easy way to generate an ac signal

[The Natural Philosopher]

A running engine on a disconnected battery does NOT guarantee a broken alternator.

A had it happen to me. Car came to traffic lights and died, and the lights went out..a seconds thought revealed the most likely problem was a loose battery terminal. It had in fact shaken right off over a bump.

Putting it back on and bashing it with a boot heel restored normality.

There are ways to blow and alternator,, but total loss of a battery to charge is not one of them.

A shorted battery is a different matter.

[The Natural Philosopher]

However harry is MOSTLY right in that a so called 'DC' motor is in fact an AC motor driven by a 'mechanical inverter'.

Oh and a DC pulse has an AC component :-) and 'motor' implies more or less continuous movement which a railgun is NOT.

[Dave Plowman (News)]

No - you said with a 'flat' battery. A very different matter from an open circuit one. The first is likely why someone would jump start a car - the second a fairly unusual occurrence.

An breakdown service would have jump started it too with the same result.

If the battery was totally open circuit it will never take a charge. Or rather if it will, needs a very long slow charge at high voltage - days.

[Andy Dingley]

Nope.

Nope^2

Nope^3

[The Other Mike]

the regulator and the diodes".

years. Alternator design has moved on a bit since then.

Didn't you used to be called thirty six or something similar?

[Onetap]

So what new technology could I expect to find inside my alternator that would stop the diodes getting fried? I didn't notice any such gizmos. Please tell me and I'll have a look. I still have it in the shed.

No. You're confusing me with someone else.

[Onetap]

Yeah. Been there and the alternator survived. I've jump started cars dozens of times and never had an alternator problem, except this once.

[Dave Plowman (News)]

Pretty well nothing guarantees anything. But it can be the cause of some makes failing.

[Dave Plowman (News)]

Anyone with sense would have looked to see if there was any sign of life from the battery - like dim panel lights. If absolutely nothing, get professional help. Since you obviously didn't know of the possible hazards of attempting to jump start an engine with an effectively disconnected battery.

[Dave Plowman (News)]

Modern semiconductors could stand a higher peak voltage.

[Onetap]

It wrecked my alternator. But I'm highly delighted to learn that it can't happen and that I'm wrong. I have a Mondeo alternator in my shed that cannot have been damaged in this= manner, so it must be worth at least =A330 to one of you internet alternator exper= ts.=20 You'll get double that on E-bay. Any offers? No? Thought not.

[Onetap]

No, I didn't. I do now.

But this discussion has resulted from the internet alternator experts' assertions that it could not happen.

I do not recall whether the lights were dim or off completely, but the car had run some weeks previously. You're saying that in those circumstances you wouldn't have jump started the car, since you knew the risk of damaging the alternator.

I say you're full of bullshit.

[Onetap]

Clearly, not high enough.

So, there are no alternator-saving gizmos in modern alternators, other than better semiconductors?

[Dave Plowman (News)]

Never say never.

So at least you've learned something.

Why is it so difficult for you to understand that many have far more knowledge about these things than you?

I've known for years it was bad practice to disconnect the battery with the engine running - due to the possibility of damage to the alternator and other car electrics.

You've only recently learned this. Get over it and move on.

[Dave Plowman (News)]

It's easy to make things fool proof. Making them c**t proof is the difficult part.

[Onetap]

I'm well aware that others have more knowledge about their field of expertise than I do, electrical stuff being one of many such fields.

That is not the issue.

Splendid; so why didn't you put in a post saying something to effect that what the Onetap bloke is saying is quite plausible?

The issue here is the many others who THOUGHT they knew a lot about alternators and have persistently questioned the truth of what I said.

So will you be telling them that they were wrong, they should learn the error of their ways and get over it?

I did, years ago. Read the posts above, many posting on here still do not believe it is possible.

[tony sayer]

Yes, I've flow a few thanks...

[Dave Plowman (News)]

Because batteries don't often go totally open circuit. It suggests you left a modern car for a long time unused. The quiescent current drain being what caused it to get into this state. It's been covered here and elsewhere often - depending on make and model the only safe way to do this is by disconnecting the battery. And even then, not all cases will result in damage to the alternator. So advising a total ban on jump starting just in case this happens is being too extreme.

[whisky-dave]

How do you hold the blades still while rotating the fan at 5000 RPM that sounds far more dangerous.

[Onetap]

You have contradicted yourself. In a post above you said; "An breakdown service would have jump started it too with the same result."

'Professional' help would have had the same result, you've said, and a fruitless argument about "it was alright before" would have followed.

My experience, with the AA (see above), is that they do something differently.