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Jump-starting a car made complicated?

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Jump-starting a car made complicated?

Posted by Denis McMahon on Jul 12, 2019 8:02 am

How to start a car with a flat battery using jump leads. I have seen these directions in more than one user's handbook, across various makes.

Assuming both cars have negative battery earthing (not that I assume anything, especially if there is the likes of a Morris Minor involved):
  1. Connect positive of one car to the positive of the other.
  2. Connect the negative of the car with the good battery to some convenient point on the engine block of the car with the flat battery. 
No explanation given of why do it this way. It works. But it is not exactly an easy instruction to remember if the handbook is not available. My guess is that the vast majority of motorists simply connect directly to both battery terminals, red to red and black to black, which works perfectly well in all circumstances.

So why connect to the engine block instead of the battery terminal?  I figured out that this might give a lower resistance path on the "jump" circuit by eliminating the resistance in the connection from the battery negative to the earthing point and providing a more-direct path to the starter motor. But this resistance would be very low any way - far less than the resistance in the jump leads.

Any better suggestions, anybody?
Denis4711

Re: Jump-starting a car made complicated?

Posted by Roger Bryant on Jul 16, 2019 11:03 am

Denis McMahon:


Hydrogen gas explosion? Someone needs to go back and look at the chemistry. When you continue to drive current backwards through a fully-charged lead acid battery, the water content of the electrolyte is broken into hydrogen and oxygen, which is emitted, giving grounds for caution. A flat battery does not give off hydrogen.
 
I agree that a well managed battery will not release much hydrogen however one with a faulty cell, the most likely failure mode, may well do so. If one cell has shorted attaching a good battery being charged by an alternator will push 10s of amps (maybe 100+) through the faulty battery overcharging the remaining cells. When the second engine starts this will add to the current. Depending how long this procedure takes (connect the leads, run the 'good' engine at fast idle or above for a few minutes, start the second engine and wait a minute to ensure it will continue to run, disconnect the leads) there is a risk of an explosive mixture being present. If a cell has gone high resistance due to cracking of the plate structure the charging current will be concentrated in a small area of the plates which will then effectively be overcharged and start to release hydrogen.
The cracking problem was greatly accentuated with the move to maintenance free batteries. To reduce the water consumption/loss the alloying material for the lead was changed from antimony to calcium which had a detrimental effect on the mechanical properties.

Best regards

Roger

Re: Jump-starting a car made complicated?

Posted by Roger Bryant on Jul 12, 2019 8:51 am

My understanding is that this is to keep the spark when you connect and disconnect away from the battery to avoid a hydrogen explosion. The handbook for a VW I had also warned about trying to jump start in very cold (-15°?) conditions as the electrolyte may have frozen again resulting in a possible explosion.
Batteries do explode. I had one used for traction purposes that appeared flat. I charged it for a few hours and tried again. The battery, luckily in a shielded from me, exploded blowing the top off a cell and spreading acid all around. The cause was a crack in one of the interconnectors (a common problem with low maintenance batteries) which allowed me to charge at a few amps but when I put a load on the connection opened up and sparked.

Best regards

Roger

Re: Jump-starting a car made complicated?

Posted by mapj1 on Jul 12, 2019 1:27 pm

Actually jump starting is one of those things that is not unconditionally safe - there is a risk of misconnection causing 24V across a few milliohms, risk of welding of the clips to the point of attachment, and no clear idea of how much current will flow for how long.
However, in practice it seems to work very well, and I certainly do often go for the battery terminals if that is the easiest spot, as I do not fancy burning the paint on a thin section by clipping to the wrong thing..
regards Mike

Re: Jump-starting a car made complicated?

Posted by Sparkingchip on Jul 12, 2019 9:30 pm

Many years ago a driver working for a aggregate merchant could not start their 7.5  tonne steel bed topper wagon. He fetched a freshly charged battery and jump leads from the workshop.  
The battery on the wagon had actually froze and when connected to the jump leads it exploded blowing a hole in the lorry bed and the driver lost an eye.
I saw both the lorry bed and the driver after the event, both were a mess.
My understanding is you should put the leads on the dead battery first and the charged battery last making sure you are well clear of the dead battery. 

Andy Betteridge 

Re: Jump-starting a car made complicated?

Posted by Denis McMahon on Jul 13, 2019 11:19 am

Roger Bryant:
My understanding is that this is to keep the spark when you connect and disconnect away from the battery to avoid a hydrogen explosion. . .

