metal jewelry from your hands, and be very attentive about what you and your tools touch. I don't work on cars or batteries enough to know how dangerous this actually is, so I don't at all recommend it.
posted by aubilenon at 11:18 AM on July 16, 2014

Err, don't stress about it - if the car is off and the positive wire isn't connected to anything there isn't any source of electricity for you to cause a short with. It'll almost certainly generate a spark or three when you connect it, don't freak out - the same thing would happen if you were connecting the negative terminal second. Just be careful when you are tightening the nut that the wrench doesn't touch anything other than the nut. And as TinWhistle said - if you listen to the advice of strangers on the internet you may end up making a lot of sparks and having your skin smell like roasted marshmallows.

The truth is, your neighbor obviously doesn't think it's a big deal to disconnect/reconnect a battery - if you're nervous about it just go ask them for help.
posted by foodgeek at 11:20 AM on July 16, 2014

Yeah to be clear the wire isn't dangerous, it's the positive battery terminal that's the thing you need to be careful with. The main danger would be while you're actually making the connection and tightening the bolt.
posted by aubilenon at 11:30 AM on July 16, 2014

It's not that big a deal as people are making out in this thread. Just make sure you don't touch anything other than the positive terminal with the wrench/spanner (anything metal) when you are doing it back up and also don't have tools in the same hand when you're putting the positive terminal back on the battery. As long as nothing metal touches the positive battery terminal to the body you're fine.

So just put the tools down, put the terminal on the battery (small sparks are fine and normal, but being hesitant will put more sparks into the equation than just confidently jamming the thing on). Make sure the terminal is seated fully and tighten it up, making sure the wrench/spanner doesn't touch the body while you do it.

An easy job.
posted by Brockles at 11:33 AM on July 16, 2014 [1 favorite]

So there's no real risk of electrocution, 12v by itself isn't going to do anything bad to you. However, there is a lot of energy stored in that battery, so a short circuit can weld metal and get very hot. You want to minimize your chances of that (although anyone who's done their own work on cars has accidentally shorted batteries, and as long as you pull the short away quickly you'll be fine).

The reason for the "positive first" rule is that the rest of the car, frame and all, is the ground, and connected to the negative lead.* It's much easier to avoid shorts if the rest of the car isn't connected to the battery.

The only other thing to worry about is if the battery has been heavily depleted, it may have outgassed some hydrogen, and if this collects under shielding (like the way modern battery mounts totally wrap up the battery in plastic and felt and crap), the spark from reconnecting the battery may ignite the hydrogen, causing you to swear, react suddenly, and bang your head on the hood as you reflexively try to escape.

Which is why you'll see people blow over the battery before they connect jumper cables or connections.

But, yeah: No big deal, just reconnect it.

*there is at least one historical exception to this, you're unlikely to run into it
posted by straw at 11:35 AM on July 16, 2014

The rule to connect the positive first and disconnect the negative first is to minimize the danger of short circuits and electrocution.

This is a bit of an overstatement and hardly going to calm the situation. Unless you are messing with the main battery bank of a hybrid car, a standard 12V car battery will not electrocute you. It may give you a bit of a jolt but in all my foolish years the worst I've had is being burned by the (shorting out) wrench between the battery and the body when I dropped it and had to pull it out quick before something worse happened.
posted by Brockles at 11:36 AM on July 16, 2014 [3 favorites]

Okay okay, sorry!

I guess sparks and damage to the car & battery (and I guess small burns) is the realistic extent of the danger, then?
posted by aubilenon at 2:31 PM on July 16, 2014

You (probably) cannot be electrocuted by a car battery (under normal circumstances). (but you can get burned etc.)
posted by RobotVoodooPower at 2:44 PM on July 16, 2014

Got any electrical tape? If so, take whatever wrench you're using for this, and wrap up all but the business end of it in a layer of tape. You still want to avoid touching anything but the positive terminal with it, but the tape gives a bit of extra safety factor in case of a mishap, like dropping the wrench.
posted by FishBike at 4:13 PM on July 16, 2014 [2 favorites]

Shorting the terminals of a car battery can produce sparks and a lot of heat, possibly burning you or starting a fire. You can get a very high current out of a car battery (low internal resistance), which is why they are good for starting cars: lots of current to produce a lot of torque in the starter.

The other danger is not electrocution but explosion. Lead acid batteries produce hydrogen as a byproduct of the charging cycle, and any hydrogen that is around can be ignited by sparks caused by connecting/disconnecting the battery. This burning hydrogen can cause the battery to burst spraying sulphuric acid all over you. So, particularly when jump starting a car, the order of connection is positive to positive, then negative to negative (preferably NOT the battery terminal itself, but an exposed piece of bare metal frame if there is one, though there usually isn't) — during jump starting the dead battery will start to charge, increasing the chance of there being hydrogen about.

The charging issue related to jump starting is not really relevant to you, so as long as you avoid shorting the terminals or producing sparks you should be fine whichever order you connect the terminals, even if best practice suggests one particular order. It’s still not a bad idea to avoid putting your face to near to the battery, and wearing safety glasses if you have them, when connecting and disconnecting terminals just in case.
posted by Quinbus Flestrin at 4:58 PM on July 16, 2014

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