As pointed out in Jump starting a dead battery: Connect black wire to negative pole of battery or grounded metal?, you connect the black jumper cable to the dead car's chassis to avoid sparks made during the connection near the battery, which could be surrounded by hydrogen gas. As Paul points out in Why should I connect the red/positive terminal first when jump starting?, this seems extremely unlikely, but okay, safety first.

However, the advice is to attach the black cable to exposed metal on the chassis or engine block. The engine block on the average car is not that huge. I'm wondering, if the battery is surrounded by a cloud of hydrogen gas, and I connect the black terminal somewhere on the opposite end of the engine block, would it not still ignite the cloud of gas? Is the cloud of gas that tightly focused around the battery? Also, what about when you actually manage to start the car? Why is there no risk of igniting the gas cloud from the various heat- and spark sources when a car is started?

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    Its at risk from any ignition source.
    – Moab
    Commented May 11, 2016 at 0:23
  • There shouldn't be any sparking under the hood when you start your car under normal circumstances. Other sources of heat (e.g., the exhaust manifold) simply aren't hot enough. The temperature of a spark is thousands of degrees. Commented May 11, 2016 at 19:08

3 Answers 3


I am no expert in this topic, but I don't think a "cloud of gas" forms around the battery. Hydrogen gas is the lightest gas of them all (0.089 g/l) and won't pool up if leaked. It will simply rise as long as the bonnet is open because it is less dense than air, which is mostly much heavier nitrogen (1.25 g/l) and oxygen (1.4285 g/l). My guess is that the practice of connecting recipient's ground to the chassis rather than the negative battery post simply protects from igniting some minute leakage from the battery itself.

There shouldn't be any open spark sources when the car is started. Alternator, spark plugs, spark plug wires, distributor and alternator shouldn't openly spark if they're in good condition, and they're usually far enough from the battery anyways. The heat shouldn't be nearly enough even if for some reason the engine you're jump starting is hot. Hydrogen spontaneously ignites in the atmosphere at ~500 °C, under hood temperatures shouldn't go much above ~100 °C

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    Hydrogen will float straight up, and move pretty quickly. There's just no way for it to "make its way" over to the engine block.
    – JPhi1618
    Commented May 10, 2016 at 12:43
  • @JPhi1618 That's true. But I've been wondering if theoretically some hydrogen from an old battery could pool up under a closed bonnet (assuming it has no holes in it) and be ignited by old and frayed arcing spark plug wire while driving slowly. Commented May 10, 2016 at 13:24
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    I bet that it can. When I was a kid, the battery in my moms car exploded and dented up the hood right when she tried to start the car. I bet there was a collection of gas that was ignited by something sparking when the key was turned. There might be too much air flow for this to happen when the car is moving, but it certainly happens to a car sitting still.
    – JPhi1618
    Commented May 10, 2016 at 13:27
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    The line "under hood temperatures shouldn't go above a fifth of that" isn't quite right. The boiling point of water at standard pressure is 373.15K ,nearly half of 773.15K (aka 500 °C). And only absolute-value-based measurements make sense to describe "a fifth of" a temperature. Commented May 10, 2016 at 18:52
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    @MontyHarder You're right, that wouldn't make sense in a scientific background. I'm sure there would be a bunch of people stating that "100 °C is not half of 500 °C", though. I'll just rephrase if it's that confusing. Commented May 11, 2016 at 7:02

This is going to get a bit technical but should still be understandable even if you slept through chemistry classes.

When is hydrogen dangerous?

Just as with the fuel-air mixture in an engine, hydrogen is combustible only when it's within a range of concentrations. We use what are called the Lower Explosive Limit (LEL) and the Upper Explosive Limit (UEL) which are the concentrations of gas (in air) that will ignite. For hydrogen gas, the LEL is 4% and the UEL is more than 75%, meaning that if the concentration of hydrogen gas reaches 4%, it is at the level that could ignite with a spark. For comparison, the natural concentration of H2 in the atmosphere is about 0.01%.

How does a car battery produce hydrogen gas?

A car's battery is filled with a mixture of 36% sulphuric acid (SO4) and 64% water (H2O). Hydrogen gas is released when the battery is charging due to a process called electrolysis in which water decomposes into its constituent hydrogen (H) and oxygen (O) in the presence of a voltage that is greater than the decomposition voltage equilibrium. That voltage is 1.227V for water. The normal voltage for a single lead-acid battery cell is 2.1V and there are six of them in series in a car battery (6 x 2.1 = 12.6V). Because the cell voltage is higher than the decomposition voltage for water, very small amounts of gas are almost always being produced. However, the worst case (most hydrogen) is produced when forcing maximum current into an already fully charged cell. It's also a temperature-sensitive reaction, so higher temperatures will tend to produce more gas.

