Using jump leads

[Ala Larj]

I was reading something earlier about how to use jump leads to start a car with a flat Battery. It said that you should attach the positive lead to the batteries and the negative lead to a metal part of the car. I have always just attached the leads to both positive and negative poles of both batteries. Am I doing something detrimental to either vehicle? I am aware that you shouldn’t use a hybrid to jump start a non hybrid and understand why. But what am I doing wrong and what could be the consequences?

[LittleCasper]

19 minutes ago, Ala Larj said:

I was reading something earlier about how to use jump leads to start a car with a flat battery. It said that you should attach the positive lead to the batteries and the negative lead to a metal part of the car. I have always just attached the leads to both positive and negative poles of both batteries. Am I doing something detrimental to either vehicle? I am aware that you shouldn’t use a hybrid to jump start a non hybrid and understand why. But what am I doing wrong and what could be the consequences?

You could seriously dammage some of the ECU's and other electronics in the car by doing that, it's a bit like getting a bulb that can only take one AA Battery and connecting two together, something's got to give but thankfully Fuses exist although I wouldn't rely on them working.

I've seen the back plate behind the Battery literally melt A joint possibly due to this, seen people destroy ECU's when connecting things up wrong too. 

[Herbie]

19 minutes ago, Ala Larj said:

I have always just attached the leads to both positive and negative poles of both batteries. Am I doing something detrimental to either vehicle?

It's a question of safety rather than anything electrical.

A charging Battery can give off hydrogen gas and if it's present in the correct proportions it can be ignited or explode. The rationale is that if you place both leads on the Battery terminals, if there are any sparks as you disconnect the leads then this could cause an explosion. If the negative lead is placed far enough away from the Battery then any sparks created will be inconsequential.

I have to say though, that in more than 45 years of driving and being an amateur hobby mechanic and messing about with batteries, I have never known anyone to actually cause an explosion in this manner, nor do any family members or friends know of it ever happening in the real world.

I have always connected directly to the Battery terminals and will always continue to do so.

[Herbie]

11 minutes ago, LittleCasper said:

You could seriously dammage some of the ECU's and other electronics in the car by doing that, it's a bit like getting a bulb that can only take one AA battery and connecting two together, something's got to give but thankfully Fuses exist although I wouldn't rely on them working.

Sorry Liam, but just in case someone worries about what you've written and it frightens them, I have to say that it's not correct in this context.

The negative Battery terminal is strapped directly to the car body, so in effect the whole of the body becomes the negative terminal anyway. From an electrical point of view, a few inches of steel isn't going to affect the passage of current in any appreciable way whatsoever.

As mentioned above the only reason for this advice is to avoid the (infinitesimally small) chance of an explosion.

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I've seen the back plate behind the battery literally melt A joint possibly due to this, seen people destroy ECU's when connecting things up wrong too. 

Now that is true. If the jump leads are connected with reverse polarity, big bangs are indeed likely and a few things may even need replacing.

[Ala Larj]

11 hours ago, Herbie said:

It's a question of safety rather than anything electrical.

A charging battery can give off hydrogen gas and if it's present in the correct proportions it can be ignited or explode. The rationale is that if you place both leads on the battery terminals, if there are any sparks as you disconnect the leads then this could cause an explosion. If the negative lead is placed far enough away from the battery then any sparks created will be inconsequential.

I have to say though, that in more than 45 years of driving and being an amateur hobby mechanic and messing about with batteries, I have never known anyone to actually cause an explosion in this manner, nor do any family members or friends know of it ever happening in the real world.

I have always connected directly to the battery terminals and will always continue to do so.

Thanks. That makes sense.