---------------------------------------------------------- AeroElectric-List Digest Archive --- Total Messages Posted Thu 09/13/12: 3 ---------------------------------------------------------- Today's Message Index: ---------------------- 1. 10:10 AM - Backup battery for fuel pumps? (Robert L. Nuckolls, III) 2. 10:47 AM - Mag noise (Robert L. Nuckolls, III) 3. 03:26 PM - Re: Backup battery for fuel pumps? (Robert L. Nuckolls, III) ________________________________ Message 1 _____________________________________ Time: 10:10:33 AM PST US From: "Robert L. Nuckolls, III" Subject: AeroElectric-List: 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. 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 . . . ________________________________ Message 2 _____________________________________ Time: 10:47:01 AM PST US From: "Robert L. Nuckolls, III" Subject: AeroElectric-List: Mag noise Noise in radio. Newly overhauled Bendix mags, new ignition harness new P leads, Noise only appears with mag switch on Both, is not there when switch is on left or right individually. Rechecked all ground connections, shielding is grounded at both ends. Engine is grounded to engine mount, engine mount grounded to airframe. All connections at switch have been checked. Aircraft is a Glastar with composite fuselage. Someone has suggested using double shielded wire for P leads. We have tried everything to solve the problem and have run out of options. Would really appreciate your help. I have your loose leaf book but could not find anything that specifically addressed this problem. Thank you. Do this experiment. Disconnect both the p-lead center conductors -AND- ground wires at the switch and leave them hanging. Fire up the engine and see if the noise persists. Kill engine with mixture. If noise goes away, reconnect wires to the switch but REMOVE local ground for the shields. Wiring data in the book is figure Z-26. Emacs! The shields should be grounded at the engine end only. I think this may fix your problem. Bob . . . ________________________________ Message 3 _____________________________________ Time: 03:26:24 PM PST US From: "Robert L. Nuckolls, III" Subject: AeroElectric-List: RE: Backup battery for fuel pumps? At 01:16 PM 9/13/2012, you wrote: >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. >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 . . . ------------------------------------------------------------------------------------- Other Matronics Email List Services ------------------------------------------------------------------------------------- Post A New Message aeroelectric-list@matronics.com UN/SUBSCRIBE http://www.matronics.com/subscription List FAQ http://www.matronics.com/FAQ/AeroElectric-List.htm Web Forum Interface To Lists http://forums.matronics.com Matronics List Wiki http://wiki.matronics.com Full Archive Search Engine http://www.matronics.com/search 7-Day List Browse http://www.matronics.com/browse/aeroelectric-list Browse Digests http://www.matronics.com/digest/aeroelectric-list Browse Other Lists http://www.matronics.com/browse Live Online Chat! http://www.matronics.com/chat Archive Downloading http://www.matronics.com/archives Photo Share http://www.matronics.com/photoshare Other Email Lists http://www.matronics.com/emaillists Contributions http://www.matronics.com/contribution ------------------------------------------------------------------------------------- These Email List Services are sponsored solely by Matronics and through the generous Contributions of its members.