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N509RV(at)eckenroth.com
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 Posted: Tue Feb 17, 2009 1:10 pm Post subject: Figure Z-10/8 compared to Z-11 + Aux Battery and Diode Bridg |
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I had previously written asking for advice on trouble shooting my system which is based on Z-11 with the addition of a aux battery to prevent brown out during engine starts.
"I have a RV9A wired as per Z-11 with a 7A aux battery as an EFIS keep alive
during start. Lately the EFIS has been graying out during engine start which implies that the aux battery is being tapped for other than the electronics during engine start.
The batteries are isolated using two diode bridge rectifiers from B&C. The E bus which feed the EFIS and other electronics gets it's power from the main bus through one of the bridge rectifiers. Both of the batteries power the main bus through individual contactors. The E bus is also powered through the second bridge rectifier by both batteries direct and then controlled by the E buss switch. Normal start is main battery contactor on, aux battery contactor off, and E bus switch on. This should isolate the aux battery to the E bus until the aux battery contactor is turned on after engine start.
I have checked both diode assemblies to see that they are allowing current
to flow in the correct direction and they are not allowing backfeed. Everything checks out properly using the voltmeter. However it seems to me that the problem must lie with the diodes since they are the only link between the batteries with the aux battery contactor off. Can a diode function correctly under low load conditions and then temporarily break down under start conditions."
The replies to my questions were that the diode cannot sometimes fail and is the aux battery fully charged. This did not solve my problem. I discussed my schematic with an EE friend who thought the problem was due to the 2 diodes feeding one buss and that a diode could temporarily leak back if the voltage on the anode was a certain amount less than the voltage on the cathode. This would be the condition with two batteries feeding the same buss with voltage drop from the diodes. He thought that this could be enough to sometimes brown out the EFIS. If this is true then my system design needs to be changed.
I notice that the new Z-10/8 utilizes a brown out battery with the use of a relay to isolate the two batteries during engine start. I can easily change over to this schematic and will if the 2 diodes are the problem.
I know there is a lot of education and experience out there. Can my problem be due to the design utilizing the 2 diodes.
Regards
Paul Eckenroth [quote][b]
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floyd wilkes
Joined: 11 Jun 2007
Posts: 39
Location: spring branch, tx
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| Posted: Tue Feb 17, 2009 2:11 pm Post subject: Figure Z-10/8 compared to Z-11 + Aux Battery and Diode Bridg |
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Paul,
Do you have the aux battery connected to the connected to the "external backup power" on pin 15 or just to the keep alive on pin 2. When I only had one battery, I would get the brown out you mention. When I added the aux battery connected to pin 15, my problems went away.
Floyd Wilkes
601XL
[quote] ---
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Floyd Wilkes |
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user9253
Joined: 28 Mar 2008
Posts: 1927
Location: Riley TWP Michigan
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| Posted: Wed Feb 18, 2009 9:24 am Post subject: Figure Z-10/8 compared to Z-11 + Aux Battery and Diode Bridg |
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Paul,
I have tested diodes at work that check out fine with a voltmeter but fail under load. I think the reason is that the voltmeter is testing the diode with 1.5 volts, whereas the actual circuit operates at a much higher voltage. So using a voltmeter is not a good test. Here is a test you can try: Disconnect the main battery to eliminate any possibility that it is affecting your tests. Connect one lead of a 12 volt test light to the aux battery negative or ground. With the other lead, touch each terminal of the diode. The light should illuminate on the battery side of the diode but not on the other side. The fact that you are using two diodes should not cause the symptoms that you describe.
Joe Gores
[quote][b]
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Joe Gores |
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user9253
Joined: 28 Mar 2008
Posts: 1927
Location: Riley TWP Michigan
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| Posted: Wed Feb 18, 2009 2:13 pm Post subject: Figure Z-10/8 compared to Z-11 + Aux Battery and Diode Bridg |
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Paul,
I think your EE friend is mistaken about diodes conducting when reversed biased, no matter if there are one or two or more diodes. Are you sure that aux battery contactor is open during engine cranking? Have you tried disconnecting a wire off from it to see if the E-buss voltage still drops during starting? I was wondering if any part of the aux battery ground circuit is shared with the starting curent from the main battery? Have you checked for bad connectons? It is difficult to comment on your proposed changes without having a schematic to view and understand better.
Bob,
I admit that diodes rarely fail. The one that failed at work was in an industrial battery charger for 48 volt electric hi-lo. The charger put out 150 amps when initially connected to a discharged battery. The batteries are as big as a school teachers desk and were located remotely from the chargers. One time I shut off the 480V supply to change a fuse but forgot to unplug the dead battery. My wrench shorted out between the fuse stud and an aluminum heat sink. The wrench immediately welded itself in place and turned red hot. Luckily I was wearing leather gloves. I was leaving the area as the wrench turned white hot and vaporized the center section of the wrench. Lessons learned: disconnect ALL power sources before working on equipment. And even "dead" batteries have some power left in them.
Joe
[quote][b]
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