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Backup battery for fuel pumps?

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nuckolls.bob(at)aeroelect
Guest

Posted: Thu Sep 13, 2012 9:10 am Post subject: Backup battery for fuel pumps?

Bob,



I recently purchased your schottky diode with the intention of using 

it as follows.  I wondered if you might be willing to share with me 

whether this is a good idea, and if not, why.



I am building an RV9, and am planning an all-electric fuel system.  I 

have a carbureted O-320 with no mechanical fuel pump.  I have two 

Facet cube fuel pumps in the cabin in parallel, just down-stream of 

the fuel selector.  Each pump has a check valve immediately 

downstream of the pump.  <snip>  Start the engine on the "aux" pump, 

and once it's running the "main" pump will come online.



         Understand



My question to you is about the electrical system for the pumps.  I 

am planning a dual alternator, single battery system similar to your 

"Z" diagrams, and have already purchased the B&C 60-amp main and 

SD-20 backup alternators (12V system). However I recognize one 

failure mode that I remain uncovered for: the loss of the negative 

terminal or wire from the battery.  It is my understanding that 

alternators don't behave well without a battery online and can 

produce erratic voltage swings which will render the alternator all 

but useless.  (I could be completely wrong about this.)  I recognize 

that the likelihood of an Odyssey 680 battery shedding a terminal is 

quite low, however I've had an automotive battery that did that exact 

thing and as a result I feel a need to account for that possibility.



         I think you're worrying too much. There are many

         critical features of our airplanes that are

         crafted to the robustness and reliability

         of prop bolts. For example, wing spars, flight

         controls, fuel lines, and yes . . . battery wires.



I also recognize that due to my electrically-dependent fuel system, I 

cannot live with wild voltage swings and the loss of my only 

battery.  I don't however, feel a need to carry the weight of two 

full-size Odyssey batteries when a single fuel pump consumes slightly 

more than an amp of current.  Instead I am considering a smaller, 

lighter, and cheaper 7AH AGM battery on a DP3T switch to one pump so 

that I could choose between running the pump off the battery bus or 

the 7AH battery.  I would like to simply run a perhaps 12GA wire from 

my main bus to my 7AH battery through a fuse/breaker/fusible-link and 

then the schottky diode.  Battery ground would go to the forest of 

tabs.  Peak reverse voltage would never be more than bus voltage, and 

I believe that even if the 7AH battery was in a pretty discharged 

state the current through the diode wouldn't be damaging (but I could 

be wrong about this as well).



         There is no good reason to believe that your

         fuel pumps will ever be totally deprived of

         of useful energy. If it were my airplane, I

         would power one pump from the battery bus and

         the other from the main bus. Each pump to enjoy

         its own breaker/fuse and switch.



I realize that the 7AH battery would be charged at something slightly 

less than bus voltage due to diode drop, but the Schottky minimizes 

that.  I also realize that when turning the master switch on before 

starting the engine, some amount of current from the main battery 

might flow to the 7AH battery if it is in a discharged state.  This 

might make it difficult to start the engine on the remaining 

energy.  But due to the typically short duration that the master is 

turned on before the engine is started, I don't expect this to occur 

for very long and the bus voltage will drop below the voltage of the 

7AH battery as soon as the starter is engaged.



My question is, is this a bad idea?  Is there something you see that 

I haven't considered?  I really wanted to avoid the complexity and 

weight of another contactor just for the 7AH battery, but a contactor 

would give me more choices on how and when to charge the 7AH battery 

and ensure it received as complete a charge as possible.  A contactor 

would however also increase system complexity for the pilot, which I 

want to avoid if possible.  I really would like to make my electrical 

system operation as simple as, say, a Skyhawk or a Cherokee.



         Separate sources and switches within the

         as-published Z-12 architecture should

         get the job done and without added complexity

         and cost of ownership for a second battery.



         As the builder -AND- pilot for this project,

         you'll be aware of value for fabrication

         and maintenance of robust connections to

         critical connections. If you have an urge

         to inspect things like prop bolts, fuel

         and oil levels . . . then due diligence to

         installation and maintenance of some critical

         wires will not add substantially to the

         burden . . . and it benefits MORE than

         availability of power to the pumps. Failure

         tolerance goes hand-in-hand with good craftsmanship

         for avoiding 'bad days in the cockpit'.

