Jump-starting a car: connecting black to the engine block

Question

I recently found out that when jumping a car, you should not connect the black connector to the recipient vehicle's battery, but rather to a grounded point on the car. The expressed purpose is to avoid a spark near the battery, which may potentially be producing hydrogen gas. Some have argued that you can connect to anywhere on the frame. Others have expressed problems with this (probably due to thin grounding wires) and recommend attaching to the engine block.

As an electrical engineer, I am taught to never rely upon a body ground to transmit current. There's plenty of issues with doing this, including the fact that mechanical connections and grounding wires are rarely designed for the high amperage. But this seems to be a special case.

Do automotive manufacturers plan for this sort of jump start as part of their requirements? For example, this would mean making sure that there is a sufficiently robust connection between the engine body and the ground side of the electrical circuitry to handle that sort of amperage. Arguably this connection would need to be as low of a gauge wire as the wires to the starer motor themselves.

Answer 1

The question says

mechanical connections and grounding wires are rarely designed for the high amperage

The ground cable goes directly (and only) to the frame in this picture

^{Image from Quora}

So all the power requirements take this route, notably the starter motor.

In my experience, it is usually easier and more secure to connect to a frame point than to the battery terminal.

Answer 2

Compare the circuits formed by the two alternatives:

• positive jumper cable - positive recipient battery terminal - starter relay - starter windings - engine block - chassis - battery ground cable - negative battery terminal - negative jumper cable
• positive jumper cable - positive recipient battery terminal - starter relay - starter windings - engine block - negative jumper cable

The difference highlighted in bold will certainly have some positive nonzero resistance, so skipping it reduces the losses in the high-current path through the cranking starter. In contrast to that, charging the dead recipient battery is not a primary goal while jump-starting, so a slightly higher resistance in that circuit does not matter.

Answer 3

If you examine your car or any other vehicle, try tracing battery ground cables. Most, if not all, battery ground connects to the chassis nearby, Chassis has a ground cable to engine block. From your engineering background and business sense, using the 4 or 6 gauge stranded copper wires would fulfill electrical demand (greatest) from starters, meet electrical designs for power distribution and reduce wire lengths for reasonable cost savings. At least one auto manufacturer recommends boosting a vehicle; connecting the positive cable to battery positive (both batteries), negative boost cable from boost battery to the engine block lifting bracket or other engine bracket mechanically bolted to the engine block for ground connection to the starter.

Answer 4

The question was "Do automotive manufacturers plan for this sort of jump start as part of their requirements?"

And the answer is: yes. Every car I've ever owned has a section about jump starting in the owner's manual. If that isn't an indication they've thought about the possibility, I don't know what is.

I guess I'm tempting fate every time I rescue someone by jumpstarting their car, but I routinely hook to the battery terminal and I've never blown up yet.

Finding an bare piece of metal on the frame under the hood of a stranger's car is hard enough. It's usually at 10:00 at night in the grocery store parking lot using the light from my cell phone, too. Everything is painted, powdercoated, or covered in grease, so the engine block or the negative battery terminal is how I roll.

I look at it this way: The danger is igniting hydrogen gas created from a charging battery. If the last connection I'm making is the negative terminal on the dead battery, the charging hasn't started yet; how much hydrogen could possibly be there waiting to spark? Obviously, the amount is greater if they've been trying to charge the battery some other way before attempting the jump start.

I know, I know, someone will link something in the comments proving it's happened before. And people blow up all the time using their cell phones at the gas pump, too... ;)

Answer 5

On many vehicles, where the battery is not in the engine compartment, specific jump start terminals are provided; so you don't have to dig around under the rear seat or in the boot [trunk].

This is how my Merc E-Class is set up [though mine is slightly different & doesn't have the second battery marked here in the engine compartment, only one in the boot.

Picture from MBWorld - Mercedes-Benz E-Class and E-Class AMG: How to Safely Jump Start Your Battery

An older BMW 7-series I had used a similar system.
You'd have to assume if the manufacturers though this was a good idea, then it ought to be 'safe', as well as considerably more convenient.

Answer 6

Might be getting mixed up between jump starting and battery charging.

With battery charging, there's obviously a charger connected somewhere. And during charging, the battery will give off explosive gases. So it makes a lot of sense that when the charger terminals are connected, one is nowhere near the battery itself. This obviates the chance of sparks - take off the one on the block, or elsewhere first, them the positive can be removed from the vehicle's positive post, producing no spark to potentially ignite said gas.

With jump starting, the leads are on for a very short time, comparably - once the vehicle has started, there's no point at all leaving the leads connected. So both terminals could happily be connected to the battery - probably with less power loss to the poorly vehicle. So no opportunity for gas build-up.

Answer 7

That is correct, you cannot rely on chassis electrical continuity anymore.

The main reason is the widespread use of aluminum body panels. If aluminum contacts steel, galvanic corrosion results. Therefore, aluminum components are isolated from steel, thus they may have little electrical continuity with the electrical grounding network and engine block.

In modern vehicles, the manufacturer provides a designated negative stud which is intended for jump starting, and should be utilized.

You should also utilize this stud for charging. Do not connect to the negative battery terminal, as there is typically a current sensor on the negative lead. Bypassing this sensor confuses the battery management system, resulting a code and disabled features like auto-start-stop, self-driving, and software updates.