nuckollsr(at)cox.net
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 Posted: Sun Sep 10, 2006 2:17 pm Post subject: "Oversized" RG batteries and small generators |
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Received a clarification from Skip Koss at Concorde on the
a topic earlier this week where it was suggested that putting
"too large" and RG battery in an airplane with a small generator
(<35A) was potentially harmful to the generator.
Skip forwarded me a note that he'd sent to AOPA Pilot magazine
that included the following:
-----------------
My reference to small capacity generating systems is not for the protection
of the generator but for the airframe wire and C/B. I have had several reports
from operators of post WWII light airplanes (Ercoupes and Stinsons etc,) of
"After
an engine start, I have to reset the C/B around 5 times before the battery
charge
current is low enough to not pop the breaker"
Typically the RG-35AXC will accept 50 amps at 14 volts for a minute or so after
a start, before the current tapers down to less than 25 amps, so I recommend to
these operators to only use the lower plate count batteries that have a higher
internal resistance, so they won't over tax their wiring etc.
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So it seems that Skip's recommendations were mis-interpreted. The
circuit breaker tripping phenomenon in these small generator airplanes
is a predecessor to the nuisance-tripping of b-lead breakers in
modern airplanes.
Customer's airplanes were "peddling hard" to recharge the larger,
lower impedance battery which held the generating system at or
slightly above the nameplate rated output. As one customer noted,
he had to reset the breaker several times before it would stay in.
Skip's concerns were (based on tripping of the breaker) mostly
for wiring . . . but we know that as much as 200% overload beyond
the as-installed rating of the wire doesn't place it in imminent
danger.
I wasn't aware that the generator systems would suffer from the
same types of nuisance trips that alternators in most TC aircraft
do. The 60A breaker on a 60A alternator is designed to nuisance
trip. It took some serendipitous installations of modern hardware
in older airplanes to bring the condition forward. It seems the
phenomenon goes back a lot further than the beginning of the
alternator era. It's seldom a problem because most generators
and alternators are normally lightly loaded. A modern, perhaps
larger sized RG battery offers an opportunity to tax the system to
nameplate ratings of the equipment for extended periods of time.
If the system is designed to nuisance-trip, then this scenario
will get breakers pop'n. Interestingly enough, upgrading an older
airplane fitted with a 60A alternator could produce exactly
the same effects; a relatively 'trouble-free' airplane could
suddenly exhibit the symptom by simply changing out a flooded
cell battery for an RG battery.
Bob . . .
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< What is so wonderful about scientific truth...is that >
< the authority which determines whether there can be >
< debate or not does not reside in some fraternity of >
< scientists; nor is it divine. The authority rests >
< with experiment. >
< --Lawrence M. Krauss >
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