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Back-Up Battery

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trigo(at)mail.telepac.pt
Guest

Posted: Tue Jan 02, 2007 10:31 am Post subject: Back-Up Battery

From: "Robert L. Nuckolls, III" <nuckollsr(at)cox.net>



 	  Quote: 	
		      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.



    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.



    With the popularity of all-electric panels on the

    rise,


	


I must say that my airplane will be double-electrically-dependant. I have a 

Subaru electrically-dependant engine and an all-electric panel (well, I'll 

still have 2 "steam-gauges": Altimeter and Airspeed Indicator). Since I have 

no possibility of installing a second alternator or generator, and the 

engine factory installation calls for 2 (equal) batteries, no wonder I am 

thinking about a 3rd (small battery), also fuelled by the possibility of 

having to put some wheight in the butt.



Opposite to your opinion, I think that, if I have to put some "dead" lead 

down there, why not put some "live" lead (aka battery), which will do both 

jobs? I believe it's a very good return on investment, like you love to say.



 	  Quote: 	
		  

    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.




	


OK, thanks. I think I already got the "ground loop" thing.



Carlos

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pilot4pay

Joined: 01 Jan 2007
Posts: 171
Location: Louisville, KY

Posted: Tue Jan 02, 2007 1:53 pm Post subject: Back-Up Battery

Subject: Re: Back-Up Battery 

<trigo(at)mail.telepac.pt>

From: "Robert L. Nuckolls, III" <nuckollsr(at)cox.net>

 	  Quote: 	
		      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:

    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.

Opposite to your opinion, I think that, if I have to put some "dead" lead 

down there, why not put some "live" lead (aka battery), which will do both 

jobs? I believe it's a very good return on investment, like you love to say.

Maybe it needs to be presented in another way.

Carlos, the issues that (I think) Bob is trying to address is not so much

about dead vs. live weight but the utility of having a battery that is so

small in capacity to be of marginal "real" value. The battery will require

periodic maintenance, so you will need reasonable access. Will that access

then require additional modifications to the plans/airframe. Now, again look

at your return on the modification. Add to that the associated circuitry and

wiring.

Is this mounting engineering task worthy of 4.5 Ah?

Recently I was researching another issue regarding electrical code and

aircraft hangars and (can't remember if it was in FARs or NEC) came across

some rules that prohibit charging aircraft batteries installed in the

airframe. (that finally answered a nagging question I'd always had as to why

my external power connector was electrically isolated from the ships

battery).

The "back-up" system installed in the 182 is a substantial system. I don't

recall the exact AH capacity of the battery but it is physically at least as

large as the primary battery under the cowl. Therefore there is an access

panel on the tail for the purpose of servicing the battery and avionics rack

there. So the system in the Cessna is a substantial piece of engineering,

but also quite massive.

It appears from your descriptions that the battery would actually end up

being just added system AH capacity, not a true "back-up electrical power

source". The Cessna system will power the entire avionics system for a solid

45 min. There is a test circuit that is part of every preflight, in order to

verify that the system is charged and available (another piece of

engineering that a reliable back-up would reasonably require).

Now for the few pounds of weight needed for balance, does it still seem

reasonable to jump these hurdles? I guess there still are personal

considerations involved, an aversion to unitaskers, or having something that

is just weight on an airplane where dead weight just seems like a waste of

useful load. But honestly, YOUR efforts would be better spent on

construction of your aircraft. This is not sarcasm, or ridicule, but a

discussion of the merits of an effort. Back up systems are great, but they

entail a level of engineering/cost/maintenance that needs consideration

before proceeding.

Believe me, in my profession(machine tool manufacturing), we constantly look

for more efficiency and utility in our equipment, and in that quest I've

come up with a lot of ideas, not all of which stood up under scrutiny.

Fortunately there were others to look at my ideas and point out factors I

may not have been aware of, or looked at from another viewpoint. I'm

grateful that I was saved the embarrassment of having spent time and MONEY

on features of little practical use.

Bob is correct in that all of the contactors/protection needs to be

considered in your endeavor, some are regulatory as well as for your safety.

Craig Smith

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_________________
Craig Smith
CH640 builder
SN: 0078

"Just think how stupid the average person is,
and then realize that half of them are even stupider!"
--George Carlin

trigo(at)mail.telepac.pt
Guest

Posted: Tue Jan 02, 2007 4:08 pm Post subject: Back-Up Battery

From: "C Smith" <pilot4profit(at)sbcglobal.net>



 	  Quote: 	
		   The battery will require periodic maintenance, so you will need reasonable 

  > access. Will that access then require additional modifications to the

 plans/airframe.


