> I have experiance with marine electrical systems. Their are some very
>light, vapor sealed rotary battery switches. They would need to be modified
>for aviation use. But they are very reliable and are capable of high current
>loads. For two battery systems, the rotary switch is labeled left, right,
>both.
It's very easy and in fact recommended that multiple batteries
each have their own contactor for connection to the system. Furhter,
there are connections to each battery that do not go through the
contactor for running components of an electrically dependent
engine. I.e., your electrically dependent engine should operate
whether or not the DC master switch(es) are ON or OFF . . .
>> It's not an issue of electrical demand its the reliability and redundancy
>> that concerns me. I have the Stratus Subaru engine and I'm getting the
>> dual ignition but obviously don't have mags. So the second battery
would be
>> primarily backup power for the ignition, but potentially you could use two
>> batteries of the same size and provide complete redundancy. Of course on
>> the other hand adding that reduncy could also increase the complexity and
>> reduce the reliability of the system. I'm just interested if there is some
>> experience out there with some real simple redundant electrical systems.
This topic has been discussed at length on the lists, in our book,
in articles downloadable from our website and illustrated in numerous
wiring diagrams downloadable from the website. Two-battery installations
are no big deal . . .
>The concern you have for flying with your soob electrical system is
>legitimate. If auto systems are going to be used in aircraft, then you must
>build in redundant systems to match typical aircraft systems.
I'll suggest the LAST thing we want to do is match "typical" aircraft
systems . . . the architecture, components and pilot's understanding
of those systems have not changed in 50 years.
>If a battery fails in most aircraft, the engine will continue to run,
>In your auto set up if the battery fails so does your engine. A
>two battery system duplicates the two magneto system pretty close
>as far as redundantcy is concerned. Batteries can and do fail with
>out warning.
Batteries do NOT fail without warning. It's just that most of us
don't pay any attention to what the battery is trying to tell us.
We replace tires when the tread is gone, overhaul cylinders when
the compression gets low, file nicks out of propellers when noticed,
etc . . . . but we beat a battery until it fails to crank the engine
. . . and replace it after we've propped the airplane for the third
time.
Very rudimentary preventative maintenance techiques will insure
that nobody reading these words will EVER experience battery failure.
>To reduce the weight penalty two smaller bateries can be used,
>but they must be sized with absolute precision.
Don't know about "precision" but some consideration must be given
to what a battery's task is. Batteries have three duties: (1) crank
the engine, (2) stabilize alternator(s) and (3) provide power for
essential goodies should alternator output be lost. The BEST
hedge against alternator failure is two alternators . . . dump
the sucky vacuum pump and install a second alternator. THEN the
batteries on board no longer have to be sized for standby power.
Total system weight can be much reduced.
>This topic is outside my area of knowledge, so educate me. I have an HDS
>with Stratus Soob. I have a small motorcycle battery (14AH) which has
>cranked me up without hesitation for 18 months and 98 hours of flight
>time. If my alternator light comes on and my instruments haven't indicated a
>problem, I think I can turn off my master and fly a long time on battery.
I'd encourage this builder to replace THINKING with KNOWING how long
his airplane will stay aloft battery only. Your battery should be
no smaller than your fuel tank. If you do not KNOW that the battery
capacity on board will allow you to use up fuel on board, then I'll
suggest further investigation, personal education and perhaps
some changes to your system are indicated . .
> something is wrong with the battery, shouldn't there be early indications.
> For thirty-one bucks I can get a new one. I have dual ignition but never
> thought I needed dual batteries. What are the odds of loosing all
> electrical if you are maintaining your airplane and monitoring your
> instruments? My empty weight is 602 and I like that.
There's no pat answer to this . . . a number of options exist for
insuring your flight system reliability. My personal goal for
system reliability is, "From the time I break ground to the time
I land, I don't want to break a sweat." This doesn't have to mean
nothing ever fails. It means that I have to architecture a system
for failure tolerance and educate myself in its operation and
maintenance to sustain that level of reliability. Dual batteries
and indeed dual alternators can often make for a LIGHTER airplane.
