[alpha1]

Originally Posted by R2D2

Modern cars should not be push started.

I have read this in many forums, but could never understand the underlying reason for this advice.

I have been forced to push start my car a few times, and it did without any hue or cry. (The battery had been very weak).

[Jeroen]

Originally Posted by DerAlte

It is not about the voltage, which the voltage regulator takes care of to keep in the 13.2-14.4V range. It is about the current that the alternator produces, and that is engine RPM dependent.

Add to this the voltage drop across the jumper cable, which usually has a gauge governed by low cost. This is what @sgiitk has written about above (voltage drop across the jumper cable x current flowing through it = power loss which appears as heat).

Since the cranking current of the recipient car is being supplied by the donor alternator in all this, it is better to keep the donor alternator around the max. operating point.

Just a few points, I don’t want to go into a to detailled technical debate about the current being RPM dependent. You are of course correct, in the sense that is the theory. But at the same time I’m not so sure how much more output you actually get by raising the RPM from idle to 2.500 on an average modern car alternator.

If I check a few diagrams from say Denzo, I would say maybe 20-25%. Modern alternators are pretty good at providing a high output even at idle it appears. Raising the RPM allows the output to go higher, but not by much.

My main question is really around why do we think it is “better” to rev the donor car engine? Better for whom or what. What is the quantifiable “better”?

There is nothing wrong by putting the full starting load on the donor battery. Why would it be good (‘better’) to put it on the donor car alternator? Potentially there could be some problems, me thinks!

Lets take an example. Lets say, somehow, I left the glove compartment light on in my Jaguar XJR. (SC V8, 4.0L) A week later I get into my car and try starting it, nothing happens.

So my friendly neighbour has a little Nano and offers me a jump start. Now the battery and the alternator of the Nan, compared to the Jaguar, are of course, minute. If not in size, at least in formal capacity and rating.

However, my Jaguar is maintained to perfection, ambient temperature is fine. So it really doesn’t need that much to get going, other than it is a big heavy engine to crank. Of course you never know.

Now, we hook it up to the Nano, using the largest gauge jump start cables known to man. Should you rev the Nano when starting the Jag or not ? Would it be “better” to have the Nano’s alternator take the full brunt of the Jag’s starting current?

To the other point on jumper cables, fully agree. Always and I mean always get the biggest ones money can buy. I have witnessed one case where a couple of idiots tried to jump start a car. Pathetic little jump cable, insulation melted during their sorry attempts, shorted the whole belly lot.

But better yet, get a booster pack!

Jeroen

[R2D2]

Originally Posted by alpha1

I have read this in many forums, but could never understand the underlying reason for this advice.

I have been forced to push start my car a few times, and it did without any hue or cry. (The battery had been very weak).

Push starting damages the emission control system. This is according to my car manual. This is probably due to the possibility of unburned fuel/air mixture travelling down the exhaust.

Jeroen - The reason I suggest raising RPM to 2000-2500 RPM is because many cars' alternators (at least for Jap cars and mentioned in the workshop manuals) have their tests for V & A done at that engine speed range. And I agree, raising the engine to over this limit is futile because the regulator kicks in and drops the voltage.

[Jeroen]

Originally Posted by R2D2

Jeroen - The reason I suggest raising RPM to 2000-2500 RPM is because many cars' alternators (at least for Jap cars and mentioned in the workshop manuals) have their tests for V & A done at that engine speed range. And I agree, raising the engine to over this limit is futile because the regulator kicks in and drops the voltage.

Thanks. I still like to understand the quantifiable benefits of revving the engine of the donor car during the jump start. I don't see why that would make the jump start better.

If the only problem was a dead battery it will start fine just being hooked up to another battery. Revving whether it might pour a few amps into the dead battery is irrelevant unless you leave it running for at least 15 -20 minutes.

Actually, what with modern cars and electronics you can find more and more jump start procedures calling for caution. It is being referred to as a last resort. Preferably jump start a modern car with a booster pack. Or just hook up the donor car battery with jumper cables but don't start the donor car engine at all.

Jeroen

[sgiitk]

Let me add my tuppence worth. Unlike a Dynamo which controls both the current and (then) voltage, where the peak output my be reached only above 5000rpm, an alternator regulates only the voltage (current being self-limited). An alternator runs at higher speed (may be 12000 rpm max with the pulley ratio taken into account) and approaches full output above 2000-2500rpm (engine) as has been correctly pointed out.

[Jeroen]

Originally Posted by sgiitk

Let me add my tuppence worth. Unlike a Dynamo which controls both the current and (then) voltage, where the peak output my be reached only above 5000rpm, an alternator regulates only the voltage (current being self-limited). An alternator runs at higher speed (may be 12000 rpm max with the pulley ratio taken into account) and approaches full output above 2000-2500rpm (engine) as has been correctly pointed out.

