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Old 02-06-2005, 01:15 AM   #1
Lachlan
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Default Please explain jumping...

I'm just curious, why do you connect the + first when you jump a car? It seems like you would want to ground the cars commonly first, but you don't.
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Old 02-06-2005, 01:53 AM   #2
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Actualy you realy shouldn't jump a car now-a-days (the alternator doesn't like it)

But the order in which you connect the cables realy doesn't matter, just make sure they are conneted(on both cars) correctly!
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Old 02-06-2005, 01:54 AM   #3
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So...how do you start a dead car?
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Old 02-06-2005, 01:58 AM   #4
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You're technically supposed to connect the jumper cables as usual. Then leave the working car running for a few minutes to charge the battery. Then turn off the running car and try to start the car with the dead battery. The reason is because the amperage draw from starting the dead car can sometimes damage the alternator on the running car.

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Old 02-06-2005, 11:42 AM   #5
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Old 02-06-2005, 11:46 AM   #6
Lachlan
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Thanks, but that tells me how to attach the cables, which I already knew. I'm looking for an exlanation of why that order.
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Old 02-06-2005, 12:11 PM   #7
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Quote:
Originally Posted by Lachlan
Thanks, but that tells me how to attach the cables, which I already knew. I'm looking for an exlanation of why that order.
you are just connecting the batteries in parallel - the order really isn't too important - one reason for doing positive-positive, then negative-ground (or negative - same thing) is so that you minimize the odds of shorting the positive and negative together or hooking the cables up incorrectly (positive to negative). you could just as easily do negative-negative, positive-positive.

I usually hook up the cars, have the good car running for a little bit (at elevated rpm - 3k or so) to charge the battery on the other car a bit, then have the other driver start their car while keeping the good car revved up. Once the other car is started, I turn off the good car, and disconnect the cables.
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Old 02-06-2005, 03:09 PM   #8
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Hi guys.
This is my first post here so it may read a bit messy.
The reason for connecting the + lead first is simple. If the - lead is connected first and the + lead gets dropped there is a BIG flash and possibly an equally big bang as it shorts out the slave cars battery. By connecting the + lead first, you get that all secure before waving the - lead around. If you drop the - lead it simply conects as normal. No short circuit.
The process of jump starting a car is simple. If the jump leads are heavy enough to take the current, connect them and start the dead car. The problem is that most cheap leads will not take the current needed for a starter motor. The answer then is to connect the leads and use the good car to charge the battery on the dead car. The dead car can then be started using a combination of both batteries still connected together. The time to charge depends on how flat the dead battery is to begin with.

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Old 02-06-2005, 03:24 PM   #9
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Thanks John! That was the logical explanation I was looking for! I guess it's the same reason you always disconnect the negative battery terminal first!
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Old 02-06-2005, 03:47 PM   #10
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Similar logic. In that case it's so that the spanner on the + terminal doesn't short out the battery if it touches metal.

John
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Old 02-06-2005, 05:34 PM   #11
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Quote:
Originally Posted by John UK
Similar logic. In that case it's so that the spanner on the + terminal doesn't short out the battery if it touches metal.

John

Translation; Spanner= wrench in the U.S.

I'd like to also add that you should make the last neg connection to the chassis away from the battery to minimize the chance of a battery(hydrogen gas) explosion from the spark you will get.
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Old 02-06-2005, 05:52 PM   #12
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Slightly off topic, but I recently used a portable 300amp jump starter to start a dead jeep. Both positive and ground connections are made before power is even supplied to the cables/clamps so the chance of one of the leads striking a surface and sparking is greatly minimized. I was extremely pleased with the no-hassle factor.

Picture here.
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Old 02-06-2005, 06:12 PM   #13
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Battery charging is a whole different matter. When a car is jump started, the highest voltage applied to the battery is from an alternator. These are stabilised at 14.2 volts max so the battery will not gas.
When a mains charger is used, they are rarely stabilised and ferquently supply voltages in excess of 14.2. As a result, the battery produces gas. Oxygen from one plate and Hydrogen from the other. Don't ask me which plate is which, I can't remember. However, this mixture of gasses is highly explosive. One spark and there's a big bang. This spark can come from disconnecting the charger leads at the battery before the mains is switched off. I have seen a couple of batteries literally blown apart by this. The spray of battery acid isn't too nice either.
ALWAYS unplug a charger before disconnecting it from the battery.

John
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Old 02-06-2005, 07:37 PM   #14
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Quote:
Originally Posted by GDR
I'd like to also add that you should make the last neg connection to the chassis away from the battery to minimize the chance of a battery(hydrogen gas) explosion from the spark you will get.
gas production from an inactive, dead battery isn't generally a concern.
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Old 02-08-2005, 04:52 PM   #15
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Quote:
Originally Posted by John UK
Similar logic. In that case it's so that the spanner on the + terminal doesn't short out the battery if it touches metal.

John
Oh man. I did that once when I was working on my old Camaro. Nothing broke, but man, that scared the crap outta me.
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Old 02-09-2005, 05:51 PM   #16
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Oh man. I did that once when I was working on my old Camaro. Nothing broke, but man, that scared the crap outta me.

I have seen spanners (wrenches) arc to brake pipes and blow a hole in them. It's bad news if you don't spot the hole before you drive off!

John
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Old 02-09-2005, 06:56 PM   #17
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at work, we have small arrays of monster lead acid wet cell batteries for starting up turbine engines. It's roughly the equivalent of about 12 large car batteries and they are hooked up in 24V configuration. If you accidentally cross the terminals on these, they have enough current supply to vaporize the wrench - we actually have a crescent wrench that is missing most of its jaw from this.
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Old 02-09-2005, 07:44 PM   #18
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At work we have a small 48V array for our phone switch that i manage. It will keep the switch up for about 1 hour. There are 125 sealed lead acid batteries in the array. A technician once dropped his wrench and cross the 2 large terminals. Large flash. Bad smell. Mostly vaporized wrench .
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Old 02-11-2005, 05:45 AM   #19
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Quote:
Originally Posted by DrD
gas production from an inactive, dead battery isn't generally a concern.
No...but if you've charged the battery for awhile using the donor car, and the problem car is running - at high charge rate - then it's a bit more possible.

A bigger problem is the combination of dead battery and very cold weather: I've read that if the dead battery is mostly/partially frozen when you connect the leads the battery can explode. I would imagine this would be even more possible if the donor car were running and full alternator amperage was available at connection time.

In this case connecting the ground lead at a point where you are as far away from the battery as possible would be a really good idea
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Old 02-11-2005, 08:58 AM   #20
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Quote:
Originally Posted by GravelRash
No...but if you've charged the battery for awhile using the donor car, and the problem car is running - at high charge rate - then it's a bit more possible.

A bigger problem is the combination of dead battery and very cold weather: I've read that if the dead battery is mostly/partially frozen when you connect the leads the battery can explode. I would imagine this would be even more possible if the donor car were running and full alternator amperage was available at connection time.

In this case connecting the ground lead at a point where you are as far away from the battery as possible would be a really good idea
There is almost no chance of a battery producing gas when charged normally by a car alternator. Lead acid batteries are charged from a constnt voltage source. This is what an alternator produces, between 13.8 - 14.2 volts. The current drawn by the battery is a function of it's own internal resistance. This is decided by (amonst other things) it's state of charge.

I have no experience of frozen batteries. We don't get temperatures that low here.

John
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