"proper" use of jumper cables

[SteveDallas]

I've wondered about this for a while. I was reminded of it this morning when I found I had left my car keys on in the "accessory" position overnight, resulting in a call to my local auto club.

If you read the instructions for using jumper cables (read the instructions? BWAHAHAHA ) you'll find that you are supposed to attach the black clamp from the cables onto some grounded part of the car's frame, NOT to the negative terminal of the battery. Why is this recommended? And if it's so important, how come nobody ever does it?

Because if you fuck up, the battery can explode, leaving you with battery acid in your eyes.

[That Guy]

It's to prevent the hydrogen cloud from exploding when jumping the dead battery. The cables on the end of the live battery need to contact both posts, but the dead end needs to contact the positive post and a ground of the car (engine block, frame, etc) to prevent arcing.

[Tobiasly]

In more English terms, the last contact you make is probably gonna spark. Better for that to happen away from the battery, where an explosive hydrogen cloud could conceivably exist.

I'd bet the reason almost no one does it is that red to red, black to black is easy to remember. Live red to dead red, live black to dead ground is not as easy.

Considering how well-made today's batteries are, I'd say the likelihood of the battery blowing up is far lesser than the likelihood of your average driver forgetting which color goes to ground.

[xoxoxoBruce]

Considering how well-made today's batteries are, I'd say the likelihood of the battery blowing up is far lesser than the likelihood of your average driver forgetting which color goes to ground.

It's not a matter of the quality of manufacture. It's a function of lead/acid batteries to outgas hydrogen. Even the "sealed", never add water models do it. The "gel" batteries don't, plus they can't spill and don't corrode anything. But, "gels" cost almost twice as much.
I've seen batteries explode. It ain't pretty.

[BrianR]

I once dropped a wrench on to the hot terminal of a bettery that already had the ground hooked up. Welded the thing to the car frame and blew up the battery after three seconds or so.

Made a heck of a mess., as well as making me use up four boxes of baking soda to clean it all up.

Sheesh

I'm more careful now.

[SteveDallas]

That's kinda what I thought. But what I don't understand is, why do people like tow truck drivers do it? I've jumped maybe half a dozen cars in my time. Surely somebody who answers roadside assistance can do several per day. I'd assume they understand the risks, and that they don't want to "get lucky" and have the battery blow.

Oh, yeah. Most people are dumb as posts. I forgot. Never mind.

[juju]

So, you just hook the ground up to the car frame?

[xoxoxoBruce]

The best place is where the cable from the neg post attaches to the engine or frame. Unfortunately that's quite often on the engine near the fan, which is a hazard to cables and hands. When you hook the cables up the engine's not running, but hopefully it will be when you're done. It also helps if the car doing the jumping, is running.
One other caution. Some older foreign cars are POSITIVE GROUND. It's rare, and if you have one you're probably aware of it. BUT, if someone offers you a jump, be aware of their car.

[wolf]

The only time I ever had a problem jumping my car (postive to postive, negative to ground is a mantra for me) was when some "helpful" man connected the battery cables for me.

Nice guy, dumb as a post. As I was yelling, "You're doing it wrong" he cross-wired the cables, and blew a fusible link (only, thank goodness) in my car.

Dumbass.

[russotto]

Gel batteries (and AGM) CAN outgas, particularly during overcharge. They don't in normal operation, though.

As for the connect-to-ground advice, I've tried it. On most cars I've tried, it doesn't work; battery to battery is the only way to do it. Positive on good to positive on dead, negative on dead to negative on good, in that order.

Connecting to the starter housing or other well-grounded thing should work too, but getting to those parts is more of a hazard than a possible spark.

[xoxoxoBruce]

Gel batteries (and AGM) CAN outgas,

How so? I thought these were completely sealed, hence safe to use in the trunk or passenger compartment, even upside down??

[tw]

Hydrogen gas problem is but another minor reason why a spark is not desireable adjacent to a battery. (Where is hydrogen concentrated when fans and engine move so much air?) But if the battery connection is via a frame, then a possible short circuit is not really a short circuit. That wire (battery to frame) slows the battery discharge meaning that battery is either less likely to explode or that the explosion would not occur as immediately - human having time to correct his mistake.

Also connecting a black wire to chassis makes it difficult to short two batteries negative to positive - making destruction of vehicle electronics less likely due to jumper leads reversed.

Problem; cars provide no good point to clamp that ground wire. Either good clamping point does not exist or jumper wire gets tangled up with fan belts, etc.

Batteries have an interesting design. Built so that batteries tend not to explode up. Instead weaker points are on the side where an explosion is less likely to take out a human face.

[Griff]

If you have two cars with metal bumpers, you can hook up the positives and make the final connection from your drivers seat. Saftey first kiddies!

[xoxoxoBruce]

(Where is hydrogen concentrated when fans and engine move so much air?)

If the fan was running we wouldn't have to jump it, would we.

That wire (battery to frame) slows the battery discharge

Not so. That wire is larger than most jumper cables so it's not the limiting factor.

Also connecting a black wire to chassis makes it difficult to short two batteries negative to positive - making destruction of vehicle electronics less likely due to jumper leads reversed.

Nope. See above or better yet weld one yourself.

Built so that batteries tend not to explode up.

What explodes is the hydrogen gas which above the liquid level. It blows the top off. I've witnessed it twice and seen the aftermath of many more.
The problem with clamping to the frame or even through metal bumpers is that the resistance can be to great. If the battery is marginal it may work but if the battery is way low, part of the juice is trying to charge the battery and part is trying to start the engine. The low battery provides the path of least resistance thus starving the starter.
Quite often the reason the battery is discharged in the first place is corrosion between the battery and the cable ends which prevents charging. This corrosion can also prevent jumping. I had to needlessly buy a starter to learn that one.