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Jump-starting a car made complicated?

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.

  • 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?
  • 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
  • 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..
  • 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

  • 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.

  • 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.

  • 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.
  • 91c765583660667b8cc178623472d42e-huge-20190607_125211.jpg


    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
  • 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.

  • 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
  • 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.