In yesterday's Telegraph, HJ wrote the following in response to a question about jump starting:
'Connect the starter car's positive battery terminal to that of the stricken vehicle, then attach the former's negative terminal to a metal earthing point on the latter. Linking both cables battery to battery might cause ECU damage.'
Does this rule apply to all modern cars? Is the positive/negative earth thing irrelevant now as all cars are the same? Is it still OK to attach battery charger leads directly to the battery terminals, or do similar rules as for jump starting apply?
Comments would be very welcome. Cheers.
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Point 1) It is very difficult (almost impossible) to generate spurious voltage across a good battery, with the emphasis on the word good. Because it is almost like a short circuit inside in terms of its internal resistance and so suppresses any voltage spikes. But this assumes the battery connections are making good contact, because the jump leads are probably going to be made onto the connections rather than the battery lugs themselves -- ideally onto the lugs.
2) If you have a positive earth system and want to follow the procedure the negative would be connected first to the negative of the battery.
3) The safest method of battery charging is off the car -- but you lose the settings, such as for the CD etc. If you charge on the car you need to know the battery has not gone open circuit before you apply the leads. If it has, you will be applying excessive voltage unless you use one of the special voltage controlled chargers, of which much has been written in previous threads on HJ.
4) I am curious as to why the jump leads are connected HJ style. Is it something to do with the transient voltages generated on the supply leads when the starter turns? Or that the battery might be duff? I would have thought in either case the result would be much the same. Spikes across starter motor means spikes across duff battery terminals.
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I agree;connecting to the battery or the engine shouldn't make the slightest difference-however the handbook on both my cars says different.
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Yes, you are right, my own handbook shows the method but no other comment. I have also just looked in a 95-97 Haynes manual and it is in there also.
If there is a spark when the negative connection is made, because things have not been turned off, by connectng to the bodywork, that spark will be well away from the battery explosive gasses.
Also, if the battery connection to the bodywork is less than good, it could also be a way to improve the starting prospects by bypassing that.
But no electronics damage reason, as yet.
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dont think it would do any harm to electrics but attaching battery to battery will let the flat battery soak up power away from the starter,if the battery is very flat couple battery to battery and run engine for a few mins,this will run some charge into flat battery then move neg lead away from flat battery on to engine earth point this is how we start large diesels with flat battery .charging on car will be ok.had an unfortunate connection to 11000 volts to the extent tyre went on fire with current trying to earth but no electrical damage despite radio arial touching power supply,lucky or what !
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Didnt read it but is perfectly correct.not just in possible damage to ECU but could set off air bags if fitted.shouldnt charge or use a booster unless it is made for the newer cars.
for same reasons as cable jumping.IIRC BMW have special earth point for this reason.
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Was mech1
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I've always taken the "last negative connection to the car body" as the rule to stop a spark igniting any hydrogen generated by the battery.
I can't see it making any difference to any on-board electronics.
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"possible damage to ECU but could set off air bags if fitted"
This is a re-statement of the reason for my point 4 posting. It is not an explanation of it happening. Where is the convincing evidence for such statements?
There has to be a sound technical reason as to how this happens, if it does happen. All things can be explained technically if they are correct. The whole of technology is built on it.
I can understand 'using only a specially designed booster'. The battery may be duff, as I said, and excessive voltage could then be applied because the battery is not capable of holding the volts down to the region of 12-14.5 volts.
But, that would happen whether you put the booster negative on the battery negative terminal or the car bodywork.
Hope no one take offence at my defiant stance. It is only a technical argument.
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When you jump start any new or nearly new peugeot which is multiplexed, as soon as the car starts turn the lights on so the surge is taken away from the BSi unit
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When you jump start any new or nearly new peugeot which is multiplexed, as soon as the car starts turn the lights on so the surge is taken away from the BSi unit
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Why cannot life be simple.
Mondeo II manual says
Once the engine has been started run both vehicles for a further 3 mins before disconnecting.
To reduce voltage peaks on disconnection, switch on the blower and heated rear screen
DO NOT switch on the headlights in place of the heated rear screen. The peak voltage could blow the bulbs.
Haynes says swith ON the lights.
I plan to stick with the Ford instructions.
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This subject gets more weird by the post.
A starter motor, taking sort of 200 amps is connected across the electrical system and disconnected at each car start attempt without damaging the electronics, yet if you disconnect a jumper lead carrying much less current you are advised to first switch on your headlamps or rear window heater! It is not April 1st is it?
Was the prototype electronics rig on which the 'technician' discovered this need also able to tolerate a car starter motor? Somehow I doubt it.
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A starter motor is not the same, as the alternator is not going while the starter is used. The procedure is to prevent over-voltage from the alternator due to sudden removal of a load and with a battery that may not be providing much of a load due to being defective. In effect it could be disconnecting the battery with the engine running, if the battery has broken inside or is dead not just flat.
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If the ground strap is in good condition then there will be only milliohms resistance between the car chassis and the battery negative - hence it makes no different whether the negative jump lead is connected to chassis or battery. There IS an argument about sparks and hydrogen though.
The other issue (switching on electrical loads) is all to do with 'helping out' the alternator's voltage regulator. The voltage regulator controls alternator output by duty-cycle control of an energising current passed through the rotor. It is difficult for the duty cycle to be varied very quickly (although reponse time will vary from one model of alternator to another) and so by putting a load on the alternator at the time of disconnection you reducing the amount by which the rotor energising current has to be reduced. This 'helps' the regulator to adjust down to a lower load and thereby reduces the chances of the output going over voltage. Most modern ECU's do include overvoltage protection circuitry in the power supply stage, although once or two do not (e.g. Honda springs to mind).
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St & A
Yes I accept that. A good point. I had momentarily lost site of the duff battery situation.
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