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nuckolls.bob(at)aeroelect
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 Posted: Wed Feb 18, 2009 6:05 am Post subject: Figure Z-10/8 compared to Z-11 + Aux Battery and Diode Bri |
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At 03:07 PM 2/17/2009, you wrote:
| Quote: | 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.
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Are your sure the battery is good? If the
diodes are preventing back-feed to the system
during cranking -AND- EFIS supply voltage
falls anyhow, then it seems the battery is
incapable of doing it's job.
Put a voltmeter on aux battery during cranking
and note that it should not drop below 12.0
volts for a fully charge battery.
Bob . . .
----------------------------------------)
( . . . a long habit of not thinking )
( a thing wrong, gives it a superficial )
( appearance of being right . . . )
( )
( -Thomas Paine 1776- )
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nuckolls.bob(at)aeroelect
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| Posted: Wed Feb 18, 2009 9:43 am Post subject: Figure Z-10/8 compared to Z-11 + Aux Battery and Diode Bri |
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At 11:21 AM 2/18/2009, you wrote:
| Quote: | 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
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I'd be pleased to put my hands on any diode
that exhibits any failure mode. I'll pay $5 plus
postage for any carcass you can send me.
Bob . . .
----------------------------------------)
( . . . a long habit of not thinking )
( a thing wrong, gives it a superficial )
( appearance of being right . . . )
( )
( -Thomas Paine 1776- )
----------------------------------------
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