Jump starting car with leads

Can some kind sole explain why there is conflicting advice on jumpstarting cars?

To my shame, I ran out of fuel last night while driving home from work. Hadn’t clocked I’d left the lights on so, when wife turns up with petrol the battery is flat, flipping balls!!

Didn’t have jump leads so this morning grab them from the cellar, quick google to refresh my mind as been 15 years since last used them. First three sites all say attach positive to positive and negative to chassis. I’m scratching head as was sure negative connects to negative but hey ho, three sites can’t be wrong.

Get to car this morning, attach leads and instructions in box reiterate the above advice. 10 minutes of trying to no avail. Attach negative to negative and starts first time.

I’ve treated myself to bacon butty and fancy coffee for the 10 minutes of annoyance, was very nice.

You can use the chassis for negative, but you need to get a good contact, which means unpainted metal, of which there isn’t normally much on show. I just use the two battery terminals myself.

Always just used the battery terminals.

as above, unless there’s bare metal easily available elsewhere then using the battery terminal makes sense.

I think they advise chassis as there’s likelihood of dropping the negative clamp onto the positive terminal…

Make sure you read the manual in anything modern, some new cars are picky about how you do it

The theory is that the last connection is likely to spark, and there’s a low risk of explosion when you have big sparks in the vicinity of batteries. So you always connect the flat battery last, and ideally connect to an earthing point further away from the battery rather than the battery itself. If that didn’t work for you, you didn’t choose a very good earthing point.

Connecting all terminals to terminals directly will work, of course, but it’s “less safe”.

Nuts on the suspension turret are my favoured jump point.

That’s all very well if you’re tall enough…

Always used terminal to terminal with no problem.

But don’t do what I did yesterday and connect them the wrong way round. Lots of smoke and burning plastic as I fried my father in laws car!

Let the working car rev for a bit when connected, even throttle it but not much.
Wait about 3 minutes until you try the other car.
When it starts and cables are disconnected do not switch off the dead battery car until you have driven it for about 20 mins (at least) or it will not start again.

Lots of smoke and burning plastic as I fried my father in laws car!

Automotive wiring never seems to work the same after you let all the smoke out of it.

Interesting ……have jumped many batteries in my time & always carry a set of heavy duty jump leads in all cars in the family. Always do battery to battery – being very systematic – pos to pos then neg to neg. Ensure the cars are close – but not touching (was bigger problem with metal bumpers) if you can get someone to rev the car with the good battery it helps – H&S = handbrake on & in neutral of course!

Again once car has started – systematically remove jump leads – one at a time………keep the flat battery car revving as much as possible & with a good alternator & battery is should charge up reasonably quickly.

The theory is that the last connection is likely to spark, and there’s a low risk of explosion when you have big sparks in the vicinity of batteries.

With modern sealed Lead Acid batteries this is no longer relevant and even with an old one, the amount of Hydrogen gas hanging around would be minuscule.

A lot modern cars have alsorts of bits and pieces connected around the negatice terminal so it’s actually a bit tricky. There’s usually plenty of places though engine brackets and suspension struts or if you have an Audi a nice point that they’ve added to connect it to.

Just a slight hijack in that some modern cars (as Drac points out) have things directly attached to the battery terminals and jumping from the battery can result in expensive fried electronics.
In that case there are usually some additional points (not the battery) to jump from so best to check the manual first.

That’s all good till you get a **** transit with the battery under the seat in a box! Special live terminal is supplied under the bonnet. Utter shite!

Underneath the seat on my wife’s Beetle was really badly scorched.

Turns out someone hadn’t fitted any terminal covers on the battery that lurked underneath it.

If some one was sat in the back then every time the car went over a bump the metal springs hit the live terminal and shorted the battery out.

“Mum, I can smell burning!”

Was a right pain to jump start too.

Right’o, the part of the chassis I used did have paint on but will just go from battery/battery next time me thinks.

Good call on the spring bolts as they are just next to the battery should I need to.

Will check the wifes electrics tonight to make sure nothing has stopped working/gone Pete Tong as she has a brand new car and had multiple wires coming off the battery.

Good call on the spring bolts as they are just next to the battery should I need to.

If they’re right next to the battery then what advantage do they offer over just using the terminals…

If they’re right next to the battery then what advantage do they offer over just using the terminals…

As there’s not lots of electronic gubbins connected to them waiting to be fried or getting in the way.

If they’re right next to the battery then what advantage do they offer over just using the terminals…

See above re extra bits attached to negative terminal…

thegreatape – Member
That’s all very well if you’re tall enough…

thanks for that TGA, i have just come from reading the last page of “obesity” and the lol was much needed.

As there’s not lots of electronic gubbins connected to them waiting to be fried or getting in the way.