Thank you for your thoughts on this.  If this sort of consultation 

advice would be something you would typically charge an independent 

consult fee for, please let me know what your fee might be for 

something of this nature.  I don't wish to require significantly of 

your time without compensation.



         Let's post this to the AeroElectric-List and

         make it a classroom exercise as opposed to

         a fee for service.

   Bob . . .

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nuckolls.bob(at)aeroelect
Guest

Posted: Thu Sep 13, 2012 2:26 pm Post subject: Backup battery for fuel pumps?

At 01:16 PM 9/13/2012, you wrote:

 	  Quote: 	
		  Thank you for your thoughts, Bob.  It's not the battery cable or 

fittings I was concerned about as much as the failure of the actual 

threaded insert which is potted into the battery case.  That is what 

happened to my auto battery - the entire female threaded insert 

departed the battery case with the cable still bolted to it - 

spilling acid all over my engine compartment.  Different battery 

technology, of course.


	


    Your RG battery will never spill liquid and it's

    unlikely to suffer a terminal shedding event as long

    as that terminal has not been damaged by accident

    or over-torqued installation of hardware. I don't

    think I'd worry about that one. Use soft, 4AWG welding

    cable jumpers to the battery terminals and they'll

    be the most mechanically pampered of their species

    anywhere.

 	  Quote: 	
		  I do agree with you that the likelihood is less than that of a prop, 

prop bolt, engine mount bolt, or other rotating engine part failure, 

so as long as the electrical supply to the fuel pump is not the 

LEAST reliable component in the chain that keeps the fan running I 

should not go to extensive lengths to mitigate the risk.



Do you have any experience with the operation of alternators when a 

battery is removed from the electrical system?  I would love to 

conduct an experiment with my SD-20 and an oscilloscope, but 

unfortunately a scope is not in the airplane fund.


	




    Consider the fact that already running alternators

    are unaware of the existence of a battery except

    for transient events that might tend to 'stall'

    the alternator. Pitot heater, gazillion watt landing

    light, PM pump motor on gear, etc. These loads cause

    a sudden onset of load that MIGHT drop the bus enough

    to encourage support from the battery (below 12.5

    volts). But in steady state conditions the battery is

    essentially out of the loop.



    Many if not most alternators will come on line self

    excited assuming bus loads can be reduced sufficiently

    to allow residual magnetism to tickle the alternator to

    life . . . in fact, the Bonanza and Baron alternators

    are intended to come up and run self excited. Hence

    the independent alternator and battery switches in these

    airplanes. Unlike airplanes with the 'split rocker' independent

    operation of alternator is allowed in the piston Beechcraft

    models . . . and described in the emergency procedures

    for the airplane.



    If a battery were to become disconnected in flight,

    you're not likely to even know it happened especially

    in daytime where the bumps in bus voltage are unlikely

    to be observed -AND- assuming you don't have any of

    the high inrush loads I cited above.



    Most builders wouldn't know the battery was unhooked

    until next pre-flight.



    This is an easy experiment to conduct when your

    airplane is sufficiently assembled to run the engine

    and exercise the electrical system. I wouldn't add

    any hedges against failure until the risks for

    those failures are confirmed to be significant.



   Bob . . .

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user9253

Joined: 28 Mar 2008
Posts: 1928
Location: Riley TWP Michigan

Posted: Fri Sep 14, 2012 6:56 am Post subject: Re: Backup battery for fuel pumps?

This is not an electrical question, but relates to the previous question about fuel pumps.  What are the pro and cons about plumbing fuel pumps in parallel compared to series?  My concern is the failure of a check valve when fuel pumps are in parallel.  The pilot might not be aware of a check valve failure until its associated pump also fails.  Then the good pump will be circulating fuel in a loop through the failed check valve.  The fuel pressure to the engine could be reduced.  Or is this concern unwarranted?

Joe

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_________________
Joe Gores

rlborger(at)mac.com
Guest

Posted: Fri Sep 14, 2012 7:20 am Post subject: Backup battery for fuel pumps?

