David -
No, I mean an alternator. Alternators put out plenty of power, relatively
independent of RPMs, IF the battery can supply them with 12 volts of
"exciter voltage". This makes them great for keeping a battery in good
shape.
However if the battery is dead flat (10 volts or so) the alternator output
drops dramatically. It will still make power (because it will
"self-excite" to some extent), but it won't make significantly more than is
being sucked by the ignition system. Generators, as you noted, require a
bunch of RPMs to make power, but put out about the same amount of power for
a given RPM, no matter what the battery voltage is.
If you want to get really technical, electricity gets made when a wire
passes through a magnetic field. A generator uses fixed magnets that
produce a constant magnetic flux, and has the copper wires would around a
spinning core. The wires are connected to a commutator and brushes so that
as the core spins the connections get switched back and forth, yielding
pulsed DC. Higher RPMs make more pulses, more stable voltage, and more
amperage per unit of time.
An alternator has the wires fixed in coils, while exciter voltage is used
to energize a set of electromagnets in the spinning core. Since the coils
aren't switched as the core spins, the output is AC, which has to be
rectified to DC in order to charge the battery and run the ignition. Since
the magnetic field is generated by electromagnets, reduced exciter voltage
reduces the strength of the magnetic field, which reduces the output
voltage. Since the resistance is (relatively) constant, amperage drops as
well.
(Technically, an alternator also produces slightly pulsed DC, but since the
electromagnets in an alternator can be made significantly smaller than the
commuatator windings in a generator, at a given RPM, the pulse rate is
enough higher to not matter.)
Chris Kotting
ckotting@iwaynet.net
(Whose Electrical Engineer boss has spent the past 8 years teaching me
about power engineering.)
On Thursday, June 04, 1998 11:12 AM, David Ramsey
[SMTP:dwramsey@worldnet.att.net] wrote:
> Hi Chris,
> I think that you mean a GENERATOR, don't you?
>
> r. An alternator will not charge a flat
> > battery unless you drive for many hours at a continuous high speed. An
> > alternator is great for KEEPING a battery from going dead flat, but not
so
> > good at recharging one that already is.
> >
>
> With a generator you can only charge the battery at rpms above 2000 and
less than 4500. If you have the lights on, your lucky to break even and not
lose any charge at speed. I have a amp gauge in my bug-i and have watched
it for years. I just put a alternator in following Franks instructions on
PDLJMPR, now I get 15 amps charge at idle with or without the lights on.
The gauge will go to 0 when the battery is reaching full charge. The
Mazda alternator I put in will charge my battery faster than my 10 amp
Sears battery charger.
>
>
> > Hey, a place that sells Interstate batteries may even honor the
warranty,
> > despite the fact that the dates aren't punched out. If you do want to
go
> > to an Interstate dealer, there's a dealer locator on their web page:
> > http://www.interstatebatteries.com/ BTW, I have no connection with
> > Interstate, (I don't even have one of their batteries). I just like
their
> > attitude, and they have a really helpful web page.
>
> I do have a Interstate battery in my Bug-I, they are good batterys. If
the battery has a dead cell no amount of charging will do any good. You
can ruin you generator or alternator if you have a dead cell as they will
try to charge the battery, when they can't. While you can take the battery
in to have it checked, you can also buy a battey Hydrometer, even a cheap
one with the floating balls will tell you if you have a dead cell. They
range in price from $15 for a good one to $2 for the ones with the floating
balls. You can buy a cheap 2 amp charger and it will charge any good
battery overnight.
> Crash
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