Can I clamp two jumper cables together to jump a car?

December 30, 2009 5:34 PM

Dead car battery, cable isn't long enough to reach - but I have another cable. Can I combine the two?

I've got a car in a garage that I cannot get out of park (in order to push it back out), and its battery is dead because I left a light on. I have two jumper cables that, combined, would be long enough to reach another car parked right behind the dead one. Can I simply clamp the two cables together? Does this work? Or will I have to go find a longer one?

Thanks!

posted by ORthey to travel & transportation (24 answers total)

Yes, should work fine, just be careful not to let the red clips touch the black ones.
posted by Long Way To Go at 5:35 PM on December 30, 2009 [1 favorite]

Yeah, this is essentially like combining two extension cords.

But the point about keeping the red and black clips separate is critical.
posted by dfriedman at 5:38 PM on December 30, 2009

Why don't you just switch the battery out with the one from the other car, get the cars lined up, switch the battery back in & jump it?
posted by torquemaniac at 5:39 PM on December 30, 2009

Batteries are heavy. It would be a lot easier to just connect the two sets of jumper cables, rather than swapping batteries back and forth.
posted by Forktine at 5:43 PM on December 30, 2009

Feel totally free to clamp the cables together.
Just MAKE SURE that they don't short together (black touching red by accident) and make sure they don't hit the body of the car. If the red cable contacts the metal body of the car while it's all hooked up, you'll get a spark show.

Also, your car that's stuck in park will probably have a manual release. On most consoles, by the shifter, is a little panel marked "shift interlock release" (or something similar) If you pop it open and put your car key in the slot, you can shift out of park, even if there's no power to activate the shift release solenoid (which is what's normally activated by pressing the brake). So, you might find it easier to do that, put the car in neutral, and roll it out of the garage rather than clamp your jumper cables together (which could get a little risky).
posted by Jon-o at 5:46 PM on December 30, 2009

This sounds like a disaster waiting to happen.

Generally speaking, a starter can draw 250 amps or more to start the engine. The most dangerous range of currents is from 0.1 to about 0.2 amp. Currents in this range can cause death by initiating fibrillation (uncontrollable twitching) of the heart, which stops the regular flow of blood to the rest of the body. Currents much larger than 0.1 amp do NOT result in fibrillation and instead stop the heart.
posted by JujuB at 5:47 PM on December 30, 2009

...and there is no way on God's green earth that any human body is ever going to draw more than 0.1 amp from a 12V power source without help from large adhesive electrodes and contact gel.

Jump-starting cars can be dangerous to their engine management computers, but it's not going to stop a human heart.
posted by flabdablet at 6:08 PM on December 30, 2009 [3 favorites]

Leave it charging for a good 5-10 minutes before trying to start. Usually makes a big difference even with a short distance between the cars.
posted by sammyo at 6:09 PM on December 30, 2009

Seconding what sammyo just said. This is good practice for any jump start. Jumper leads make far better fast-charge cables than actual starter cables.
posted by flabdablet at 6:13 PM on December 30, 2009

Wait a minute, here. You're not going to kill yourself with a car battery. A starter motor has a much lower resistance than a human body. A car battery doesn't have even remotely enough voltage to push those amps through you. The most likely kind of injury that's likely to occur when handling a car battery is when you accidentally cause a spark (with a wrench or jumper cables) and you're burned or zapped by the arcing current. When removing and installing car batteries, I've touched both terminals simultaneously with bare hands and nothing has happened. Not even a tingle.

In this situation, with the multiple jumper cable set up, the most damage I see happening is burning the paint off the body when the cables accidentally contact.
posted by Jon-o at 6:15 PM on December 30, 2009

Please feel free to ignore JujuB's ridiculous assertion that your heart will stop with all this.

The only problem you may have (assuming you are super careful with no shorting out of the cables to each other or the car body) is too much cable resistance preventing a fast enough turn over on the starter to get the car to fire. As mentioned, let the primary car idle, connected, to the secondary car for a few minutes first to allow a basic charge to be put in the flat battery and you should be fine.
posted by Brockles at 6:16 PM on December 30, 2009

JujuB's concern is actually disproved* in their link but there are two practical concerns. Everyone above has already flagged the first: Most jumper cables have the red & black clips at equal lengths, so if you clip one cable to another all the clips will be right smack against each other and it becomes stupid-easy to have a short.

The second concern is the contact resistance when you clip the cables together. 250A makes for a pretty good stick welder if there's any resistance in the splice. Make sure you get really good contact between all the clips, or you may accidentally weld them together (that's actually best-case; worst-case is a fire).

*.1A is certainly enough current to kill you, but the presence of 250A in a wire doesn't mean it's getting into you. For starters, you'd need a path through you body back to the battery to do any real damage, and you'd have to really put your mind to it to get good contact between your body and the wires.
posted by range at 6:20 PM on December 30, 2009 [3 favorites]

This sounds like a disaster waiting to happen.

This isn't even a mild jolt waiting to happen. This person has no idea how electricity works OP, this isn't going to hurt you.
posted by floam at 6:21 PM on December 30, 2009

This sounds like a disaster waiting to happen.