Joe,



It all depends on the pump.  On my Rotax 914 the standard arrangement is for two pumps in series, each paralleled with a check valve.  In case a failed pump will not pass fuel there's a parallel check valve to ensure fuel flow.  

[img]cid:A16AC493-1F3E-46DC-8400-B95D029C5247[/img]

Odds of a check valve failure are approaching zero.

You can make up a similar parallel setup with check valves on the output of each pump to ensure there's no back flow through a failed pump.



Blue skies & tailwinds,

Bob Borger

Europa XS Tri, Rotax 914, Airmaster C/S Prop.

Little Toot Sport Biplane, Lycoming Thunderbolt AEIO-320 EXP

3705 Lynchburg Dr.

Corinth, TX  76208-5331

Cel: 817-992-1117

rlborger(at)mac.com (rlborger(at)mac.com)

On Sep 14, 2012, at 9:56 AM, user9253 <fran4sew(at)banyanol.com (fran4sew(at)banyanol.com)> wrote:

--> AeroElectric-List message posted by: "user9253" <fran4sew(at)banyanol.com (fran4sew(at)banyanol.com)>



This is not an electrical question, but relates to the previous question about fuel pumps.  What are the pro and cons about plumbing fuel pumps in parallel compared to series?  My concern is the failure of a check valve when fuel pumps are in parallel.  The pilot might not be aware of a check valve failure until its associated pump also fails.  Then the good pump will be circulating fuel in a loop through the failed check valve.  The fuel pressure to the engine could be reduced.  Or is this concern unwarranted?

Joe



--------

Joe Gores





Read this topic online here:



http://forums.matronics.com/viewtopic.php?p=383060#383060



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mrspudandcompany(at)veriz
Guest

Posted: Fri Sep 14, 2012 7:44 am Post subject: Backup battery for fuel pumps?

This is not an electrical question, but relates to the previous question

about fuel pumps.  What are the pro and cons about plumbing fuel pumps in

parallel compared to series?  My concern is the failure of a check valve

when fuel pumps are in parallel.  The pilot might not be aware of a check

valve failure until its associated pump also fails.  Then the good pump will

be circulating fuel in a loop through the failed check valve.  The fuel

pressure to the engine could be reduced.  Or is this concern unwarranted?

Joe



	I would think that you would normally do a fuel pump test as part of

your preflight, i.e. turn each pump on one at a time and check pressure.

This will check the pump op and the check valve.  It is highly unlikely that

you will have both a pump and a check valve failure simultaneously.



Roger

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nuckolls.bob(at)aeroelect
Guest

Posted: Fri Sep 14, 2012 8:51 am Post subject: Backup battery for fuel pumps?

At 10:20 AM 9/14/2012, you wrote:

 Joe,



 It all depends on the pump.  On my Rotax 914 the standard arrangement is for two pumps in series, each paralleled with a check valve.  In case a failed pump will not pass fuel there's a parallel check valve to ensure fuel flow.



 

   Has anyone explored the cracking pressures for the

   two check valves built into Facet/Purolator style

   pumps? As Joe suggests, the probability of failure

   for a check valve (48 and 5 is very low. It might

   stick OPEN due to some obstruction and filters are

   supposed to stop that. 



 

 [img]cid:.0[/img]P



   For a pair of pumps plumbed in series, it seems that either

   pump would function with the other powered down unless the

   cracking pressure for valves in the inactive pump are too

   great and a low-pressure by-pass valved is called for.



   These pumps are MOST likely to quit due to lack of power.

   Given their all solid state, free-running excitation philosophy,



   See:  http://www.freepatentsonline.com/7150606.pdf



   . . . there is no part of this pump that is highly stressed or

   subject to wear. Dual pumps would address a HOST of ancillary

   failures like broken wires, bad switch, bad breaker, loss of

   buss, etc. Two, totally independent pumps/controls/protection/

   power source pretty much covers the bases. This extra plumbing

   and check valve thing may be an exercise in unsubstantiated

   worrying. Some simple tests would confirm/deny validity

   of the concern.



 

   Bob . . .

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