But that’s not the issue for the OP – he’s just suggesting it seems a good alternative in general.

But that’s not the issue for the OP

Sorry I didn’t realise you’d seen his car.

The battery on my car is in the boot…there’s a positive terminal under the bonnet and then you put the negative on the engine somewhere.

The manual says a particular location on the engine and it seems to matter on my car as it’s the only location that seems to work – I don’t know why. Previous, less modern/expensive cars didn’t matter as long as it was bare metal.

If I recall correctly, the order of connecting up the positive/negative jump leads is to stop you from frying the primary circuit on the car, i.e. all the electrical gubbins that aren’t run on the secondary circuit.

I can’t remember really…I last worked on cars about 15-20 years ago.

I don’t know why.

Aluminium heads and engine block?

I thought that, but I tried all sorts of metal bits (as there wasn’t a neg battery terminal), then had to do the unmanly thing and look in the manual.

This avoids all the ‘where do I put the jump leads’ questions;

If I recall correctly, the order of connecting up the positive/negative jump leads is to stop you from frying the primary circuit on the car, i.e. all the electrical gubbins that aren’t run on the secondary circuit.

Sounds like an old wive’s tale…

Sounds like an old wive’s tale…

I can’t remember where I got that from, i.e. did I imagine it or was I told when I was an apprentice (for clarity I worked 1.5-2 years at a garage after school so was never a qualified mechanic).

Here’s where my memory gets fuzzy…the battery output is 12v – this powers all the various gizmos that run on the primary circuit. The secondary circuit converts 12v to 20,000+ volts (via the coil).

It’s possible to fry some of the gizmos on the car being started, I think this is why the leads should be connected in a particular order. Although I can never remember if it’s live first or neg first, and when I was at the garage I was never given any particular instruction when jumping cars. I usually assume it should be neg first, but I’m probably wrong.

One thing that’s in my head, when a car starts the voltage can spike on ignition, which means that if you have the jump leads connected to the other car you could blow fuses in the car being started or something.. So the engine should be running when you connect the leads.

Possibly…Someone qualified might be able to confirm or deny all this nonsense I’m spouting.

Possibly…Someone qualified might be able to confirm or deny all this nonsense I’m spouting.

You’re talking nonsense.

footflaps CEng

My 2002 focus has a ‘special’ battery (not lead acid) and a smart charge system driven by the vehicles power control module.

I.e. when the car starts, the charging system can push up to 18V if it senses a cold engine or poor battery condition.

Now, if I jump a car with a ‘poor battery’, my alternator/pcm will possibly push 18V into the other car and my car and possibly fry all manner of modules.

Unless stranded, I’d probably pull the battery and pop down to the dealer and have them put it on their test station and charge it up for me 😉 5 year battery warranties are useful for some things at least.

With modern sealed Lead Acid batteries this is no longer relevant and even with an old one, the amount of Hydrogen gas hanging around would be minuscule.

You’ve clearly never witnessed a tractor battery go pop!

Automotive wiring never seems to work the same after you let all the smoke out of it.

I’m using that 😆

It’s really quite simple. Just connect the jump leads directly to the battery terminals, positive to positive and negative to negative. It is good practice to connect negative first. You can connect to the car body or suspension strut, but I never did cause you never know how good the earth connection is – with the battery terminals at least you know what you’re getting.

I’ve never fried anything when jump starting cars, even when connecting the positive terminals first. You’re connecting across the battery terminals do no gizmos in the circuit to fry.

The only thing to be aware if is if your car is positively earthed. I suspect all cars are negatively earthed these days anyway.

My car is an SMax and the negative battery terminal is not accessible but there is a slave negative terminal bolted onto the bulkhead and directly connected to the battery negative terminal and it is specifically for the purpose of connecting jump leads to.

I’m sure you’re right footflaps, but not sure being a chartered engineer necessarily is evidence of that. Unless you’re a chartered car mechanic of course.

As I said, I’m trying to recall something I may have learnt or dreamt in the 2 years after leaving school…so about 20 years ago. So happy to be wrong!

It is good practice to connect negative first.

I don’t think so. If you connect negatives first, the chassis of the car being started is live negative – so if you accidentally touch it with the positive, you have a short.

Connect positives first, and all that’s live is the wiring. If you accidentally touch the chassis with the negative, that’s what you were going to do anyway.

^^^^^
this

pos pos then neg neg. terminal to terminal.

NEVER let someone else handle the other end, even if the claim to be a mechanic.

As above ++/–. And be sure your leads can carry the current. Particularly important for diesel cars which have a higher compression ration. Found this out in France about to drive back to Calais. Our leads went in the bin and some decent lorry leads came out from a helpful French van driver.