	


Not at all. It will be installed on the tail "deck" of the RV-9A, between 

the 2 halfs of the horizontal stabilizer. I don't know if you are familiar 

with it, but that zone is covered by a fiberglass fairing which makes the 

aerodynamic transition between the H.S. and the vertical stabilizer, and is 

attached with some 12 screws easily removable.



 	  Quote: 	
		   Add to that the associated circuitry and wiring.


	


Very simple. Since I was "authorized" by the experts-on-duty to connect the 

(-) terminal to the local ground, I will only have to squeeze in an AWG#14 

wire from the tail to the instrument panel (in fact, I already did, in case 

I decide for this battery), and connect it there to the avionics I decide to 

have back-up power, and to the main battery, through a Diode, to take 

charge.



 	  Quote: 	
		   Is this mounting engineering task worthy of 4.5 Ah?


	


I don't know how to do the exact calculations, but I suppose that it will be 

enough to power the Comm Radio, the Transponder, the EFIS and the Auto-pilot 

(or some of these 4, depending on the pilot's decision) for the 45 min you 

mentioned. Well maybe I should go for something a little larger (6 Ah ?), 

providing it stays within the C.G. limits

 	  Quote: 	
		   It appears from your descriptions that the battery would actually end up

 being just added system AH capacity, not a true "back-up electrical power

 source".


	


Not so, as you can see from the above descriptions. My idea is to use this 

battery always at the start-up procedure, to initially power the EFIS (to 

keep the battery used and to control its voltage), and from then, remain as 

a pure back-up only for the avionics, in case of alternator-Off and the 

engine-ignition-fuel pumps running only on the Main and/or Aux batteries.



 	  Quote: 	
		   There is a test circuit that is part of every preflight, in order to

 verify that the system is charged and available (another piece of

 engineering that a reliable back-up would reasonably require).


	


Above answered



 	  Quote: 	
		   Now for the few pounds of weight needed for balance, does it still seem

 reasonable to jump these hurdles?


	


Yes, it does to me. Does it already seem to you (no sarcasm here, I mean, 

after my explanations) ?



 	  Quote: 	
		   But honestly, YOUR efforts would be better spent on

 construction of your aircraft.


	


Don't worry, I'm not spending my efforts on this. Today I was riveting my 

canopy



 	  Quote: 	
		   Back up systems are great, but they entail a level of

 engineering/cost/maintenance that needs consideration before

 proceeding.


	


That's exactly the reason why I brought the subject to this knowleadgeble 

forum ....



 	  Quote: 	
		   Bob is correct in that all of the contactors/protection needs to be

 considered in your endeavor, some are regulatory as well as for your

 safety.


	


What I did was to install a 10 or 15A (I can't remember which) fuse on the 

tail, at the positive wire, right "after" the (+) future battery terminal. 

Isn't that enough protection?



Carlos

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nuckollsr(at)cox.net
Guest

Posted: Tue Jan 02, 2007 5:12 pm Post subject: Back-Up Battery

At 06:27 PM 1/2/2007 +0000, you wrote:



 	  Quote: 	
		  

<trigo(at)mail.telepac.pt>



From: "Robert L. Nuckolls, III" <nuckollsr(at)cox.net>



>    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.

>

>    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.

>

>    With the popularity of all-electric panels on the

>    rise,



I must say that my airplane will be double-electrically-dependant. I have 

a Subaru electrically-dependant engine and an all-electric panel (well, 

I'll still have 2 "steam-gauges": Altimeter and Airspeed Indicator). Since 

I have no possibility of installing a second alternator or generator, and 

the engine factory installation calls for 2 (equal) batteries, no wonder I 

am thinking about a 3rd (small battery), also fuelled by the possibility 

of having to put some wheight in the butt.



Opposite to your opinion, I think that, if I have to put some "dead" lead 

down there, why not put some "live" lead (aka battery), which will do both 

jobs? I believe it's a very good return on investment, like you love to say.


	


   If you already have two batteries, how about moving one of them aft

   to satisfy W&B requirements without having to add all the overhead

   for installing and maintaining a third battery?



