jump leads and jump starting

Hello,

I am looking you buy some jump leads "just in case". What do you recommend? What is the minimum size cable I should buy?

Just for family cards so up to 2L should be sufficient but this does include diesels, but then you never know who else will ask to borrow them!

The method I learned (from Haynes) for jump starting is:

  1. connect dead positive to
  2. good positive
  3. connect good negative to
  4. somewhere other than bad negative.

Can I ask a silly question: Why do you have to connect the final negative lead to the engine block rather than the battery?

If there is a reason, why doesn't the same apply to the "good" car? Why isn't the negative lead connected to the block at the good end?

I have searched google for jump starting and each web site appears to recommend a different method!

They all agree that you start the good car and then the bad one but then what?

Some sites say to idle both engines others say to press the accelerator a little and use a fast idle (2000rpm?). Is this necessary? Why? Isn't the alternator output constant regardless of revs?

I think I read that you are supposed to leave both cars running for ten minutes to charge the flat battery. Do people do this? I thought they just drove off!

During this time isn't the flat battery being charged by both cars? Why would this be necessary? I can only think it's in case the flat car has a bad alternator (hence the flat battery)?

Some web sites talk about switching on all the electrics in the flat car. Others do not mention this at all. Should this be done? What's the purpose? To test that the battery works under load? I suppose it's no good if you start the engine but stall half a mile later once you switch your lights on!

Thanks, Stephen.

Reply to
Stephen
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Stephen gurgled happily, sounding much like they were saying:

Jump leads. Just buy some jump leads.

Not really cheap horrible thin hard ones, but some decent quality ones. You'll soon twig which are a big of shit as soon as you start looking.

Good as anything.

Because you're removing one "link" in the chain - the resistance of the block earth lead on the car you're starting. It shouldn't make much difference, though.

Because you're pulling power from that battery to the starter motor on the other car.

No. Once the revs get below a certain point, yes - but at idle, the output may well be lower.

If the alternator on the started car's working OK, then it's a non-issue.

Quite.

Seems counter-intuitive to me. You want to keep the load relatively low, in order to maximise the current available from the alternator to charge the battery.

Just buy some jump leads, and if you need to use 'em, do so. Some people worry FAR too much about it all...

Reply to
Adrian

If you can afford it, go for 25mm2 ones with decent beefy clamps.

Because the final connection will be between two points at a different potential. This is likely to cause a spark; it's not a good idea to create a spark in the vicinity of a recently-flattened battery due to the gas it gives off being explosive.

It's only the final connection that may create a spark.

Your method above is the correct one.

Much better to get as much charge in the dead battery before you try and crank it to maximise your chance of success. It will also reduce the chance of spiking. (See below.)

Whilst the jump leads are connected, the dead car's charging system sees a nice healthy voltage, so has a little rest. At the instant the leads are removed, it suddenly sees a low battery voltage, and goes into hard work mode. This can (and in spite of what you may be told!) does, cause a voltage spike sufficient to cause damage to the dead cars electronics.

By putting as much charge as possible in before you attempt a start, you minimise the risk. The idea of turning on electrical load is to reduce the amount the alternator can flood the system with. BTW, the added load should not include headlights, which might be blown by the spike.

To minimise the risk, it is possible to buy either a separate surge protector to connect across the battery of the dead car before you start, or leads with such protection built in.

The methods I have mentioned are the way AA patrols are instructed to use; they also have to use surge protectors.

This has been discussed before. There will be those that dispute the chance of a spike causing damage. If so, I will ask my step-daughter's partner if I could post his phone number. He would be delighted to relate the fact that at the end of last year he had a bill of over 600UKP for a repaired ECU in his VW T4 having failed to avoid said spiking!

Chris

Reply to
Chris Whelan

I was recently given one of those portable power pack things, and although I had been sceptical about them, I am a convert now. I'd recommend it over jump leads any day.

Ian

Reply to
Ian

I wouldn't jump a car these days and my car doesn't even have an ECU etc. If pushed I might allow them to take some charge for 10 minutes via the leads and then try their car after disconnecting them, but not connect and try to start the dead car woth my car connected.