. . . Batteries do explode. I had one used for traction purposes that appeared flat. I charged it for a few hours and tried again. The battery, luckily in a shielded from me, exploded blowing the top off a cell and spreading acid all around. The cause was a crack in one of the interconnectors (a common problem with low maintenance batteries) which allowed me to charge at a few amps but when I put a load on the connection opened up and sparked.


Hydrogen gas explosion? Someone needs to go back and look at the chemistry. When you continue to drive current backwards through a fully-charged lead acid battery, the water content of the electrolyte is broken into hydrogen and oxygen, which is emitted, giving grounds for caution. A flat battery does not give off hydrogen.

Modern cars manage the battery charging electronically in a much more controlled way than the old voltage regulators used to, and do not continue to pass charging current through a battery once fully charged producing hydrogen. That is the reason why nowadays we rarely need to top up the electrolyte - little generation of hydrogen and oxygen so the water does not get used up.

This sounds a bit like "nanny state" advice. A bit like that about switching off your mobile phone at a petrol station, for instance.

I have fortunately not experienced a battery explosion but I have known of cases of "sudden death". This I presume is due to  failure of an interconnector. In such cases it is wise to take extra precautions (wear goggles for instance). However the choice of how to connect to jump start is not relevant to this.
Denis4711

Re: Jump-starting a car made complicated?

Posted by Denis McMahon on Jul 13, 2019 11:35 am

mapj1:
Actually jump starting is one of those things that is not unconditionally safe - there is a risk of misconnection causing 24V across a few milliohms, risk of welding of the clips to the point of attachment, and no clear idea of how much current will flow for how long.
However, in practice it seems to work very well, and I certainly do often go for the battery terminals if that is the easiest spot, as I do not fancy burning the paint on a thin section by clipping to the wrong thing..

 
Yes there is a lot to be said for the KISS (Keep It Simple, Stupid) principle with instructions. Red to red and black to black is as simple as it gets and covers all circumstances. A misconnection could include connecting a red terminal to the engine block - short circuit and lots of sparks about to happen! And there are some older cars with positive earth still around - and these are the ones more likely  to have flat batteries.
Denis4711

Re: Jump-starting a car made complicated?

Posted by Denis McMahon on Jul 13, 2019 11:54 am

Sparkingchip:
Many years ago a driver working for a aggregate merchant could not start their 7.5  tonne steel bed topper wagon. He fetched a freshly charged battery and jump leads from the workshop.  
The battery on the wagon had actually froze and when connected to the jump leads it exploded blowing a hole in the lorry bed and the driver lost an eye.
I saw both the lorry bed and the driver after the event, both were a mess.
My understanding is you should put the leads on the dead battery first and the charged battery last making sure you are well clear of the dead battery. 

Andy Betteridge 

 
Nasty experience for the driver. The handbook I have does give precautions for batteries that may be frozen, and advises use of goggles and other protective clothing. However the order of connecting the jump leads is good battery first. Andy's understanding of connecting to dead battery first does make good sense to me.
Denis4711

Re: Jump-starting a car made complicated?

Posted by Sparkingchip on Jul 13, 2019 11:58 am

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I have a Nissan diesel van and a little Daihatsu Hijet pickup as an additional run about that is surprisingly useful, both of these have batteries that can be charged with the charger as I have owned for many years.
My wife has a Nissan Micra with stop/ start on it, if I use my charger to try and charge this battery it will probably kill it.
Like all technology, battery design and construction has moved on and you need to consider all the possible issues. 
I called the RAC to the pickup when it refused to start back in the winter,  the mechanic put the analyser on it and announced that a cell had gone and I probably needed to tighten the alternator belt to ensure it was charging at the correct voltage, that's technology of the Morris Minor period. The Micra is within the new battery technology period.

Andy Betteridge 

Re: Jump-starting a car made complicated?

Posted by Denis McMahon on Jul 14, 2019 1:40 pm

I too once had a single cell go flat, but the battery was pretty well spent anyway; it had gone flat a couple of weeks before, which does a lead-acid battery no good - sulphation of the plates and all that. It would not start the car using the starter motor. I had it on charge overnight then started the car by bump-starting it down a hill. I then drove to a fast-fit place to get a new battery.

