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Scot
Joined: 20 Jul 2009
Posts: 8
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 Posted: Wed Aug 24, 2016 5:07 pm Post subject: EFIS power switch - part 2 |
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Rather than hijack the previous thread I thought it prudent to start a new discussion.
My RV-7 has Dynon background devices (ADAHRS, autopilot panel, etc) connected to an Advanced Flight System EFIS. The backup power supply for the AFS EFIS is a TCW battery that will also power the Dynon devices connected to the Dynon network.
My intent was to install an 8 Amp secondary alternator to my vacuum pad for emergency backup power. With the TCW battery I had hope to have the option of driving the EFIS and Dynon network powered devices to the backup battery via a switch. That would allow the secondary alternator to support radios and any lighting I chose to leave on in that situation.
However the TCW battery system has an internal device that allows power to be delivered when the system voltage drops below 11.5 volts. Herein lies my problem. If I activate the secondary alternator via a field switch the voltage will not drop below 11.5 volts therefore the battery would not deliver power to the EFIS.
I suppose I could switch "off" the battery, that is to say disconnect it from the buss, and then disconnect the EFIS via a second switch. Not a complex solution but not an elegant one either.
I do not know of another solution, maybe the brain trust here could devise an elegant solution that is not prohibitively expensive. I know I could just install a larger secondary alternator but I would like to explore this option fully before abandoning it completely.
Thank you for your assistance.
Scott
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user9253
Joined: 28 Mar 2008
Posts: 1920
Location: Riley TWP Michigan
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| Posted: Thu Aug 25, 2016 4:20 am Post subject: Re: EFIS power switch - part 2 |
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I do not understand the problem. Why do you prefer to draw current from the TCW battery when the secondary alternator is operating?
If the secondary alternator is not capable of supplying the desired load current, then the alternator voltage will drop until it is equal to the battery voltage; at which point current will be drawn from both the battery and alternator.
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Joe Gores |
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Scot
Joined: 20 Jul 2009
Posts: 8
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| Posted: Fri Aug 26, 2016 7:21 pm Post subject: Re: EFIS power switch - part 2 |
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The total demand of the electrical system exceeds the output of the secondary alternator. By driving the EFIS and Dynon devices to the TCW battery I have more options in the event of of a primary alternator failure.
Additionally by disconnected the EFIS from the buss during start up, I eleminate any potential for brownout as well as giving me the option to drive the EFIS with the backup battery for preflight data entry without reducing the starting power of the main battery.
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nuckolls.bob(at)aeroelect
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| Posted: Sat Aug 27, 2016 1:44 pm Post subject: EFIS power switch - part 2 |
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At 10:21 PM 8/26/2016, you wrote:
| Quote: | --> AeroElectric-List message posted by: "Scot" <1975jsa(at)gmail.com>
The total demand of the electrical system exceeds the output of the secondary alternator. By driving the EFIS and Dynon devices to the TCW battery I have more options in the event of of a primary alternator failure. |
Normally, the secondary alternator is a get-home
with only the most useful things running for
en-route navigation thus saving what is supposed
to be a prudently maintained battery in reserve
for descent and approach to landing.
This is what the all-flight-conditions load analysis
is about . . . ENERGY MANAGEMENT in less than
optimum conditions. The artfully crafted airplane
can be cruised for hours on a fraction of the
power needed for a 'full up' compliment of
electro-whizzies.
| Quote: | | Additionally by disconnected the EFIS from the buss during start up, I eleminate any potential for brownout as well as giving me the option to drive the EFIS with the backup battery for preflight data entry without reducing the starting power of the main battery. |
Avionics haven't been a significant detriment
to adequate cranking energy for decades. Getting
the engine started takes a lot of POWER but rather
modest ENERGY. Modern SLVA (and later LiPO) cells
offer very low internal impedance which is really
what gets your engine started.
Remember the Start Stick? Recall my fussing
about Lead Acid Equivalency in LiPo batteries.
The purveyors of LiPo batteries were capitalizing
on the engine cranking ability of an itty-bitty
lithium battery when in fact, the ENERGY cotained
in the 'equivalent' battery was a small fraction
of that for the SVLA.
It only takes a few percent off the top to get an
engine started . . . running modern avionics during
clearance delivery and pre-flight is no longer an
operational concern.
Bob . . .
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