Need ways to start a car with a dead battery.- 92 Civic Auto.

"Alternator" is another name for "AC generator".

Both AC and DC generators can use either permanent magnets, or field coils.

The DC generator is an alternator modified with a commutator and brushes to mechanically rectify the output.

Both devices go back to the 1800's.

What allowed the switch to AC generators in cars was the invention of semiconductor diodes to rectify AC to DC. If you can call anything modern, the diode would be it.

Or would it?

Rectifying semiconductors were used to make "crystal set" radios in the very early 1900's. Ironically, they were then made obsolete by vacuum tubes in the 1920's. :)

For one thing, if an alternator needs power to its field coil in order to work then how did the car run without the battery?

you probably can't push start an alternator car completely without a battery, unless there is enough residual magnetism in the field coils to allow the alternator to bootstrap itself.

nate

Automotive alternators can self-excite. Once they are running, some of their own generated current is used to power the field. So if you disconnect the battery at that point, the engine keeps running. I don't think that's the case in every alternator design, certainly not historically.

At which point, the diodes will probably go up in smoke, leaving you with a dead (if present) battery, *AND* a dead alternator.

Running an alternator no-load/micro-load is a great method of letting the magic moke out of the diodes.

You are 'very' far out of date and way off on this one. GM made a 'one wire' alternator that will self excite if the rpm get up high enough. Once running, some alternators can feed off their own power output, but this isn't 'self excite' at all.

On 'modern' vehicles, if you disconnect the battery, the open ended alternator surge 'will' destroy the computer, ignition module, stereo if turned on and any other electronic part active before the alternator itself smokes out.

That just plain don't work no more, same as push starting a dead or really low battery car. The battery has to have over 10 volts in it to turn on the electronics and still have enough to turn the fuel pump on.

Mike

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When referring to the parts in automotive applications, the term "generator" always refers to the fixed-magnet device using a mechanical commutator, and "alternator" to the one with field coils and rectifying diodes.

If you start a car and disconnect the battery, it will remain running off the alternator. Once it is running the alternator does not need _external_ power to its field coil. However, note that the battery does provide some voltage regulation to the whole system, so electrical devices may behave erratically if you don't have the battery in there. (seen it myself)

The other electrical accessories are enough to keep the diodes from going up in smoke. Been there, done that, when a battery wire terminal corroded all the way through.

No, it won't. I've had a battery terminal corrode through and nothing of the sort happened. Granted, that was not a completely modern car (electronic ignition, but carbeurated). I've also run a modern car with a battery which was damaged and not taking a charge; while the gauges acted erratically and the car idled badly, it was not instantly damaged.

I have seen a mess of newer ones killed like mentioned....

Carb engines are different animals.

Mike

Matthew Russotto wrote:

Tell that to the alternator that died with a very visible (and aromatic...) cloud of smoke, a pop that sounded about like a .22 going off, and a "bacon frying" sound that was easily audible over the normal engine noise when, instead of just twisting the ground cable on the battery post as I was trying to do, I managed to lift it off the post and fumble it instead.

There might have been ten seconds, if that long, of "no battery connection" involved as I retrieved the cable end, got it lined up, and stuck it back on the post, but by the time that was accomplished, the smell of burnt electronics was overwhelming. Testing it on the local Kragen's machine an hour or so later showed a bad diode trio, and the pre-rebuild inspection later on revealed that one of them had been reduced to a lump of char, another was split open, and the third was just plain GONE - As in no sign of it to be seen other than a scorch mark in the empty spot where it should have been.

(Yeah, I know... I shouldn't have been dicking with it with the engine running - We've all got 20-20 hindsight.)

Pick up something like this:

"Mike Romain" wrote

Years ago, my 260K miles, 1988 Honda Accord fuel injection battery cable was cut by the fireman who was at the scene of the accident. The car idled smoothly for maybe 10 minutes until I'd shut it off. I still drive it today.

I believe there is a button you can push on these alternator to go full field. Otherwise, they're self regulated. Last I'd checked, the voltage never exceeded 15V when idling with the battery cable unplugged.