nuckollsr(at)cox.net
Guest
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 Posted: Tue Jan 02, 2007 8:14 am Post subject: Back-Up Battery ground (The multi-word answer) |
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At 08:24 PM 12/26/2006 +0000, you wrote:
| Quote: |
<trigo(at)mail.telepac.pt>
Bob, Larry et all
Please forget the reason why I may want to put the Avionics Back-Up
battery in the tail, and please answer the electric questions:
- Can I connect the (-) terminal of that battery to the tail's ground lug,
which is isolated from the fuselage and is directly connected to the (-)
terminal of the "Main" battery ?
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Can't imagine what you're describing here. Ground the
battery to local structure.
| Quote: | - Can I continue to use that same ground lug to connect the other grounds
( from the tail light, the strobe beacon, and the elevator trim motor)
there?
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Ground these devices to local structure. You don't
need a "tail ground" in a metal airplane for items
located in the tail.
| Quote: | - Isn't there any possibility of "ground loop"?
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No.
Do you plan a "mini battery contactor" for this "mini-battery"?
Even thought it's a relatively small battery compared to
the ship's battery, it is capable of significant fault currents
and should be treated like any other battery with respect to
positive disconnection via local contactor.
It would be helpful if you could publish sketches of your
proposed wiring.
------******------
Now that there's a bit more time to consider the questions,
let's do a quick review of the techniques and rationale for
installing "backup" batteries:
When you use the phrase "backup" there is an implied
notion that what ever power supply is normally expected
to carry some load aboard the aircraft suffers a low order
of confidence that it will always be there to do its job.
For decades, we've willingly launched into the grey with
a dependence upon alternators and batteries with less than
stellar performance records. Certain essential radios were
similarly plagued with propensities for failure. These
demonstrable facts led us to adopt a variety of fall-back
plans that included dual radios, hand-held radios in the
flight bag, back-up batteries either installed or portable
that could be pressed into service should our worst nightmare
become a reality. It's also a certainty that public
perceptions of aviation and systems that support comfortable
flight are polluted with Hollywood's
The present state of the art in electrical system architecture
and quality of materials have demonstrated that modern
alternators and batteries are a factor of 10 better than
the devices we grew up with. Further, as members of the
OBAM aviation community, we've come to understand that with
some failure mode effects planning and the crafting of
practical preventative maintenance programs, we can
justify a high order of confidence in an aircraft
electrical system.
The classic idea of a "backup" power source has been
with us since day-one. The engine drive generator or
alternator is primary, the battery is a secondary energy
source. If we consider nothing beyond the classic
alternator/battery based electrical system, modern
components and reasonable preventative maintenance offers
great reliability.
With the popularity of all-electric panels on the
rise, many airplanes have an open pad on the engine
were a vacuum pump came off . . . and for a very small
$time$ and weight budget, we can enjoy a second
source of engine driven power with essentially
unlimited endurance.
If for whatever reason, a builder finds it useful
to add a second battery, a technique for adding a
battery to any system are illustrated in Z-30 of
http://aeroelectric.com/articles/Rev11/AppZ_R11J.pdf
Major features are an always-hot bus where feeders
from that bus are protected at 7A or less by fuses,
5A or less by breakers. The feeder between the battery
and its bus is kept short. The fat-wire between the
battery and the rest of the system is controlled
by some form of disconnect . . . either a contactor
where the battery is large and used to aid in cranking
an engine or perhaps a plastic, 20A relay where the
battery is small and not useful during engine cranking.
Of course, a small battery COULD be charged-only via
a low voltage drop diode (Schottky) in cases where
the battery is never expected to deliver energy back
to the charging system.
Let's explore the original questions that launched
this thread:
There was some concern about "ground loops" . . .
Ground loops are not created by the mis-application of
a battery ground. Ground loops are the mis-application
of grounds in TWO systems . . . the antagonist and
victim systems. For the most part, ground loops are
avoided by careful crafting of the wiring for potential
victims . . . so while battery charge/discharge
currents for the small stand-by battery under discussion
are a potential source for miniscule noise currents
carried on the battery's ground path, good grounding
practice for potential victims makes the ground-loop
question moot. So yes, the battery can share a ground
to the airframe with all other non-victim devices
that might also ground to that location.
If I understand correctly, this battery is being considered
as "useful ballast" and was not a high-priority
item in a failure mode effects analysis. This begs
the question as to why the ballast should become a battery
as opposed to a chunk of dead-lead. If the builder's
confidence level in the rest of the system is so low as
to make addition of this battery attractive, then I'll
suggest that some attention be paid to the rest of the
system first. Perhaps the perception of poor reliability
is not justified. If the goal is to simply make a piece
of necessary ballast "useful" I would suggest caution.
System complexity goes up which increases likelihood
of generating maintenance issues with the extra battery's
installation. I.e, you now have a piece of ballast with
an ongoing maintenance requirement for the lifetime of
the airplane. You also have battery disconnect and hot-bus
structures co-located in the tail with the battery.
Would it not be an improvement on the $time$ expensed
in the ownership of this airplane if the ballast were
"dead lead" as opposed to "useful"?
This presumes, of course, that all the appropriate homework
has been accomplished on the rest of the system that
drives real usefulness for this "backup" battery to zero.
In other words, if this battery has real value as a backup
energy source, does this not speak poorly of any efforts
to make the additional battery unnecessary in the first place?
Bob . . .
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