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sgfreeman(at)smyth.net
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 Posted: Sat May 17, 2008 8:06 am Post subject: Schematic Review |
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I am building an RV-7A. The panel will be all electric and set-up for light IFR. I have completed my schematic and would like some input since I am new at this electrical stuff.
The basic configuration is a single alternator/two battery system. The primary and aux battery are the same size (17 amp-hr) and can both be used for starting. There is a main and essential (avionics) buss that can be isolated on either battery.
I envision the modes to be:
1) Normal operation: Start with primary batt, run system off alternator and primary batt, only turn aux batt on long enough to "top off" its charge and then isolate it back off.
2) Alternator failure and/or main batt failure: Power the avionics buss with the isolated aux battery for emergency power until able to land. This will be up to 2 hours for the avionics. Note: If there is remaining power in the Primary Batt- I would be able to get some time with it also.
3) Ground Start- If the Primary Batt is drained during a hard start situation, the option to switch to the aux battery for start is there. Since these are smaller dry cells, I like this option.
4) Plan is to rotate the aux battery to the Main Battery every other year or as necessary and replace the aux battery with a fresh one. This will provide preventative maintenance for the batteries.
Thanks for any input you can provide to make sure the schematic doesn't have errors.
Scott Freeman
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N246SF_Electrical_System_Design_Rev_2-_for_Review.xls |
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terryml5c2p6(at)sympatico
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| Posted: Sat May 17, 2008 9:35 am Post subject: Schematic Review |
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Scott, a couple of things I noticed; you have 2 unprotected unswitched
circuits directly connected to the Aux battery, namely the Aux Battery Bus
and the Aux Battery Switch. These both pass thru the firewall to the
cockpit, and if a fault to ground occured they could not be turned off. My
suggestion would be an in-line fuse for the Aux Bat Bus, and move the feed
to terminal 2 of the Aux Bat Master Switch to the other side of the Aux Bat
Relay. As shown turning on the Aux Bat Switch will energize both the Main
and Avionics buses, so presumably you would manually turn off the loads on
the main bus. To me it would make more sense to wire the fuse-protected lead
from the Aux Battery direct to the Aux Bat Avionics Master, this way it
would be an emergency-only switch, and the Aux Batt Master switch would just
be used to parallel the batteries for starting or to charge the Aux battery.
Personally I see it as a bad idea to mix emergency control functions with
routine functions on the same switch. FWIW, a good suggestion I've seen is
to put the DC power receptacle on the alway-hot bus. One advantage of this
is that you can recharge the battery on the ground without opening the
cowling.
Terry McMillan
RV7 FADEC dual electrical
| Quote: | From: "Scott Freeman" <sgfreeman(at)smyth.net>
Reply-To: aeroelectric-list(at)matronics.com
To: <aeroelectric-list(at)matronics.com>
Subject: Schematic Review
Date: Sat, 17 May 2008 12:00:56 -0400
I am building an RV-7A. The panel will be all electric and set-up for
light IFR. I have completed my schematic and would like some input since I
am new at this electrical stuff.
The basic configuration is a single alternator/two battery system. The
primary and aux battery are the same size (17 amp-hr) and can both be used
for starting. There is a main and essential (avionics) buss that can be
isolated on either battery.
I envision the modes to be:
1) Normal operation: Start with primary batt, run system off alternator and
primary batt, only turn aux batt on long enough to "top off" its charge and
then isolate it back off.
2) Alternator failure and/or main batt failure: Power the avionics buss
with the isolated aux battery for emergency power until able to land. This
will be up to 2 hours for the avionics. Note: If there is remaining power
in the Primary Batt- I would be able to get some time with it also.
3) Ground Start- If the Primary Batt is drained during a hard start
situation, the option to switch to the aux battery for start is there.
Since these are smaller dry cells, I like this option.
4) Plan is to rotate the aux battery to the Main Battery every other year
or as necessary and replace the aux battery with a fresh one. This will
provide preventative maintenance for the batteries.