How much hydrogen is produced?

The reaction we're interested in is the one for hydrogen ions (H+) becoming hydrogen gas (H2). Chemists would write it like this:

2H+ + 2e- --> H2

This just means that two hydrogen ions plus two electrons yields one molecule of hydrogen gas. (It also implies that the the gas comes from the negative electrode, but remember this is per cell.)

Skipping much of the mathematics and chemistry details, we can calculate that at a temperature of 25C (77F), we produce about 0.45 liter of hydrogen gas per cell per 1 Ah (amp-hour) overcharge. So pushing 10A through a fully charged standard 6-cell battery for one hour would produce 0.45 l/Ah x 6 x 10A x 1 h = 27 l of hydrogen gas at 25C. For that to be dangerous, we'd need at least a 4% concentration, so we'd need a total volume of air + hydrogen to be 675 liters or less. Jump starting doesn't usually occupy a whole hour and usually doesn't involve overcharging an already charged battery.

Where does the hydrogen go?

As anyone who has ever heard of the Hindenburg disaster knows, hydrogen is both lighter than air and also combustible. Because it's lighter than air, any hydrogen gas released from the battery will tend to rise.

The bottom line

In the typical under hood placement of a car battery, most of the hydrogen will rise harmlessly into the atmosphere as soon as the hood is opened, but more may be produced as the car is being jumpstarted. That's why the hoods should stay open and why it's also good practice to first connect (and first disconnect) the negative connection as far from the battery as practical.

Automotive designers have already gone through these calculations and the concentrations under the hood are much much lower than 4% even in extreme situations of charging system malfunction. For instance, standards such as IEEE 484 describe a design target of no more than 2%.

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    Great answer! As I understand the amount produced while jumpstarting should be ridiculously small and the only real danger is if the alternator's voltage regulator is overcharging the battery while driving. Commented May 10, 2016 at 13:42
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    It's not even a danger while driving because air will be flowing over the battery due to the forward motion of the vehicle preventing any buildup of hydrogen gas. The danger is highest when attempting to jumpstart the car with a rapid charging external charger, but the odds of a free-space explosion are exceedingly small -- the real danger is an explosion inside the battery.
    – Edward
    Commented May 10, 2016 at 13:48
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    "the real danger is an explosion inside the battery" I have replaced a few of those exploded batteries, Usually the electrolyte is really low when they explode.
    – Moab
    Commented May 10, 2016 at 14:13
  • 1
    @Moab: Yes, that's right. The lower the electrolyte, the more volume for gas buildup within the battery. It's one of many reasons that the electrolyte should be topped up before charging.
    – Edward
    Commented May 10, 2016 at 14:16
  • This is an incredible answer, and I wish I could upvote you more. Many thanks!
    – Bas
    Commented May 10, 2016 at 14:40

Firstly, the reason for connecting the positive jumplead before the negative jumplead is that so that if it slips off, you will notice this before connecting the negative one. If you connected the negative one first and then the positive one, one of the positive crocodile clips could slip off, make contact with the chassis of one of the vehicles, thus shortcircuiting the battery and causing an immense current to flow, resulting in a fire. With modern shrouded jumpleads this is less of an issue.

Secondly, the most important reason for connecting the negative jumplead to the dead vehicle's engine block is for a better connection to the starter motor. The starter motor's ground is connected via its chassis to the engine block, therefore most effective starting occurs when the jumplead is connected there. Connecting to the battery terminal introduces a longer path from the jumplead to the starter motor, so is to be avoided as it makes starting less effective.

On the other hand, connecting the negative lead to the engine block increases the resistance to the dead vehicle's battery, avoiding excessive charging current. Connecting the negative lead to the battery of the dead vehicle would give a higher charging current (but as explained above, a lower starting current.)

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    If there are significant ohms in the natural path from the starter back to the battery, why would it be done that way at all? I think it's a very large path of metal, thus very low resistance. The actual connection resistance would be dominated by how well the clip makes contact, and the petina and grime on the block is worse than a terminal designed for that purpose.
    – JDługosz
    Commented May 11, 2016 at 5:10
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    @JDługosz there's thick wire between battery & chassis, so there's under 0.1 ohm. But when starter motor uses 100s of amps it can make a difference. Designer will have sized that wire to be "enough" with a charged battery. With discharged battery & corroded wire working against you, it can be easier to start if you connect direct to the engine block instead of the battery. Every little helps. mechanics.stackexchange.com/a/1424/11767agrees with. I've also seen someone have more success by connecting the positive direct to the starter motor but that's inconvenient and potentally dangerous Commented May 11, 2016 at 21:13

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