Grounding the negative clamp when jump-starting your car

December 29, 2011 4:43 PM

Question about jump-starting a car: why did my neighbor absolutely insist I ground the negative clamp... to the battery? That seems unwise.

This was the first time my friend and I had ever tried to jump-start a car (but I've been around it several times), and it wasn't working, so I asked a neighbor that happened by to take a look. He insisted that I ground the negative by hooking it up to the battery of the dead car - that's how he always does it, and claimed that the negative side is grounded to the chassis. He was very confident, and acted like I was an idiot when I refused.

We figured out the problem on our own, but it makes me question: was he right? Is my info old and are car batteries now built this way? If it doesn't work, why would anyone who's jumped a car before ever insist on it?

FWIW I might have been swayed except for the directions on the jumper cable that also read IN BIG CAPITAL LETTERS not to do that.

posted by subject_verb_remainder to Travel & Transportation (26 answers total)

Your question is a bit unclear but the negative terminal of car batteries has always been wired up as ground. It is easier (and generally safer) to use the chassis frame ground as sparks are hopefully far enough away from the batteries to not ignite a hydrogen explosion from a charging battery. The hook up order I was taught was "positive to positive, negative to ground", where the "ground" meant the chassis of the dead car.

There are some very old cars that used positive ground but you are unlikely to encounter a 1959 Jaguar.
posted by chairface at 4:55 PM on December 29, 2011 [6 favorites]

The negative terminal is generally grounded to the chassis, which is why that would work. It's safer to connect it to the chassis because then the spark that may form won't be near the battery. If the battery is giving off hydrogen gas (unlikely, but it can happen), then such a spark could cause a fire or explosion. I think modern batteries don't vent that way, but that's only a vague recollection.

I'm not aware of any downside to connecting the cable to the chassis rather than the battery itself. I suspect there isn't one, and your neighbor was just insistent because that's the only way he knows of. But he can safely be ignored.
posted by spaceman_spiff at 4:56 PM on December 29, 2011 [1 favorite]

Response by poster: Oh, so there's no actual problem with clamping the negative onto the negative of the dead car battery in itself, but just the danger of sparks near the battery?
posted by subject_verb_remainder at 5:04 PM on December 29, 2011 [1 favorite]

It's the sparks that are the issue, yes.
posted by plinth at 5:07 PM on December 29, 2011 [2 favorites]

Right. The ground/negative of the battery is the "source" of the ground for the chassis. So the only reason to not hook it right to the battery is the supposed hydrogen explosion theory. And the time to worry about that is when disconnecting.
posted by gjc at 5:08 PM on December 29, 2011 [2 favorites]

Folks also try several different things when the car doesn't start immediately, when what is often needed is some patience while the dead battery builds up a charge.
posted by sammyo at 5:17 PM on December 29, 2011 [1 favorite]

The cables always say to not do this, and when I'm in charge I follow those instructions, but every other person I've ever seen jump start a car has simply connected the batteries and gone to town. There's some small but measurable risk of an explosion* but people generally don't worry about that, and either way will start the car.

* I have seen a battery explode, though not during a jump start, and it was both scary loud and covered everything within a dozen feet with battery acid. I would not want to be standing next to a car battery when it exploded, so I always follow the safety nerd instructions, even if the risk is very small.
posted by Forktine at 5:24 PM on December 29, 2011 [1 favorite]

I've always been taught to ground to bare metal on the frame of the car. (Hi, I am very forgetful and so very good at running my battery down and now own my very own car jump starter battery back as a result.) Seconding what sammyo says - if you, say, leave on the map light in a car, leave it parked in a cold garage for two weeks, and then try to start it - you need to leave the charger attached to the dead battery for a few minutes, then turn the key. Not that I have any experience in this situation.
posted by lyra4 at 5:25 PM on December 29, 2011

To piggyback a bit: does anyone know of documented cases of this sort of explosion occurring in the real world?
posted by contraption at 5:25 PM on December 29, 2011

Make that "battery pack". Ie one of thse guys:
Jump Starter

I hate to say it, but I've been incredibly glad I own this many times.
posted by lyra4 at 5:27 PM on December 29, 2011 [1 favorite]

Response by poster: Thanks all, that makes a lot more sense.

