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07-05-2013, 11:30 PM
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Join Date: Apr 2011
Location: KHXF
Posts: 143
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Question re standby alternator
I am considering a single bus dual alternator electrical system.
Primary alt = 60 amp
Standby = 30 amp
The standby is just along for the ride unless the primary takes the day off.
Assume the primary goes south.
Standby now on line.
What powers the plane the battery or standby alt?
The real question is do electronic items need to be load shed or does the standby alternator simply decrease the discharge rate of the battery?
Tnx
__________________
John Palese
1) AAL Captain & Check Airman (ret)
2) USAF LtCol & Command Pilot (ret)
Active:
1) ATP / CFII ASME
2) Adjunct Professor-Aviation Science
3) MKE FSDO FAAST Lead Rep & Remedial Trainer.
RV-12 Transition Training, Rental & Instruction.
RV-6/6A, RV-7/7A, RV-9/9A transition training
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Last edited by JPalese : 07-06-2013 at 05:22 AM.
Reason: Typo correction
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07-06-2013, 12:42 AM
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Join Date: Dec 2011
Location: Livermore, CA
Posts: 6,797
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It depends on your electical load. If you are using 30 amps or less, then the battery will stay charged, the standby alternator will carry the load. If you are using more than 30 amps then either shed down to 30 or accept the battery discharging.
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07-06-2013, 05:33 AM
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Join Date: Apr 2011
Location: KHXF
Posts: 143
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Another question
Thanks Bob - I wasn't certain if a load above the standby alternator capacity would fry the alternator or drain the battery slowly.
While there will be an avionics master switch, many of the latest avionics devices do not have their own power switch, so pulling circuit breakers is the only load shed available.
What about forcing an EFIS (or other device) to use it's backup battery for power? Will simply pulling the device CB do this?
How often and in what manner should the backup batteries be discharged and charged to keep them in good shape?
__________________
John Palese
1) AAL Captain & Check Airman (ret)
2) USAF LtCol & Command Pilot (ret)
Active:
1) ATP / CFII ASME
2) Adjunct Professor-Aviation Science
3) MKE FSDO FAAST Lead Rep & Remedial Trainer.
RV-12 Transition Training, Rental & Instruction.
RV-6/6A, RV-7/7A, RV-9/9A transition training
RV-7 Transition Training LODA.
Drone Detection & Counter-measures
---..
...--
DE WB9JPH
<IXOYE><
www.facebook.com/rv12wi
www.facebook.com/rv7wi
http://www.terravigilis.com
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07-06-2013, 03:40 PM
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Join Date: May 2005
Location: X35 - Ocala, FL
Posts: 3,679
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Quote:
Originally Posted by JPalese
Thanks Bob - I wasn't certain if a load above the standby alternator capacity would fry the alternator or drain the battery slowly.
While there will be an avionics master switch, many of the latest avionics devices do not have their own power switch, so pulling circuit breakers is the only load shed available.
What about forcing an EFIS (or other device) to use it's backup battery for power? Will simply pulling the device CB do this?
How often and in what manner should the backup batteries be discharged and charged to keep them in good shape?
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The alternator will only out out its rated power, so if you're pulling more an that, you'll discharge the battery.
In my experience, depending on the panel, the avionics will pull a total of 10-20 amps, with momentarily a couple of extra amps when transmitting on the radio. The alternator field will draw a few amps. 5 amps for a 60-70 amp alt normally. The best quick load shed is kits and pitot heat if able. I wouldn't pull breakers unless you see your voltage dropping. Kill lights first then think about avionics. Many avionics can be killed by a switch on the unit as well, but that depends on what you have.
I'd only go to backup battery if the secondary alt fails. An EFIS will usually draw a couple of amps max under normal circumstances.
In my experience, most items with backup batteries recommend that you test the life at annual condition inspection. If under a certain length of time, they recommend replacing. I don't think most of them have memory like the Ni-Cads of old, so regular full discharge isn't necessary.