This isn't even a mild jolt waiting to happen. This person has no idea how electricity works OP, this isn't going to hurt you.

Yes. The 12 volts from a car battery is not enough to conduct through human skin.
posted by Menthol at 6:28 PM on December 30, 2009

Thanks for all the replies - it didn't work, for some reason. The dead car flickered lights on and off and wouldn't start. I was very careful to hook everything up correctly... so I'm not sure what went wrong. I may try again, letting the good car idle for a longer period.

I thought of the override, but there's no button on this car (it's a new subaru outback). There must be a way, though.
posted by ORthey at 7:09 PM on December 30, 2009

I dunno about current Subarus but on my 2000 Forester to override the automatic trans with the battery dead you remove the plastic around the shifter and you have to push the screwdriver included in the road kit down to hit the shift interlock release. It should tell you how in the manual.
posted by ghharr at 7:26 PM on December 30, 2009

OK - I solved it. I'm an idiot. ghharr, you are right, there's a little plastic thing that I stuck the key into and got it out of park, backed it out, and jumped it normally. I really have no idea why I didn't try that earlier.

Thanks for all the responses. I'm curious as to why it didn't work with the double cables - I probably did something wrong.
posted by ORthey at 7:36 PM on December 30, 2009

I've seen jumper cables where the clamps are all plastic except for a pissy little piece of foil right in the tip, which is allegedly supposed to contact whatever you're clamping onto. I could easily understand a pair of those failing to make good contact.

To start a car, you need as little electrical resistance between the battery and the starter motor as possible. If you've got double-length jumper cables in the path, you've got six sets of metal-to-metal contact plus extra cable length in the conduction path, all of which could add up to rather more resistance than the starter motor itself has. This will mean that the target car's starter doesn't see anywhere near the donor car's full battery voltage, which means it won't draw enough current, which means it won't be able to develop enough torque to turn the motor over.

Using the jump leads to fast-charge the target car's flattened battery avoids this, because you can still get useful amounts of charge into that battery even if cable resistance limits the charge rate fairly severely. If the target battery is dead flat - i.e. it won't even light up the interior light before you charge it - you will probably need to leave the donor car jumped to it for at least ten minutes before it collects enough charge to get a start.

Best practice is to avoid jumper leads altogether, and charge the target car's battery with a low-rate (say, four amp) mains powered battery charger until it's full. This takes time. Overnight generally works.
posted by flabdablet at 7:53 PM on December 30, 2009

Glad you were able to get it out.

I also heard someone mention on Car Talk the other day that you can skip jumper cables altogether and just touch the contacts of one battery to another, but Tom and Ray seemed to think that was a pretty dangerous option and I don't think I would have the balls to try it.
posted by ghharr at 8:04 PM on December 30, 2009

It is possible to get a jolt from a car battery. My sweaty knee was touching the metal bumper (which is part of the negative ground system) of my 70 Impala, and when I happened to rest the back of my hand on the positive battery terminal, I started feeling...funny. Then my leg and hand started to buzz. Then I jumped back and figured out what the fuck.
posted by notsnot at 8:39 PM on December 30, 2009

So let's say you've got only one cable, and you're needing to jump the car. You can touch the two bumpers together (old cars you could anyways, with steel bumpers) and that would take care of the negative ground, attach the one cable to both positives on each respective battery, you're golden. I know that most bumpers are plastic anymore, just an fyi there -- any ground to any ground will work...
posted by dancestoblue at 8:55 PM on December 30, 2009

One possible reason the car didn't start even with the doubled-up cables is that there was corrosion or an otherwise bad connection, either where the cables were clamped to one another or at the battery terminals themselves. It is possible to have an electrical connection from one battery to the other that isn't an open circuit but still doesn't allow enough current through for the engine to turn over. I once had a car with corroded battery terminals that occasionally wouldn't start even with a good charge. The dome light would come on, the radio would play, it would try and crank but wouldn't quite make it, whereupon I would pop the hood and wiggle the terminal clamps around with a pair of vice grips, scraping through the corrosion and re-establishing a good connection. Then I'd get in and it would start right up.
posted by contraption at 10:36 PM on December 30, 2009

It's not that you can't feel a shock from a 12V battery, it's that you have to go way out of your way for that shock to be anywhere near dangerous. Hell, you can get "shocked" by a 9V battery, as anyone who's licked one can tell you. (Note: I do not recommend licking a 12V car battery unless you want to visit the burn ward followed by the poison control unit.)

I heard the Car Talk no-jumper-cables story too -- they forgot to point out the consequences of screwing up. In this case that means accidentally reversing the top battery (connecting + to -), which I bet is easier that you'd imagine since you have to flip the top battery upside down. If you did that, you'd see two batteries dump out energy as fast as they could, at least until (a) the terminals vaporized, or (b) one or more cells exploded, ending the chemical process. I suppose you could debate whether you'd rather get hit by flying, vaporized metal or by high-speed hot sulfuric acid, but I suspect they're both pretty bad.
posted by range at 11:03 PM on December 30, 2009

This sounds like a disaster waiting to happen.

This is such appallingly ignorant fear-mongering that I have to echo: the voltage of car batteries is not dangerous to humans.
posted by anadem at 1:18 AM on December 31, 2009