I learned to jump a car by watching my uncle several times, and he used to be an electrical engineer. He was always really adamant about grounding it far from the battery with the impression that terrible things would happen otherwise. I guess I've just never dealt with anyone else so the idea of using the battery was like telling me to stick a fork in an electrical socket.

I'll stick with the proper directions, but at least I know what he was talking about now.

@kyra4 - I saw that at Sears when I went to buy jumper cables, and I might pick one up prior to camping season
posted by subject_verb_remainder at 5:37 PM on December 29, 2011

As long as you clamp onto the battery *before* the other end completes the circuit, there are no sparks so you're safe. This is why most people clamp onto the frame for the last connection (which should be ground on the donor car). That'll also be the first connection you remove, so again, no sparks near the battery.
posted by introp at 5:44 PM on December 29, 2011 [3 favorites]

To piggyback a bit: does anyone know of documented cases of this sort of explosion occurring in the real world?

I've seen a small explosion occur during jump-starting - the battery was mounted in a plastic battery box, the lid was lifted off just enough to access the battery terminals, and the spark was enough to ignite the hydrogen & oxygen mix under the cover. A loud 'pop!', and the cover flipped over and across the engine bay.

I've seen the accident report on an actual battery explosion, though it was a stationary plant start battery. Short version is a plate was cracked internally, the operator thought it was flat and boost charged it; when that didn't work they attempted to jump start it. Because of the cracked plate and overcharging, the battery was basically full of hydrogen and oxygen; the pressure build up had caused the microcellular vent to lift out of its seat; and the spark from attempting to jump start it flashed back past the displaced vent, into the battery casing, and the battery exploded.

Which helps explains why battery explosions today are a lot rarer than they used to be. Old-style car batteries (up until ~70's) had just screw caps over each cell for topping up, and each cap had a small vent hole in it. That gives a nice path for any flash to travel back into the battery and cause an explosion; additionally, it was common for people to check the electrolyte level and leave the caps off before jump starting.

Instead of vent caps, modern sealed or "maintenance-free" batteries typically have a semi-porous microcellular vent which provides no flash path back into the battery. "Low maintenance" batteries typically have old-style caps to allow occasional topping up, but they're often hidden under a larger cap with a microcelluar vent which again provides no flash path back into the battery. If you look at a car battery and see a (typically) grey rectangular mesh/sintered insert or cover, that's the microcellular vent.
posted by Pinback at 6:05 PM on December 29, 2011 [3 favorites]

Keep in mind that, in a modern car, a lot of the metal bits may be either A) not actually metal or B) not actually attached to the car ground.
If, after letting the battery charge for a while, you still can't get a jump, make sure you're connected to the frame itself, not some trim disguised as such.
Also, you want bare metal, not the painted bits, which can interfere with getting a good connection.

Perhaps your neighbor was just cutting to the chase and connecting directly to the battery to avoid all of the above.
posted by madajb at 6:54 PM on December 29, 2011

As introp says, the basic idea is that the last connection of the four that you make, and the first you remove, can produce sparks and so they should be far away from batteries and especially from the battery being charged.

The other reason for the specific order that's printed on the jumper cables is to make it less likely that you'll short out a battery by accidentally touching a clamp to the car chassis (since you're probably working in the dark, in the rain, by the side of the road, at 3am, and haven't had to jumpstart a car since ever).

does anyone know of documented cases of this sort of explosion occurring in the real world?

A very brief google turns up plenty, eg this survey of cases from an opthalmology journal and this NIOSH case report were on the first page of results.
posted by hattifattener at 7:22 PM on December 29, 2011

Like pinback, I too have seen a battery cover filled with hydrogen go "bang". Like madajb, I'm also concerned that on a modern car I can't necessarily find a part that I can clamp to that's metal and attached to the car ground.

My policy has been to attach the negative terminal to the running car last, on the theory that the charged functioning battery is least likely to be venting hydrogen.
posted by straw at 7:25 PM on December 29, 2011 [1 favorite]

hattifattener has it.

Ground to chassis last, and well away from the battery and any gasses waiting for a spark. If a battery explodes on you it will change your life, so just clamp it over there mmmkay?

