Jump Leads - correct procedure

[anothernameitist]

Need to start a car with a flat battery this weekend.

Can't remember how to do this thinking

connect positive lead to good battery, then to positive of bad battery
Some for negative leads
start good car up
wait a few minutes
start bad car up

disconnect negative from bad and good car
disconnect positive

keep both cars ticking over, switch off good car, lock it and go for a blast in bad car. park up and lock car

Go for cup of tea

Am I missing anything or getting anything wrong, apart from wanting a beer instead of tea

[SSBB]

I have always connected the negative to a metal part of the (flat car's) engine bay, not battery to battery.

[Ozzie Osmond]

SSBB said:

I have always connected the negative to a metal part of the engine bay, not battery to battery.

This is to prevent risk of a spark near the battery which could ignite any hydrogen gas given off.

[LuS1fer]

Positive to positive.
Negative to negative.
Negative to engine block according to the AA who once jump-started my 5.7 litre Camaro.
Make sure the cables don't overheat.

[Krikkit]

Haynes guide on jump leads.

[Galileo]

So you're connecting a good battery to a flat battery without being charged?
You risk the flat battery taking enough charge out of the good battery, and you'll end up with two semi flat batteries both unable to start their cars.

[jmorgan]

First make sure you can jump start as normal. Did it to our festa and blew its brains out. 700 notes for a repair. Should have connected the earth to the turbo not the battery!

[PanzerCommander]

Ozzie Osmond said:

This is to prevent risk of a spark near the battery which could ignite any hydrogen gas given off.

Be careful with this, I'd advise the OP to read the owners manual for connecting the negative lead, some cars specify the battery and some an earthing point. One of the guys on the Mustang forums buggered his PCM by connecting it to the wrong place!!

Edited by PanzerCommander on Friday 20th December 11:01

[BOBTEE]

Don't put either negative on the battery, use bare metal on either the body, or more likely, the engine/gearbox.

It's unlikely that a spark would be a problem when use the neg' terminal but damage to a 'computer' or battery monitoring device is possible.

What car(s) is it?

[Prizam]

jmorgan said:

First make sure you can jump start as normal. Did it to our festa and blew its brains out. 700 notes for a repair. Should have connected the earth to the turbo not the battery!

Do you even know what electricity is ?

[2 sMoKiN bArReLs]

Galileo said:

So you're connecting a good battery to a flat battery without being charged?
You risk the flat battery taking enough charge out of the good battery, and you'll end up with two semi flat batteries both unable to start their cars.

That's why he's got the engine running on the good battery source.

I've jumped hundreds of cars this way.

[ouch]

I can understand the risk of spark suggesting not to connect directly to the battery, but I can't figure out how connecting the negative lead to the battery would fry any electrics.

Can someone explain this?

[jmorgan]

ouch said:

I can understand the risk of spark suggesting not to connect directly to the battery, but I can't figure out how connecting the negative lead to the battery would fry any electrics.

Can someone explain this?

Ford have a smart charging system and this sits in the path of some of the battery connections. I found this out AFTER I turned it into a brick.

[motorhole]

1) Start the car with the good battery first and let it run for a couple of minutes before connecting leads.

2) You have the correct order - positive of good battery to positive of bad battery (leaving car running)

3) Negative of good battery to suitable earthing point on car with bad batter (engine mounting arms or a decent bolt head, an existing earth strap are all good points. Try to avoid the radiator as these are often on rubber mounts [so is the engine, but that is earthed via earth straps already]). Keep jump leads well away from any magic black boxes - control units for PAS/ABS, ECU etc.

4) Once connected, wait a few minutes again for the bad battery to accrue some charge.

5) Start the car with the bad battery.

6) Remove leads - negative on bad to negative on good, positive on bad to positive on good, reverse of above.

7) Take car with bad battery for a spin!

Bear in mind if the battery is completely goosed, it may not hold any charge so once the car is switched off for more than a few minutes, it will require a jump start again. If this is the case, time for a new battery.

[davepoth]

ouch said:

I can understand the risk of spark suggesting not to connect directly to the battery, but I can't figure out how connecting the negative lead to the battery would fry any electrics.

Can someone explain this?

It probably wouldn't, but sparks are generally bad in most situations. As mentioned above, if the car to be jumped is properly flat (won't even try to crank) it's best to get the "good" car running before connecting the leads. If you're trying to jump something big with something small you may well want to rev it up to 2k or so just to make sure it doesn't stall when you connect the leads, as it's a big draw on the alternator.

[Dodsy]

Check owners manual. My jag cant be used wirh the engine running to jump start as it has a high power boost capability that will potentially fry a normal cars electrics,

[Prizam]

ouch said:

I can understand the risk of spark suggesting not to connect directly to the battery, but I can't figure out how connecting the negative lead to the battery would fry any electrics.

Can someone explain this?

Connecting directly to the battery will not fry any electrics.

When an alternator is running you get +14ve running through it... no different to jumping a car.

The spark vs battery gasses is real, but EXTREMELY unlikely. You are more likely to connect the -ve lead to something that runs through a sensor that is rated at less than 14v and 100 amps.

If your just connecting to a bit of body work, engine or gearbox. You cannot always guarantee the path of least resistance.

[conkerman]

Main reason of not connecting battery to battery is that is the dead battery is shorted out internally, bad things can happen when the good battery chucks all its current into bugger all resistance.

Can still happen when done properly, but you will be standing out of the way and can key off.

The only way I know you can fry an ECU is if your jumping cars charging system is overvolting.

ETA. If the Jump leads have been corrected properly of course!

Edited by conkerman on Friday 20th December 11:28

[ouch]

jmorgan said:

Ford have a smart charging system and this sits in the path of some of the battery connections. I found this out AFTER I turned it into a brick.

So how does this happen? As far as I can tell, when you jumpstart a car, all you're doing is adding and extra current source in parallel with the existing battery, as far as any electrics connected are concerned, the only thing that would be different to a regular starting experience is the fact the pre-staring voltage is higher (14.4v from a good alternator) than that of a batter not under charge.

Could the risk be mitigated by jumping from a car that doesn't have the engine running?

[Prizam]

Dodsy said:

Check owners manual. My jag cant be used wirh the engine running to jump start as it has a high power boost capability that will potentially fry a normal cars electrics,

Imposable... If it has a 12v battery then it can jump start anything. Amperage has nothing to do with it.

(for the record, i have a Jag XJ and have jump started a few cars from it.)