Jump starting: Which lead first?
Released on: 2014-05-08

Today I show you why it matters which lead is connected first when jump-starting a car, and why it's not obvious if you have an electronics background.

The answer is that on negatively grounded cars (pretty much all cars on the road from what I'm told), the positive (red/+) lead should be connected first.

Conversely, In the rare case that you have a positively grounded car and you are jump starting with another positively grounded car, then you'll need to connect the negative (black/-) first.

The last combination: mixing a negatively grounded car with a positively grounded car. I don't know of a safe way to do this. I imagine your best bet is just to take a lot of care not to have anything unintentionally touching, which at this level of power, should be your attitude anyway. Another solution may be to disconnect the donor battery from the donor car. I've heard a little noise about this option that sounds like voodoo mentality, but just be aware that not everyone is comfortable with it.

Which ever way you do it, you should always connect positive (red/+) to positive and negative (black/-) to negative.

If in doubt, ask someone who knows. Judging by the noise on the Internet and how long it took to get a straight answer on this (even from professionals!), it may be rather hard to find, let alone qualify. Better yet, consult with the manual for your car.

Why?

And this is what this episode is really all about!

Which ever terminal is grounded is the one attached to pretty much everything metal in the car. If you connect the grounded terminal first and then bump any grounded metal with the second lead, you'll get a short circuit. Wikipedia has this to say on the subject:

"Another potential cause of explosion is when the battery terminals are short-circuited via a very low resistance path (like a wrench or other tool dropped or lying across the terminals). Apart from the sparks which usually occur in a short circuit, heating due to the internal resistance of the battery can cause the electrolyte to boil, also leading to explosion due to buildup of water vapor pressure (unrelated to electrolysis)."

Why is it less obvious with an electronics background?

Because potential difference between the two cars (static electricity) can be damaging to electronics. The usual way in electronics to dissipate this difference safely is to connect the ground of each circuit together. Ie connect the negative to negative first.

At some point a decision would have been made that it was safer to tell people to connect the positive first, and then make sure the electronics of the car are protected/isolated against unintended static.

More information

Here is a more complete and authoritative guide to jump starting a car. 

Imported image from the old system
Above: Imported image from the old system

Imported image from the old system
Above: Imported image from the old system

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