I once had a belt that was inclined to slip, especially in wet weather; after driving through a puddle the battery light would come on for a few seconds. The belt was replaced during a routine service.  If the belt slipping became more prolonged it might cause the battery to go flat but I don't see how it would cause a single cell to fail.
Denis4711

Re: Jump-starting a car made complicated?

Posted by Philip Oakley on Jul 16, 2019 12:43 pm

The sequence is mainly to prevent simple accidental short circuits between the live battery and it's chassis. The local chassis is a very big, easy to accidentally touch thing, especially when starting the circuit (i.e. accidental completion of the direct short).

I hadn't heard about the 'frozen' electrolyte issue, which adds another 'keep away' feature of the connection steps.

Making the final connection (where the bang could happen) be the chassis to chassis connection makes it the least complicated, most likely to succeed step, hence fewer 'bangs'.

What wasn't mentioned was what to do afterwards.
A: does the local (dead) battery need a partial charge - if yes, then run the good engine for a while, and then TURN OFF, before attempting to start the other vehicle, otherwise you can blow the diodes/electrics in the alternator as it tries to source the full starter load.
B: the local (flatish) battery is weak - allow the two batteries to equalise, ensure the other vehicle is OFF, start as normal, letting the jump leads provide the additional current for starting (noting that the jump leads are not as good as the direct leads from the flatish battery to the starter, hence the equalising, or step A).

Summary, The loose end of the Plus-Plus lead is at most risk of accidental short. Do it in the right direction (away from the danger battery) and get it right first. Everything else falls into place.

Re: Jump-starting a car made complicated?

Posted by Andy Millar on Jul 16, 2019 3:53 pm

Your comment on the alternator half reminds me of an issue I had a few years ago (but only vaguely remember), hopefully someone here will be able to explain this better than I as it's a right pain when it happens: After jump starting a car I found a sensor had died in one of the cars - I think the one I was starting but may be wrong. I found out this was quite common, the inductance of the jump leads allows a voltage spike to be generated on starting which can take out random lumps of electronics. Does this ring any bells with anyone?

The cure is, where possible, to disconnect the jump leads after charging the battery in the jumpee, although I seem to remember thinking that if that battery was in a fit state to start that car then it would be of low enough internal impedance to stop the problem anyway. 

I do remember that it would have been cheaper to buy a new battery than what it cost me to replace that sensor!


On another note, this was all much more fun in the days when some cars were negative earth and some were positive earth. I remember sitting indoors having a cup of tea while waiting for my positive earth Ford Anglia to charge from my dad's negative earth Renault 4 (later mine). On a hill. The handbrake slipped on one of them, it rolled down the hill, the bumpers touched, and there was an awful lot of smoke...fortunately they were very thin (probably home made) jump leads otherwise we could have had two cars with exploded batteries! Ah, we had to make our own entertainment in them days...

Cheers,

Andy
Andy Millar CEng CMgr IET Mentor / IET PRA uk.linkedin.com/in/millarandy

Re: Jump-starting a car made complicated?

Posted by Philip Oakley on Jul 16, 2019 4:02 pm

Yep there is a high voltage surge at the end of the start phase as the peak current through the starter motor inductance is interrupted. The peak level is acceptable (for properly designed components and sensors) for a regular vehicle, but there can be awkward scenarios when the leads are attached, especially if the other car has it's engine running.

Re: Jump-starting a car made complicated?

Posted by Denis McMahon on Jul 17, 2019 7:00 am

Thanks, Roger, for the detailed explanation of what can happen if a single cell has failed. Others have made points about having the engine of the rescuing car running during the jump start. Although this seems to be common practice, and even recommended in some quarters, my philosophy has always been not to have the engine running. An alternator is not designed to power a starter motor, and it does not need to in a single car in isolation. Also, as others have suggested, the surges of the jump start will affect the control circuitry of the rescue car in a manner in which they are not designed to handle.

Provided the battery of the rescue car is in good condition, it should start the other car without the assistance of the engine running.
Denis4711

Re: Jump-starting a car made complicated?

Posted by Simon Barker on Jul 17, 2019 8:31 am

I'm pretty certain the reason for leaving the engine running on the "donor" car is so that you don't end up with two cars with flat batteries.

The alternator will put some charge back into the battery between attempts at starting the other car, and even if you do run the battery on the donor car nearly flat, the engine is already running so you can go for a good drive to put some charge back into it.

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