Sure, but if you compare the output at say 1000RPM and 2000RPM how much higher is the output?

I'm looking at some Denzo literature that shows 20-25% only.

And still, back to my original question. What is "better" about reving the donor engine during the jump start? What would suffer if you dont?

Jeroen

[Aroy]

Originally Posted by Jeroen

Sure, but if you compare the output at say 1000RPM and 2000RPM how much higher is the output?

I'm looking at some Denzo literature that shows 20-25% only.

And still, back to my original question. What is "better" about reving the donor engine during the jump start? What would suffer if you dont?

Jeroen

The alternator is designed for a current rating. Once that is reached the controller will limit the current by reducing the voltage. As there is no commutator in an alternator, it can run at quite high speeds; unlike the Dynamo in older cars; hence it is geared to generate a fair amount of current at idling speed. That the literature indicates that the peak current is reached at a low speed, means that the design is for it to deliver maximum at cruising speed of the vehicle.

[Sutripta]

Originally Posted by Aroy

The alternator is designed for a current rating. Once that is reached the controller will limit the current by reducing the voltage.

So that means that there is a current sensor in the "controller" (I assume the AVR)?

Regards
Sutripta

[Jeroen]

Originally Posted by Aroy

The alternator is designed for a current rating. Once that is reached the controller will limit the current by reducing the voltage. As there is no commutator in an alternator, it can run at quite high speeds; unlike the Dynamo in older cars; hence it is geared to generate a fair amount of current at idling speed. That the literature indicates that the peak current is reached at a low speed, means that the design is for it to deliver maximum at cruising speed of the vehicle.

We are getting into very complicated territory here. When we talk about design and current ratings I believe we need to agree on the definition first. There are a few international norms (e.g. ISO 8854) that define alternator ratings. If memory serves me correct, that definition stipulates it is the maximum rated output at 6000RPM.

For the purpose of our discussion we are also interested in the low or idle RPM output. Those same standards also define how that should be measured. Long story short, your typical alternator specification could look something like this: 65/100/13.5 Which means its max output at idle (1500RPM or engine idle) is 65A, its max out put is 100A (6000RPM) and the test voltage is 13.5V.

As the voltage can vary quite a bit, especially during jump starting, its anybody's guess what the real output would be.

Originally Posted by Sutripta

So that means that there is a current sensor in the "controller" (I assume the AVR)?

The amount of field current allowed to pass through the regulator to the rotor, or field coil, is controlled by the voltage feedback from the battery.

https://alternatorparts.com/understa...ternators.html

Back to the original question and my comments: I dont believe there to be a big difference in alternator current at idle versus 2500 RPM, so whatever 'additional' current that produces, I think it is pretty small difference. To get the maximum you would need to really step on the accelerator and rev your engine at 6000RPM.

So, I still don't understand why reving the engine during jump starting is "better". Personally, I think it is just a myth cooked up by people not fully understanding how these things works. Truth is that, the fundamental physics to which some are eluding are correct. The question is, in practice, at 2500 RPM how big are the the practical effects.

Look up the specifications of your car alternator. Those two first numbers are the theoretical spread between idle (say 1000RPM) and 6000RPM. Now how much more than 'idle output' would you get at 2-2.500RPM. that number would be the theoretical delta. You still need to account for some changes in Voltages as well under Jump start conditions. It's not going to be an impressive number over and above the idle output I think. But I've been wrong before!

Its' a bit like filling your car tyres with Nitrogen. All advantage of nitrogen are true, however, on a regular car you would be hard pressed to actually get any (measurable) benefits.

By all means rev those engines, but I doubt it actually does anything. If anything, in case of big difference between battery/alternator capacity between the two cars, I can actually see some potential for trouble.

The best way to get some idea on how this really works is to take some measurements during an actual Jump start. I checked Youtube. A billion zillion video's showing how to jump start, some rev, some dont. But nobody measures anything.


Jeroen

[sgiitk]

Originally Posted by Jeroen

Sure, but if you compare the output at say 1000RPM and 2000RPM how much higher is the output?

I'm looking at some Denzo literature that shows 20-25% only.

And still, back to my original question. What is "better" about reving the donor engine during the jump start? What would suffer if you dont?

Jeroen

Remember alternator output is self limiting therefore only the 20-25% difference.

Revving the donor ensures that the donor battery does not go flat, that is all. Remember you can safely use a Nano to start a Merc or a Rolls, so one has to cater for that also.

[Jeroen]

Originally Posted by sgiitk

Remember alternator output is self limiting therefore only the 20-25% difference.

Exactly, to my earlier points, why rev at all?