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Jesse Saint
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07-29-2013, 03:23 PM
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Join Date: Oct 2007
Location: St-Jerome,Quebec,Canada
Posts: 1,125
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Back up alternator
Hi Jesse
On this note, which Back Up alternator would you recommend??
The B&C with 20 Amps and an external regulator or a Plane Power with 30 Amps and internally regulated??
The Plane Power should be easier to install I guess..
Thank you
Bruno
rv4@videotron.ca
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07-29-2013, 03:29 PM
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Join Date: Oct 2006
Location: Garden City, Tx
Posts: 5,147
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The B&C 20-amp is rated for 20 amps at 28 volts at full RPM - but on a 14 volt system it will actually put out 32 amps, according to B&C. I've got this setup on my airplane (not flying yet), with a Plane Power primary and SD20 backup.
http://www.bandc.biz/pdfs/quickfacts_sd20.pdf
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Greg Niehues - SEL, IFR, Repairman Cert.
Garden City, TX VAF 2020 dues paid 
N16GN flying 700 hrs and counting; IO360, SDS, WWRV200, Dynon HDX, 430W
Built an off-plan RV9A with too much fuel and too much HP. Should drop dead any minute now.
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07-29-2013, 09:19 PM
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Join Date: Oct 2007
Location: St-Jerome,Quebec,Canada
Posts: 1,125
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Back Up Alternator
Hi Greg
Thanks for the info
I've got a Plane Power 60 Amps Main Alternator right now and as I'm planning to fly my -4 IFR eventually I wanna have some back up if the main one ever quits on me.
I'm leaning toward the Plane Power model so far due mostly to ease of installation with its internal regulator but I could be swayed toward the B&C SD-20 if it is Superior..
Thanks again
Bruno
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07-30-2013, 05:49 AM
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Join Date: Feb 2008
Location: Pensacola, FL
Posts: 374
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Nowhere close
Plane (pun intended) ignorance here: I assume on piston engines both the main and B/U alternators are hard mounts (always spinning, no clutch type mechanism?). Why not just use 2 regular alternators, are they just that more heavy than the backups? Are there not enough mounting points? Price point? I just don't understand (lack of knowledge, no judgement).
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07-30-2013, 07:23 AM
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Join Date: Mar 2008
Location: Dublin, CA
Posts: 1,261
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I recently went through all of the alternator options as I'm planning for future configurations. I looked at the Plane Power with the internal regulator and auto alternators with external regulators. The Plane Power unit seems like the simplest solution and it does provide the overvoltage protection. The external regulator configuration is not much more complicated though. A "Ford" type regulator is available for around $20 and it is very simple to wire. These regulators can be used with the B&C alternators. The B&C alternators are reasonably priced but their regulators are expensive. I think the 20A backup unit is a good choice. You need to buy there overvoltage protection module for $36 to protect that alternator too.
By the way an auto alternator with external regulator and overvoltage protection is about $300 cheaper than the Plane Power unit.
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07-30-2013, 07:48 AM
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Join Date: Oct 2006
Location: Garden City, Tx
Posts: 5,147
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Quote:
Originally Posted by LAMPSguy
Plane (pun intended) ignorance here: I assume on piston engines both the main and B/U alternators are hard mounts (always spinning, no clutch type mechanism?). Why not just use 2 regular alternators, are they just that more heavy than the backups? Are there not enough mounting points? Price point? I just don't understand (lack of knowledge, no judgement).
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Mounting locations, primarily. The B&C backup alternators (and some others) will mount on the vacuum pad on the back of the engine. With modern avionics there are plenty of ways to build an airplane that doesn't need to suck  so you can free up the vacuum drive pad.
__________________
Greg Niehues - SEL, IFR, Repairman Cert.
Garden City, TX VAF 2020 dues paid 
N16GN flying 700 hrs and counting; IO360, SDS, WWRV200, Dynon HDX, 430W
Built an off-plan RV9A with too much fuel and too much HP. Should drop dead any minute now.
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