Thanks for any input you can provide to make sure the schematic doesn't
have errors.
Scott Freeman
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| Quote: | << N246SFElectricalSystemDesignRev2-forReview.xls >>
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user9253
Joined: 28 Mar 2008
Posts: 1935
Location: Riley TWP Michigan
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| Posted: Sun May 18, 2008 8:01 am Post subject: Re: Schematic Review |
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Suggestion 1. An avionics master switch is controversial because it becomes a single point failure for all of your avionics. Modern avionics are not supposed to be affected by voltage spikes. If concerned about expensive avionics, one could shut off individual units, not as convenient as a master switch but safer. If you must have an avionics master, consider using a double pole switch wired in parallel. If one half of the switch fails, the other half will carry the load.
Suggestion 2. Feed the Aux Battery Avionics Master Switch directly from the battery without going through the Aux Battery Master Switch. Having two switches in series doubles the chances of switch failure. Re-label the Aux Bat Avionics Master switch as "Avionics Aux Feed".
Suggestion 3. Remove the ground wire from terminal 4 of the Aux Battery Master Switch. Instead, connect terminal 4 to terminal 2 of the Main Master switch. This essentially connects the two battery contactors in parallel for normal operation. Re-label the Aux Bat Master switch to Aux Bat Isolate. This switch would be normally closed, even after engine shut down. You would only open this switch in case of an emergency. This wiring change will simplify your system. There will be only one master switch to deal with. There will be more cranking amps available. And the aux battery will be kept fully charged.
Suggestion 4. Mount and wire the "Avionics Aux Feed" and "Aux Bat Isolate" switches so that they are normally in the down position. They would be toggled up in case of an emergency.
Suggestion 5. How about using a variation of Bob Nuckolls' Z-19? I like the feature of the low voltage module automatically isolating the aux battery without pilot intervention.
These are only suggestions to help and are not meant to criticize.
Joe Gores
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klehman(at)albedo.net
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| Posted: Mon May 19, 2008 4:33 am Post subject: Schematic Review |
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I am skeptical whether paralleling a double pole switch really helps
reliability. Mechanical switch failure would take out both poles. A
lightly loaded switch might suffer corrosion and wear issues at a
similar rate for both poles. One would not know if one pole failed
prematurely. For heavily loaded contacts perhaps the second pole would
help share the arc damage though??
Ken
user9253 wrote:
| Quote: |
Suggestion 1. An avionics master switch is controversial because it becomes a single point failure for all of your avionics. Modern avionics are not supposed to be affected by voltage spikes. If concerned about expensive avionics, one could shut off individual units, not as convenient as a master switch but safer. If you must have an avionics master, consider using a double pole switch wired in parallel. If one half of the switch fails, the other half will carry the load.
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user9253
Joined: 28 Mar 2008
Posts: 1935
Location: Riley TWP Michigan
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| Posted: Mon May 19, 2008 7:15 am Post subject: Re: Schematic Review |
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Hi Ken,
I agree with you that a mechanical failure will disable both poles of a double pole switch. It has been my experience that the majority of switches fail because of high resistance between contacts, especially in switches that are not used frequently. I also agree that one would not know if only one pole failed. But who cares as long as the load keeps working? I am not suggesting that double pole switches be used in place of every single pole switch, only for essential applications. The added cost and weight for a couple of DPST switches is not much. Depending on one's panel, it could be a hassle replacing a switch. I would want to delay doing that by using a more reliable switch. I believe that a double pole switch wired in parallel has an extended life compared to a single pole switch. Maybe someone has already done tests to prove or disprove this theory.
Joe
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daveleikam(at)wi.rr.com
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| Posted: Mon May 19, 2008 2:47 pm Post subject: Schematic Review |
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I flew an Archer for a few years that had two avionics masters side by side
wired parallel. Toggles will usually fail when turned on or off. If one
failed (never did), you knew it and the other was available.
Dave Leikam
#40496 N89DA (Reserved)
Muskego, WI
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