Why do we need to attach one negative cable to bare metal when jumping a car?



Why do we need to attach one negative cable to bare metal when jumping a car?

In: Engineering

Lead acid car batteries can emit hydrogen gas (hydrogen sulfide typically) under certain conditions which is flammable. Since we are taught to attach the negative last, it usually creates a spark. Attaching the negative to a point of the car away from the battery puts distance between the spark and a potential (albeit rare) ignition. So safety is the reason. Note that even many “sealed” batteries can off-gas.

There is always a risk of a spark when connecting and disconnecting the cables and this makes it less likely that the spark will happen at the battery terminals. A spark at the battery terminals can potentially be bad because there can be some built up gases in or around the battery.

Your starter is shorted to the car frame for the safety reasons given by other users, and to prevent electric shock. However, if you’re trying to start your car, if the leads are connected to the battery the electricity must first flow through your battery and then to your starter. If you connect the ~~positive~~ negative to the car frame, the current can flow directly through the starter, which is faster and more efficient.

The reason it’s suggested to keep the leads far apart and use bare metal on the vehicle being jumped is so you don’t accidentally touch the leads together and short out the good battery.

It’s basically to “idiot proof” the process.

The negative terminal is tied to bare metal with a ground strap, it’s all the same difference.

Everything is the car is already grounded to the chassis. This saves on weight and cost. Running wires to and from everything adds up after a while. Therefore you can ground to the chassis and be fine because your circuit is complete.

The best answer is a combination of AndMarmaladeSkies and TheDevler: modern cars are all negative grounded, which means the negative terminal of the battery is connected to the metal chassis. When boosting a car, it’s possible that a spark could ignite gases from the battery. So, to be safer, you make the final connection somewhat away from the battery, ie, on the metal chassis. This prevents you from doing the connection at the battery itself, where there is a tiny risk.