Text 15537, 50 rader
Skriven 2005-11-08 10:45:00 av Michiel van der Vlist (2:280/5555)
  Kommentar till text 15493 av Raymond Yates (1:3613/48)
Ärende: Fog lights
==================
Hello Raymond,

 > I think there's a combination of two thngs then. AFAICT the
 > jeep is wired so it draws fromthe battery and then the alternator
 > circuit replaces what it draws.

That makes no sense to an electrical engineer. ;-)

No, there is no way to wire a car like that as the battery has only one set of
terminals. Current can flow in or out of it, but not both at the same time.

The generator, the battery and whatever draws current form the circuit are
basically all connected to the same point.

 > This is based on the fact that one way one could test
 > to see if the alternator was working was to disconnect
 > the battery and see if the engine would still run.

Yes, with old cars that trick worked. But... the voltage regulator would not
work very well as there is no "buffer" any more. The voltage could rise way
above the specs.

 > You can't do that anymore with the new cars.

That is because all these new electronics does not take kindly to an extreme
rise of voltage. They are protected to overvoltage, but only uo to a point.

So the new voltage regulators have a "sense" circuit that senses if a battery
is present and if not, it disables the generator output.

A side effect is that it also is no longer possible to start a car with a
completely empty battery by towing it or let it roll down a slope. You need
jumper cables.

Last time I used them was in Ireland, a year ago. It was at what they claim is
the most western spot in Europe that can be reached by car. There were a bunch
of tourists and one of them was stuck with a dead battery. No one could help as
they had all come in rentals and rentals don't carry jumper cables. I was the
only one with my own car and I had the jumper cables to help them out. ;-)

 > Second the Jeep has a 4000 cc engine, and idles at ~1000 rpm.

I think that is the explanation. The engine is too big to notice the extra
load.


Cheers, Michiel

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