If the donor car battery can start the donor car engine, it can start the dead battery's car as well. Unless you have a bad battery to start with, you can can repeatedly start a car from a normally charged and maintained battery.

The donor battery will not go flat and it will not be charged much either. Because if you know what you are doing, a Jump start is over in minutes. As soon as the car with the dead battery starts, it will charge its own battery immediately.

Originally Posted by sgiitk

Revving the donor ensures that the donor battery does not go flat, that is all. Remember you can safely use a Nano to start a Merc or a Rolls, so one has to cater for that also.

Not so sure. It comes down to capacity of the Nano battery compared to what is needed to crank the Merc or the RR to sufficient rotation speed, whilst also ensuring the Voltage is sufficient for the electronics to work properly.

I have seen batteries from petrol car engines being used on diesel engine and they could not crank them sufficiently. Or ran very hot!

[Sutripta]

Originally Posted by Jeroen

The amount of field current allowed to pass through the regulator to the rotor, or field coil, is controlled by the voltage feedback from the battery.

https://alternatorparts.com/understa...ternators.html

Hi,
My query was very specific, prompted by an equally specific observation by another member. (And its purpose was to make people think. I've always been flummoxed as to how something as simple as an alternator and its AVR can leave people so confused. This is not the first time we are having this discussion on this forum.)
Thanks for the time and effort in providing the link, but such a very basic explanation of how an alternator works does not really answer my question. And will not get people to think.

Pls. don't take this post in the wrong spirit.

Regards
Sutripta

[Jeroen]

Originally Posted by Sutripta

Hi,
My query was very specific, prompted by an equally specific observation by another member. (And its purpose was to make people think. I've always been flummoxed as to how something as simple as an alternator and its AVR can leave people so confused. This is not the first time we are having this discussion on this forum.)
Thanks for the time and effort in providing the link, but such a very basic explanation of how an alternator works does not really answer my question. And will not get people to think.

Pls. don't take this post in the wrong spirit.

First time I'm part of this discussion so I don't have the context of the previous discussion.

Alternators and AVRs might be simple, but they can be tricky to fully understand how they actually work. So we tend to loose ourselves in, not necessarily wrong, but generic statements such as "self regulating", or talking about "field coils" and "voltage feedback"


Very simplistic to my understanding:
The actual alternator current output doesn't get measured. The current that feeds the magnetic field of the rotor is directional proportional to the input voltage to that field.

Say the alternator is running at a given RPM It has a certain loading and we increase the load. The output voltage drops and the regulator will increase the voltage (and thus current) to the rotor. That increases the magnetic field, which will also increase the output voltage.

An electronic regulator provides continuous and instantaneous adjustment of rotor current by sampling the alternator output voltage and by comparing it against a internal standard reference. Which was quite an improvement over the old, relays based, bolt-on regulators.

Here is another link that shows the above in some detail, with some simple calculations:

file:///C:/Users/etmjtdo/Downloads/Alternator_Secrets-2.pdf

Having said all that, I'm not so sure how things work on a modern cars where the regulator is actually part of the engine management system. I'm sure they have found better, more elaborate ways to control this. These days on modern cars the alternator will be disconnected during engine start and brought on line gradually. All parts of the magic electronic trick. I'm sure it must do other things as well, or the old things better.

Not sure if the above and the subsequent link was what you were looking for?

I'm still trying to learn whether there is any evidence that revving your donor engine during a jump start is better, If so why? Based on my very limited knowledge and understanding I'm not convinced.

Your thoughts perhaps?

Jeroen

[srikanthns]

Has anybody heard about Battery desulphator? I am told this will avoid the clogging of anode and the deposits, thus increasing the battery life. I have seen a few models selling on e bay. Has any of our members used this? If so, what is your assessment?

[Sutripta]

Originally Posted by Jeroen

First time I'm part of this discussion so I don't have the context of the previous discussion.

So let me rephrase my question for a wider audience, and put in (some sort of) context.

In the bad old days of dynamos, burnt out (rotors = the power producing part)
were common enough.
Dynamos needed an external control unit. Normally it had three submodules:
A) a cutout (remember, no diodes)
B) a voltage control unit. Controlled field current so that output voltage did not exceed the max value set.
C) a current control unit. Also controlled field current so that output current did not exceed maximum current set.
We are not getting into esoteric controllers like the Lucas (=prince of darkness) CV regulator, which should be of interest only to the vintage and classic car buffs) which combined the current and voltage control functions in one unit)

If the settings were not properly set (and chances are if a roadside mechanic had fiddled with the settings, it was now incorrect), and the current limit was exceeded, the dynamo would burn out.

My question: how are the current and voltage limits taken care of in an alternator.

Regards
Sutripta