Jumpstart: Why does BLACK get grounded and not hooked to NEGATIVE on dead battery?

Is there a difference? Black terminal on the dead car is grounded anyway.
So what’s the deal with hooking the black cable to the chasis instead?

It is to give you one more chance at not exploding combustive gasses from the battery by a wayward spark.

A dead battery produces hydrogen gas. If the final negative connection is made on the negative battery post and a spark is created the battery can explode. That’s why they suggest the final negative connection be made somewhere away from the battery.


When I was 19 a battery exploded on me as I disconnected jumper cables from it.

  • The whole top of the battery blew off - shooting plastic shrapnel all over the place.
  • Battery acid sprayed all over me, including my face.
  • The sound was like a cherry bomb going off next to my ear.

I flushed my eyes out for a long time under the big sink we had. I’m lucky to have my eyesight.
Ever since that day, I always make the last clamp connection to a ground point - located away from the battery.

Batteries are more likely to explode when disconnecting a big battery charger without shutting it off first…But any spark around a battery has the potential of igniting the hydrogen gas that is given off by any lead-acid battery when it is being charged…The more vigorous the charge, the greater the chance of an explosion…

By not connecting directly to the battery, there is little chance of a battery explosion…

Ok, got it.

Oddly, the only spark I ever see is when I connected the RED.
I have never seen spark come from the BLACK terminal.

Oddly, the only spark I ever see is when I connected the RED.
I have never seen spark come from the BLACK terminal.

That tells me you’re connecting the RED terminal last. Not the best choice.
The last (4th) clamp should be a ground (BLACK) clamp and it should be on a grounded part of the vehicle.

…and yes, there will be a spark on the last (4th) connection to the dead vehicle’s ground, especially if the dome light/headlights/etc are still switched on (causing the dead battery in the first place)

I have never seen spark come from the BLACK terminal.

This is because of the way electricity works. A well designed circuit will switch the GROUND side of the circuit rather than the POSITIVE side because there is a higher potential at the positive side. Once the current gets through the load and to the negative side of the circuit there is less likelihood of a spark because the voltage has already had to pass through the load and do work of some form.

There is still a possiblity or even likelihood of a spark at the negative terminal, but the spark is almost always smaller at the negative end. This is why connecting the negative terminal last, and to somewhere away from the explosive gas producing battery is far safer and recommended.

JayWB, it doesn’t matter in the slightest what side of the circuit is switched for the purposes of jump starting a vehicle. You will still get a spark on the last connection you make when you complete the circuit. The electricity doesn’t get ‘tired’ after going through the load–you can measure current at either end of the circuit and it will be the same. And if the circuit is broken at either end, you should see the same potential between the points that complete the circuit when measuring with a voltmeter. You might get a bigger spark if you’re breaking the circuit on an inductive load that stores EMF, such as a motor, relay coil, solenoid, or ignition coil, but that doesn’t really apply either for the purposes of jump starting a vehicle.

The only slight difference on a DC circuit may be the direction that the spark ‘sprays’ when you make that connection. And if your hands are steady when you make that last connection, you may not even see a spark–the spark occurs when the connection is made and then broken, such as when trying to establish a connection on a painted, dirty, or somewhat corroded piece of metal. Typically when jumping a car, I wiggle the last connection around on the ground point, hoping to see a spark, because that means I’ve got a good connection and the dead battery is likely charging.

The danger of blowing up a battery is pretty minimal on a cold day, outdoors, with the hood open and the wind blowing, and the last connection made on the dead battery. On a hot summer day with the air kind of still, it’s a little worse. Still the odds are in your favor, but it’s still a good idea to make the last connection away from the battery, and turn your face away from the battery when doing it. I’ll admit that I’ve made the last connection to a battery terminal before when jumping a car and not being able to find a good connection on the body, but it’s still not the right way to do it.

Hey Used- I think the answer to your question is that there is exactly no difference (electrically) on whether you hook the negative jumper cable to the battery or to a good ground on the frame. I always hook up the cables to the dead battery first then hook up the good assist battery, this way you’re away from any potential hyd explosion, also if someone isn’t careful and while you have the dead battery hooked up to the cables and the other two cable ends touch each other, the spark and potential damage will be less severe than if the two free ends touch each other from a running vehicle.

Oblivion, I’m sure you’re right.

That would require that my college professors were wrong though when I went to school for electrical engineering. I prefer to believe them, thank you just the same.

The emf is higher on the positive side of the load. Once it has gone through the load there is no emf. The circuit is not at ground potential until it’s actually grounded, and it certainly doesn’t get “tired” as you say, but the potential is far smaller.

But hey, you connect a battery any way you like. I’m just trying to instruct the correct way, and why it’s correct.

To summarize, you should connect the RED first, and then the BLACK.

My understanding from my college classes is the way Oblivion describes it.

The voltage at the open terminal is the same whether the last connection is the pos or neg clamps. The size of the spark that jumps across the gap (as you clamp that last connection) is will be the same with either polarity clamp.

@bennyandthejets, I’ve always heard hook to the hot battery first, this has always been my way of doing it. I also like to make all the connections myself to prevent cross polarity. If someone else is hooking up one end of jumper cables and I’m hooking up the other, I ALWAYS confirm by asking if they are putting red to positive and black to ground. My jumper cable clamps and charger clamps have a plastic coating on them so the only potential hazard of them touching one another is the small area that actually comes in contact with the battery posts/ground. When using a battery charger I also wait until the connections are made from the charger to the battery before introducing current to the charger and turn off current to the charger prior to removing the charger cable clamps.

@JayWB: I also have studied electronics. While you may be technically right, when you’re dealing with 12 volts, and at the battery, not at the end of any one circuit, any difference in potential is going to be negligible at best. We’re not talking a pole transformer fed by a HV line here. Try it–you will get just as big a spark whatever connection you decide to make last. If you can tell the difference, you are blessed with far better perception than I have. For that matter, if you have a coil connected to a battery, it hardly matters which side you disconnect–the field will still collapse and you will get just as much of a surge from the stored energy. The only difference is which way the current will be flowing for the surge, and thus which direction the spark will spray. Having switches ground a circuit in an automobile may be slightly better for the longevity of the switch contacts (very slightly), but with all circuits ending at the battery terminals, it’s like trying to notice the flush of one toilet at the end of the sewer line, assuming accessories in the car have been left on.

As (hopefully) you know, the reason for making the last connection the negative one, is the vehicle’s chassis being connected to the negative pole of the battery, so there’s less danger of slipping and making a really impressive spark should the chassis be contacted with the positive cable, and ensuring that the last connection to be made is far from the battery.

As far as the direction that the electrons travel in DC current, I learned both in college and in the military, that it is all theory; some schools of thought say that the electrons travel pos to neg, and others teach the opposite. There is (to date) no way to prove which direction those little rascals travel. But in reality - what difference does it make?

My wife’s BMWs have had remote jumper terminals under the hood, as the battery is located in the trunk. It would be nice if all cars had this, even if the battery is located in the engine compartment, for the safety reasons mentioned here. It seems like the extra cost and weight would be minor in comparison.

Battery explosions are so rare, it’s really not an issue…A one in 10,000 event…

@Caddyman Unless your the one in 10,000 it happens to! :slight_smile:
The actual number of jump starts being done nowdays has to be way down too with cars having electronic ignition and fuel injection. I can’t recall the last time I had to have one or give one. My jumper cables are just becoming dead weight in the trunk.