The part I always forget to observe is the removal order AFTER the jump ...
posted by intermod at 9:15 PM on December 29, 2011

Just to make it clearer: if you follow the ground cable from the dead battery of the car to be jumped, you'll see that it connects the battery to nothing other than the chassis of that car. Clamping the jumper cable to the chassis simply does what the dead battery's cable was doing already... but further away from the battery. There's no danger in that, aside from the exceedingly rare exception mentioned above. (It's also quite useful when starting an old car with no battery.)

As well, I suspect the instructions on the cable are more of the "you can't possibly screw things up if you do it this way" than the "this is what a mechanic would tell you" variety.
posted by matlock expressway at 10:38 PM on December 29, 2011

Official Car Talk Instructions
posted by TedW at 5:18 AM on December 30, 2011 [3 favorites]

Back in the days before electronic ignition systems and computer-controlled everything, you simply hooked the cables to both posts on both batteries and started up.

Today, though, the proper arrangement is to hook the negative cable to a ground on the dead car, usually on the chassis.
posted by Thorzdad at 6:55 AM on December 30, 2011

I learned this from my high school shop teacher, who told us a story of unverifiable truthiness about it happening in his shop class in years past... luckily with no injuries.
posted by utsutsu at 7:02 AM on December 30, 2011

Even before electronic and computer controlled everything, I was trained to always connect the last negative to the bare metal frame of the running car. It was even more important then, with the old batteries with the cells you had to add distilled water to.

Another thing we used to have to do was clean corrosion off the battery terminals. We'd use baking soda and water. I haven't seen a battery in years that looked all yucky, so that may not be needed any more, either.

Nthing letting the cars sit a bit, if the dead battery is really dead.
posted by QIbHom at 7:18 AM on December 30, 2011 [1 favorite]

In general, last connection is black to frame. However, the correct answer is "Read the car's manual" which will tell you if your car is different, and if it isn't, will almost always have a picture of a place on the car that is grounded and not near the battery.

Car batteries can deliver a great deal of current -- 500A or more. It is easy to hook three of them up it series and weld steel with them. The high current is why jumper cables are so thick, and so, when you make that final connection, a lot of current wants to hit a small spot, so you can get a great deal of heat happening very quickly.

So, read the manual, do what it says -- and use good jumper cables.

And, of course, if one car is dead and your jumping with another car, rev the running car so that the alternator is delivering power. If your using battery to battery, hook them up and wait a minute -- and wait one minute before each start attempt.
posted by eriko at 7:47 AM on December 30, 2011

Anecdata: I've jump started literally hundreds of cars (work at a car dealership). We always hooked everything up straight to the posts and never had an issue.

We knew that we were supposed to hook the last negative clamp up to the frame to prevent battery explosions but I only ever talked to one person who had heard of it happening and that had been a long time ago and it can be a PITA to get a good connection to the frame.

A lot of more modern cars (and even some as far back as 1995 that I can think of) have terminals located away from the battery. Sometimes it's for safety, sometimes it's because they mounted the battery lower in the engine bay where you can't get to it without moving a bunch of stuff.

TL;DR: It's safest to hook up to the frame but you'll be fine going straight to the terminal.
posted by VTX at 8:00 AM on December 30, 2011

Your data are not old. His is. Directly connecting to the battery is the old school way. If you have never worked on a car with a carb, you are not old enough to understand. ( so say Click and Clack)
posted by Gungho at 8:53 AM on December 30, 2011

Everybody else has the connection order thing covered nicely, so I'll just throw in a tangential point: I've seen jumper cables where connection clips are mostly plastic with little copper connection pads in the tips.

If that's the sort you have, turf them and buy some nice thick leads with proper metal connection clips. The plastic ones are indeed harder to create accidental brush-by short circuits with, but they're also completely friggin' useless for creating actual connections, especially to randomly shaped lumps of chassis metal.

Provided you follow the proper connection and disconnection sequence, it's really hard to make a brush-by short anyway. For what it's worth, I usually use the hoisting ring attached to the donor car's engine block to make the last connection. And do let the donor charge the dead battery for a full minute before trying the start; that way you don't end up trying to pull the whole starting current through jumper leads and four clipped-on connections, which is a pretty good recipe for excessive voltage drop and disappointing starter performance.
posted by flabdablet at 8:59 AM on December 30, 2011