Take the 2-minute tour ×
Motor Vehicle Maintenance & Repair Stack Exchange is a question and answer site for mechanics and DIY enthusiast owners of cars, trucks, and motorcycles. It's 100% free, no registration required.

While I've given and received boosts before I've always been following instructions by rote and never really understood what's happening.

The procedure I've been told is:

  1. Attach red clamp to the positive terminal on the dead battery.
  2. Attach the other red clamp to the positive terminal on the good battery.
  3. Attach black clamp to the negative terminal on the live battery.
  4. Attach the other black clamp to the engine or chassis on the car with the dead battery.
  5. Start good car.
  6. Hopefully start dead car.

I'm somewhat confused by step 4. since it's often described as clipping the last black clamp to "ground". But the tires should be insulating the car so it's not really grounded is it?

So what's actually happening when you start the dead car? Is it drawing power from the good battery to start the car? I don't understand how that happens since I don't see how there's a complete circuit between the two cars.

share|improve this question
1  
A quick process point - I would always start the good car before attaching the wires, as otherwise if the good battery is slightly low you might have trouble starting the good car (current may be drawn through the bad battery from the good one) –  Rory Alsop Jan 24 '12 at 10:11
    
@RoryAlsop is right - you'll notice that the tow truck drivers always start their vehicle before they give you a jump start. –  Bob Cross Jan 24 '12 at 12:58

3 Answers 3

up vote 3 down vote accepted

Here's a diagram. It shows two batteries connected to each other and wires from each battery going to each respective car. What the diagram doesn't show is that those wires, "To Starter" --> "Ground" form a loop as well, and if you picture that, you will see that the path forms a full circle.

To be slightly more precise, the path forms two circles, like digit 8. The two batteries in this case serve as the common path between the circles, so both batteries end up in the path of both cars, connected in parallel.

This stuff would be a little easier to understand if you read up a bit on basic electronics. As for "ground", they don't mean "earth", this is an electric term which means common reference point, which is often the body, or the case of the device. Almost everything has a ground, your car, your TV, your laptop, even tiny little circuit boards generally have a grounding path which runs along at least one edge.

When one of the cars is running, that car has two voltage sources: 1) the battery and 2) the alternator. When the other car is hooked up, those two sources will be combined with the 3rd, negative source (i.e. drain), non-working battery, and the combination of the voltage pressure is what's going to start the second car.

This voltage pressure applies to the starter of the second car as much as it applies to the old battery. Usually this is fine, because batteries are designed not to flow backwards, but in some cases, when batteries goes bad, they drop in internal resistance and in that case, majority of current goes through the old battery instead of the starter causing the car not to start. In those circumstances it generally helps to disconnect the old battery and trying to jump start again.

share|improve this answer
    
Why is the negative terminal connected to the car frame? Doesn't this mean there's always electricity flowing through the car? I don't follow what you mean when you say you should "disconnect the old battery" and try again. Do you mean actually remove the battery? –  Gregory Bell Jan 24 '12 at 16:22
    
@GregoryBell: yes, there's electricity flowing at all times through the car body. But if you consider how much electricity 12 gauge wire can carry and compare to the amount of metal the entire car body has, you'll realize that it's really not all that much. Also the amount of metal gives all circuits a path of least resistance back the the negative terminal of battery/charging circuitry and halves the amount of wiring you would've needed to have otherwise. As far as disconnecting the battery, what I meant is unhook the ground (-) terminal from old battery and leave it hanging. –  DXM Jan 24 '12 at 16:49
    
@Gregory Bell: Why is the negative terminal connected to the car frame? The last step, connecting to the car frame, completes the circuit between the two cars. This is the connection that can cause a spark when it is made. Lead acid batteries produce hydrogen gas, and a nearby spark can cause them to explode. The last (potentially sparking) connection is made away from the battery to eliminate (or at least reduce) the chance of an acid throwing explosion. Note the disconnections should be made in the opposite order of connections, since the first disconnect can also spark. –  Shannon Severance Jan 24 '12 at 19:02
    
See the first two paragraphs at: en.wikipedia.org/wiki/Jump_start_(vehicle)#Limitations –  Shannon Severance Jan 24 '12 at 19:05

DXM's answer hits all the important points in great detail. To give you a quick answer to your specific questions at the end, though:

So what's actually happening when you start the dead car? Is it drawing power from the good batter to start the car?

Remember, the live car is running. Its alternator is being driven by a functioning engine. When you go to start the dead car, it is drawing power from that fully energized electrical system rather than from its flat battery (not the battery on the good car). That generally provides plenty of power to start the dead car.

A hard lesson follows that I've learned at least two times in my life:

Remember, however, that while the dead car is now alive, its battery is still flat and extremely suspect. Even if the battery is fine, it still has to charge for a while. If you have a battery charger, bring it home and put it on the charger for a while.

Even so, batteries are consumable parts. They have a finite lifespan and, sometimes, they are letting you know that they plan to leave you stranded again unless they get replaced. The smart people I know write the installation date of the battery right on the cover in Sharpie. If it's been five to seven years, there's a good chance that the battery is dying on you.

Sadly, I've never been that smart.

share|improve this answer

Your negative terminal connects to the frame of the car. This allows you to run a single wire to some piece of electronics and "ground" the other side to the frame of the vehicle (which runs to the negative terminal of the battery/alternator).

By making the last connection to the frame of the car, you complete the circuit just as well as you would have on the battery post (provided it's a rust free, paint free piece of frame). The last connection should cause a spark as you connect it, and by using the frame of the car you ensure that spark is away from the battery (which is full of hydrochloric acid and vents explosive fumes).

So now you should understand the complete circuit. The actual jump start is dead car's starter motor drawing power from good car's alternator. If your jumper cables aren't a thick guage, you might not be able to push enough current to actually start. In this case, you have to let it sit as good car's alternator charges dead car's battery enough such that dead car's starter can get enough current from dead car's partially charged battery and good car's alternator across crappy jumper cables.

To ensure a quick jump start, always buy the largest gauge jumper cable they sell.

share|improve this answer

Your Answer

 
discard

By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.