   Only you can make the judgement as to whether the proposed, third,

   itty-bitty battery offers a good return. But after years of

   rubbing elbows with folks in the business, I know what reactions

   we'd get if we add one more line item to the maintenance list

   for continued air-worthiness . . . especially in view of the

   relatively small benefits offered by a 4.5 a.h. device.



   This is why I wanted to discuss it in more detail. I like it

   if everyone monitoring the List understands all the ramifications

   for such an installation and understand further that I would

   recommend avoiding such an installation until all the alternatives

   and cost of ownership issues have been evaluated. I.e., is there

   a simpler way to get the job done?



   Bob . . .

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klehman(at)albedo.net
Guest

Posted: Tue Jan 02, 2007 5:49 pm Post subject: Back-Up Battery

>

 >

 > I don't know how to do the exact calculations, but I suppose that it

 > will be enough to power the Comm Radio, the Transponder, the EFIS and

 > the Auto-pilot (or some of these 4, depending on the pilot's decision)

 > for the 45 min you mentioned. Well maybe I should go for something a

 > little larger (6 Ah ?), providing it stays within the C.G. limits

Carlos



As a guess off the top of my head, consider that if you intend to draw

say 4 amps out of a 4.5 AH battery then you will only get about  2 AH or

30 minutes with a new fully charged battery. A battery has high internal

losses when working that hard. There are discharge charts available for

many batteries that will give you a better approximation.



If your main batteries are say 16 AH then they won't run the run the

engine for much more than 30 min. each if you are drawing nearly 15

amps.  You'd likely do a bit better with the batteries paralled. Adding

another 4 amps to those main batteries will only shorten the engine

running by a few minutes. You might get more benefit per lb. from upping

one of the main batteries to 20 AH than from adding another 4.5 AH battery.



However your weight and balance is certainly another consideration if 

not your main concern with this. Yet another

important consideration will be system management though. If you

are ever in the situation where the battery capacity is that critical  I

would urge you to make the system as easy to operate as possible and

ideally zero management. I would not want to have to manage 3 batteries

in the heat of the moment...  Sizing the third battery so that it always

runs your critical instruments (E bus) and will outlast the engine might

be worth considering.



Also there are many ways to power a second generator on a subaru if you

decide you want more electrical capacity. I've even seen accessories

driven off a small flat belt from the prop shaft. I used two small

multi-v belts off the oem pulley on my ej22 and also I directly drive a

vacuum pump off the pulley end of a camshaft.



Ken

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Ed Anderson

Joined: 10 Jan 2006
Posts: 475

Posted: Tue Jan 02, 2007 6:02 pm Post subject: Back-Up Battery

Having flown an "electric dependent" Mazda rotary powered installation since 

1998, I understand the concern about not having enough electrons along if 

the electron pump stops working. However,  I do agree with Bob concerning 

the very marginal benefit vs complexity of a third small battery.  I started 

out with a modification of one of Bob's excellent wiring diagrams and ended 

up with an 60 amp alternator (with abnormal voltage - too high or too low 

warning )with the crowbar circuit tripper for the alternator and a 

voltmeter.  I started with two 25 AH Concord RCG batteries totaling 44 lbs!. 

I could have probably stayed airborn on just the starter motor {:>)



After flying for 4 years with this combination and only using the secondary 

battery to help crank on cold mornings, I now fly my "all-electric" with one 

680 Odyssey battery (which I swap out every 2 years regardless of its 

condition).  I have shed 30 lbs of battery weight and I believe that on the 

east coast that the 30-45 minutes the 17 AH Odyssey will give me at minimum 

current drain configuration should enable me to easily find a suitable 

airport.  My minimum current drain configuration is all but engine 

electronics off, GPS on battery, radio on.  I pull a CB so only one set of 

spark plugs are firing (the rotary has two per chamber), only one pair of 

fuel injectors are firing (plenty to keep me airborn) and only one fuel pump 

is running.



I have learned the hard way that weight is the performance killer and have 

spent years slowly getting my aircraft's weight reduced toward a norm for my 

bird.  I would strongly suggest following Bob's suggestion of moving one of 

your two batteries toward the rear for the W&B and forgetting the third 

battery.



FWIW



Ed



Ed Anderson

Rv-6A N494BW Rotary Powered

Matthews, NC

eanderson(at)carolina.rr.com

http://members.cox.net/rogersda/rotary/configs.htm#N494BW

http://www.dmack.net/mazda/index.html



---

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_________________
Ed Anderson
Rv-6A N494BW Rotary Powered
Matthews, NC
eanderson@carolina.rr.com

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