>1. Flight over hostile territory (I fly in the Pacific Northwest w/o a lot
>of "emergency landing fields").
>2. Alternator craps out.
This happens a LOT on certified aircraft . . . just check the
service difficulty reports at faa.gov . . . the REASON alternators
crap a lot is because the overwhelming majority of the TC
fleet are fitted with crappy alternators . . . holy-watered and
configuration managed right into antiquity. TC alternators fail
routinely in obscene ways every month . . . through bolts broke,
cases cracked, bearings seized, windings burned . . . you name
it . . . it happens. By LAW, that alternator will be returned to
ORIGINAL configuration and bolted back on some poor pilot's airplane.
B&C and similar alternators (Nipon-Dienso) have DEMONSTRATED
operational reliability suggesting that most will run the lifetime
of engine with nothing more than a belt change. B&C's return rate
in thousands of sales over the past 10 years has been under 1% for
the total fleet!
>3. The above fact is discovered by the voltage dropping alarmingly low on
>the voltmeter.
Why not some form of ACTIVE notification of alternator failure?
Most pilots don't look at the voltmeter until the panel starts to
go black or the radios begin to mis-behave . . . with no ACTIVE
notification, one tootles along with everything operating and
lights blazing thus squandering a limited energy resource. By the
time you know anything is wrong, your options are all gone.
>4. Shedding the electrical load still leaves too little juice to power
>things like radio, fuel pump, and CD player.
See articles on website and chapter in book on system reliability.
>Hence, I installed a second 17 ah battery with a switch on the panel that
>kicks in the second battery and provides extra time to make a safe landing
>with needed equipment.
Dual 17 a.h. batteries is 34 pounds total. Add to this about 8-10 pounds
of vacuum system for 44 pounds. Now consider taking out two batteries,
one vacuum system and putting one 4 to 7 pound alternator and one
10 pound battery for a weight REDUCTION of 27 pounds and a net
increase in flight system reliability unequaled in ANY certified
aircraft.
>5 years ago I was on a 300 mile cross country in a Cessna 150. 30 miles from
>my destination the voltage regulator apparently failed wide open. Sparks
and
>smoke started spewing our of the instrument panel as my first indication.
In
>the short time I took to turn everything off, it was too late, basically
>everthing that was "on" failed including the electric clock. I was also
>amazed that in that short time, the battery was completely discharged. Once
>the smoke cleared and I calmed down. I continued to my destination with a
>completely inoperative electrical system and made a normal no flap landing.
>With one battery and an electronic ignition system, I would have had a dead
>engine as well.
Forgive me, I am in no way trying to demean this writer's experience
but this is typical of the "dark and stormy night" stories that
drive our design, maintenance and operating decisions on homebuilts.
I'll suggest that TC aircraft can be used only as examples of
how NOT to architecture, maintain and operate an electrical system. I've
often written that my personal mind-set climbing into a rental TC
ship is that I don't care if ANY of that stuff is working 5 minutes
after take-off. I intend to get where I need to go without breaking
a sweat. That means UNDERSTANDING the limitations of a machine designed
by government blessed factories and maintained by government blessed
mechanics and procedures . . . and outfitting myself to deal with the
worst. $30 worth of parts and a weekend's effort could elevate the
average TC aircraft into 21st century . . . but it ain't gonna happen.
This is why we need to look past our experience with TC ships to
design and operate our airplanes.
Virtually EVERY concern voiced above can be addressed with simple
choices in architecture and knowledge of how the system and its
components operate. Education and decisions based on understanding
will make it so . . .
Bob . . .
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( Knowing about a thing is different than )
( understanding it. One can know a lot )
( and still understand nothing. )
( C.F. Kettering )
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http://www.aeroelectric.com
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