🚗 Best way? Jump starting a flat battery with leads.

Fishdoodle · 08-06-2024 6:01pm #1

Today I had to get the car (petrol engine) jump started at a supermarket.

Usually Id connect the leads to the terminals of the flat battery …which is what the helpful security guy did. (He used a jumpstart kit). This is also what the RAC and Halfords instruct.

The connection sequence would be as shown in the picture. However, you’ll notice in this picture that the negative cable is connected to bare metal on the car that’s flat. This is recommended by some car manufacturers.

One reason for this is to avoid sparks and the plume of (potentially flammable) gas when connected to battery terminals (as opposed to bare metal).

I’m confused on the bare metal approach. Wouldn’t that method potentially cause an electric shock if your hands were to touch an area of bare metal in the engine bay?

So, would it be best to restrict current to battery and electrical circuitry in the car?

There might be implications however given the prevalence of electronic equipment in modern cars.

What’s considered the best/safest way (-ve terminal OR Bare Metal) ? And why?

🚗 😎

BlakeS94 · 08-06-2024 6:08pm

It goes like this.

Red lead to Battery Positive on dead battery to donor car battery positive

Black lead to battery negative on donor car, connected to earth point on dead car, advised to use bare metal so if there is a spark the two cables aren't close to each other

They get taken off in the reverse way. Black on dead car, black on donor car, positive on donor car and then positive on dead car.

You're not really supposed to use jumper cables on modern cars. Better to just buy a battery jump starter.

Fishdoodle · 08-06-2024 6:18pm

Ok …. There’s sense in that “advised to use bare metal so if there is a spark the two cables aren't close to each other”

If you get a spark it could ignite the plume of vapour when connected to terminals. 👍

TheChizler · 08-06-2024 6:23pm

With a jump starter that has a switch there'll be no spark at the terminals if you connect them when it's off, so no risk of igniting gas. Most batteries are sealed these days anyway.

You won't get a shock no matter where you connect it for several reasons. The car is floating electrically with respect to ground so if you touch it there won't be any potential across you, also your shoes will insulate you from the ground so it isn't completing any circuit, and if all that wasn't true 12 V isn't enough to shock you beyond a tingle if you were to grab the car with wet hands and lick the positive terminal. Even 24 V in a truck you'd be fine.

Fishdoodle · 08-06-2024 6:44pm

If the battery is sealed … then why the vapour discharge? It looks pretty dangerous …it’s cool how resistant the body is to the voltage . All the same, I wouldn’t go licking the terminal 😅 There’s probably someone on you tube who’s done that 🤪

BTW - I thought the current would have to run through the battery (terminals) … Don’t people do this in winter (overnight) to keep them top up ? Still getting my head around the negative cable. Could you just use the positive lead only to jump start. If the negative is attached to a piece of metal on the recipients car , what’s it actually doing?

naughtysmurf · 08-06-2024 7:20pm

The negative battery terminal is also directly connected to the chassis of every vehicle during manufacturing so connecting black jump cable to the chassis is the same as connecting directly to the negative battery terminal

Basically the donor car battery is replacing the flat one for starting purposes, once started the flat battery is still flat until it’s charged up again

Padre_Pio · 08-06-2024 7:46pm

https://www.boards.ie/discussion/comment/122235421#Comment_122235421

Sure but it's usually easier to clip to the battery negative than find exposed metal.

I wonder is it just a legacy thing that's not really relevant on modern cars.

You can lick the terminals if you want, you'd just need an awful long tongue to get both of them. I always laugh when I see movies where they're shocking someone with a car battery. Pure nonsense.

FishOnABike · 08-06-2024 7:58pm

The negative terminal of the battery is connected to the car (and engine) body. Hence negative earth system in modern cars.

naughtysmurf · 08-06-2024 8:58pm

On older vehicles it’s usually easier to connect to the battery, some of the more modern ones, less so with their location & array of covers & clips

I was told years ago that connecting to chassis was to protect the ECU, how true that is I’ve no idea

The reason why you can’t jump start with just the positive lead is because a complete electrical circuit isn’t made, the circuit is made from the donor battery + to the car electrical system & back to the donor battery - only then will 12v be available to get things going again

Fishdoodle · 08-06-2024 9:35pm

https://www.boards.ie/discussion/comment/122235748#Comment_122235748

Makes sense that a circuit is made -and as other posters mentioned, that the negative is also connected to the chassis.

Re: the ECU - I heard that also. So things seem to have changed with modern cars.

In the past I’ve seen the jump leads used with the donar car running as the leads are attached. Now I’ve seen it advised not to have the engine running until the leads attached. Something to do with the alternators of both cars.

BlakeS94 · 08-06-2024 10:10pm

I've had success in the past connecting to the bonnet latch and the strut mounting by the scuttle panel

An Ri rua · 08-06-2024 11:27pm

https://www.boards.ie/discussion/comment/122235506#Comment_122235506

Unless they use a dc ac inverter and a powerdrill…

User1998 · 09-06-2024 1:02am

I’ve been jump starting completely dead and partially dead batteries using + to + and - to - for 5+ years and never had a single issue. Never once connected to the chassis unless I had to. (Hybrid Toyota’s for example)

Edit: I use a battery booster, not a doner car. Not sure if that makes a difference?

Quay_Koncept · 09-06-2024 6:55am

The last time I needed a jump start 2 people were very hesitant to help me as they said they were afraid it would damage the electronics in their car. ie computer. Has anyone ever actually blown a CPU from jumping starting a car?

kirving · 09-06-2024 7:26am

https://www.boards.ie/discussion/comment/122236350#Comment_122236350

It's unlikely, but absolutely possible if you mess something up. I'd be hesitant to use mely car tbh, but I keep a jumper starter in the boot.

All ECUs will have reverse current protection up to a certain level, but you can have current spikes, or shorts which could damage electronics.

I tried to help a guy out some years ago with a jump start. He had leads, but my car was new to me, and one of my battery terminals was covered, so I asked him to wait while I read the manual.

In the meantime he tired to connect it himself, shower of sparks to I jumped out and told him not to touch anything.

Connect it up myself, no dice, so I hop into my car to rev to 1500rpm and help it along. After a minute I spot flames through the crack under my bonnet. Hopped out and his leads had melted and gone on fire. I ripped them off quickly and thankfully my car was okay.

Quay_Koncept · 09-06-2024 7:31am

Jasus that was a close one lucky you didnt get any melted plastic on your paintwork! Yeah was surprised I had difficulty getting a jump start i was stranded in a shopping centre. I now keep a set in my boot to help others and recently got chance to offer someone a jump start. My good deed for the day.

Fishdoodle · 09-06-2024 1:35pm

https://www.boards.ie/discussion/comment/122236376#Comment_122236376

Wow - thats not what you want to have happen! Lucky you got the clamps off without injury.
In that event, Im guessing the leads were connected to the battery terminals. The flat battery discharged Hydrogen gas and the sparks ⚡️ ignited the gas causing the fire. I guess this wouldn’t have happened if the -ve lead was on bare engine metal.

TheChizler · 09-06-2024 1:50pm

https://www.boards.ie/discussion/comment/122237492#Comment_122237492

The leads melted from the amount of current going through them, nothing to do with igniting gas. A hydrogen explosion you'd know all about if it happened.

beachhead · 09-06-2024 3:26pm

Probably cheap connectors/lead not able to take current

🚗 Best way? Jump starting a flat battery with leads.

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