Mark

Reply to
Mark

Ones where the conductors are copper and have a cross sectional area of at least 16mm² This is rated at 110 amps so should be ok for most cars - except on a very cold day where the battery is totally flat. Make sure they have large firm crock clips too. Better would be ones using 25mm² cable - they'll cost about 40 quid.

You can use smaller ones by running the donor car while coupled to the flat battery for 10 minutes or so to partially charge that battery - assuming the battery isn't totally fooked.

Reply to
Dave Plowman (News)

The batteries in those - or at least the reasonably priced ones - won't survive being used often for jump starting. Perhaps 10 times or so.

Reply to
Dave Plowman (News)

"Dave Plowman (News)" gurgled happily, sounding much like they were saying:

Some of them. My £20 jump box survived about 5-6 years of use until it got battered around the car boot and the switches were left on until it went COMPLETELY pancake.

Reply to
Adrian

Why?

BMW provide jump start points under the bonnet for easier access than the rear mounted battery. And instructions in the driver's handbook on how to use them if needed.

Reply to
Dave Plowman (News)

The battery in my one died after a couple of years, I cut the case about and replaced it with a fiesta battery, absolutely brilliant, it will start anything. Obviously much heavier but still much better than jump leads between vehicles.

Reply to
Mrcheerful

What would I need for a cold day where the battery is completely flat? ;)

Reply to
Stephen

Why? Isn't the voltage spike in the car with the flat battery rather than your healthy vehicle? From what I have read in these replies, the spike is caused from the bad car's alternator going from low load whilst connected to the good car to heavy work once the jump leads are disconnected.

I'm not too keen on the battery packs because AIUI their batteries need to be kept charged, so when you need them you may find you have two flat batteries: one in the car and one in the pack!

Reply to
Stephen

Patience, give it five minutes with the other car fast idling & it'll start.

Reply to
Duncan Wood

Nope, it's in the car which is generating the current. But if you turn the rear demist on &/or haven't got a knackered battery it's not an issue.

BTDT

Reply to
Duncan Wood

I have when needed just used some thin speaker wire as a charger lead rather than jump start, leave connected for half an hour and away it will go.

A decent set of jump leads will set you back the best part of a hundred quid and they should allow enough current to flow for a flat battery on a cold day.

A cheap set used carefully will get you out of trouble.

Reply to
Mrcheerful

Heh heh - minimum 25mm. But there can be problems getting a good connection at high currents. Best to run the donor vehicle for a while to charge up the flat battery before attempting to start it.

Reply to
Dave Plowman (News)

Yes, I've read that a healthy battery acts as a capacitor but I'm confused how the good car gets spiked?

Chris' reply said: "Whilst the jump leads are connected, the dead car's charging system sees a nice healthy voltage, so has a little rest. At the instant the leads are removed, it suddenly sees a low battery voltage, and goes into hard work mode. This can (and in spite of what you may be told!) does, cause a voltage spike sufficient to cause damage to the dead cars electronics.

By putting as much charge as possible in before you attempt a start, you minimise the risk. The idea of turning on electrical load is to reduce the amount the alternator can flood the system with. BTW, the added load should not include headlights, which might be blown by the spike.

To minimise the risk, it is possible to buy either a separate surge protector to connect across the battery of the dead car before you start, or leads with such protection built in."

So is the spike in the healthy car or the flat car? It seems to me that it cannot occur in the healthy car because (i) it has a good battery to absorb any spike and (ii) it is disconnected from the flat car when the spike occurs.

Reply to
Stephen

Thus spake Stephen ( snipped-for-privacy@invalid.org) unto the assembled multitudes:

I use a little catchphrase to help me remember: "Come to life. There and back again."

i.e. connect dead positive to 'living' positive, then back again from the negative terminal. Well, it works for me... ;-)

Reply to
A.Clews

It depends on how healthy a battery it is. The failure mode isn't the alternator suddenly trying to create current, it's the alternator sudddenly stopping needing too. However it's unlikely to cause you grief. & all the built in electronics is supposed to be protected against such events.

Reply to
Duncan Wood

Car electronics should have surge protection built in locally. At the sorts of current involved there it costs pennies. Otherwise things like a dodgy alternator to battery connection could cause exactly this. Or other problems local to the car.

Reply to
Dave Plowman (News)

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