nuckollsr(at)cox.net
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 Posted: Sun Jan 07, 2007 12:09 pm Post subject: Alternator failures |
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At 10:54 AM 1/7/2007 -0500, you wrote:
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For years I was skeptical too that a weak battery would hasten alternator
death. However eventually I developed a few thoughts on why there may be
some truth to this in automobiles.
1. Higher longer charging does tend to make the alternator run hotter and
some internal VR alternators are not well cooled. Any vehicle that is
started with jumper cables is about to ask for a serious effort from its
charging system.
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So after getting the car started. Let it stand at
curb idle for ten minutes or so with minimal accessories
turned on. This little pre-charge should have the
battery boosted beyond it's max-recharge current
draw.
| Quote: | 2. Installing a new but partly charged battery may stress the alternator
even higher than ever as it charges the battery at max current for an
extended period. An old alternator just may not be up to the effort. Maybe
the brushes are worn or maybe the solid state devices get hotter than they
have for awhile. Certainly the cooling of old greasy/dirty components is
not as good as on a clean new unit. I'm not sure that a test stand is
going to successfully imitate the service environment that I'm thinking
of. This might explain death shortly after the new battery is installed though.
3. Weak "maintenance free" batteries are sometimes low on electrolyte. I
think that further reduces their capacity to absorb any excess voltage or
current and might lead to more voltage excursions. Most people never pop
the caps off automobile batteries any more as it is often not obvious how
to do it, or that it can be done. It seems that my little (8AH) AGM
batteries will accept very little current initially when fully discharged.
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An RG battery is sealed and should not be opened over
the service life of the battery. The do-called "maintenance
free" he's referring to is a class of vented, flooded battery
which will run without service as long as they are not
abused. But this is a separate issue not related to the
thread and generally not applicable since I doubt that
anyone would put such a battery in their airplane.
| Quote: | 4. As a WAG another contribution might be abnormal operation while fooling
around with a weak battery. If I leave the ignition/key on with my ND IR
alternator on my aircraft without starting the engine, the alternator does
draw several amps of field current and it will heat up quite noticeably
with no cooling airflow.
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Yup. All externally regulated alternators will do this.
They've been commanded by a regulator (full field output
voltage) to raise the bus voltage will the shaft is not
turning. Hence, full field current of about 3A x 12v dissipates
36 watts of unproductive heat within the machine. Some
modern IR alternators sense the ac voltage from the
stator windings and won't allow the regulator to come alive
until it senses that the shaft is turning. Apparently the
one you have does not have this feature. But when
the field is fully excited this 36 watt dissipation
is a fraction of that which the alternator would dump
under full load.
Consider a 60A machine putting out 14V (840 watts)
running about 75% efficient. This means that
at full load it rejects about 280 watts of heat.
| Quote: | Anyway my personal rule now is to change out any suspicious battery with a
new FULLY charged unit and I can sometimes send the vehicle to the
wreckers with the original alternator. I do run weak batteries in my
tractor but that has an external homemade VR that hangs out in the breeze
(like the alternator) and everything runs very very cool 
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You've suggested that IR alternators are, as a class of product,
more vulnerable to cooling starvation than ER alternators. I have
no personal observations that might support this idea. All
alternators are energy conversion devices that run at less than
100% efficiency. This means that when operated at name-plate rated
loads, they will have some heat to reject. An alternator cannot
be loaded to more than a few percent above name plate rating.
I.e. unlike generators, you cannot get the machine to dump
out say 2x rated load. They're magnetically current limited
so it all comes down to cooling. The smaller and more compact
the alternator, the more difficult it is to get air to flow
to vital areas . . . but this is still an INSTALLATION issue
and not one we should have to address by how we install
or deal with dead batteries.
It may well be that many poorly installed alternators
have been saved from early demise by any number of hat-dances
performed to various tunes. But in the final analysis,
you cannot expect ANY alternator to function to nameplate
ratings under all conditions if you don't provide for
adequate cooling.
Bob . . .
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