nuckolls.bob(at)cox.net
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 Posted: Mon Nov 12, 2007 5:42 am Post subject: Z14 Bat/Alt Master switch circuit questions |
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At 03:26 PM 11/11/2007 -0500, you wrote:
| Quote: | Bob,
I m building a Z-14 (dual bat, dual alt, split bus) with a 100amp primary
alt, a 20amp aux alt (SD20), and identical Odyssey style, AGM
batteries. Thus, I ve biased my heavy/normal loads toward the primary
side/bus (heated prop, heated seats, etc).
I noted the z-14 drawing has s700-2-10 (three way) bat/alt master
switches, which are functionally equivalent to traditional Cessna style
split master switches. I understand this is a useful approach for
preventing activation of an alternator without an associated battery
on-line. However, as shown, it also means that taking a battery off-line
dictates that the associated alternator is off-line.
It seems that it would still be ok to leave an alternator on-line, with it
s associated battery off-line, as long as the cross-tie and other battery
were still on-line. However, this is not possible as shown in a std Z-14.
I m interested in knowing more regarding these potential scenarios and
potential alternative approaches&
1) What typically happens when a battery fails that might cause
someone to want it off-line?
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Doesn't happen in a adequately mainatined RG battery.
| Quote: | a. Shorts a cell, which causes it to drop approx 2 volts (???)
i.
Would I even know this while the alternator is working??
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| Quote: | ii. What
happens if it (and the associated alt) is left on-line???
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Not a scenario to consider.
| Quote: | b. Battery cable becomes disconnected, or contactor fails, etc -
which causes the battery to become off-line
i.
What happens to the alternator that is now working without an associated
battery?
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Voltage regulation might become a bit less stable . . . not big deal.
Alternator at-risk for being "stalled" by hi-inrush load but easily
accommodated by closing crossfeed contactor after the stall-event.
| Quote: | ii. How
would I know to turn on the cross-tie (assuming it was normally off) to
tie the lone alternator to the remaining good battery?
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Any time one of the two busses goes dark -OR- if the alternator
is known to be off-line (low volts warning light).
| Quote: | c. Other Failure scenarios???
2) It seems the wiring alternatives would include:
a. Do nothing (leave z14 as-is) since the potential loss of a battery
is low, and even so I could simply either 1) leave the failed battery and
associated good alt on-line (with the cross connector) and/or 2) complete
an expedited landing with only the aux power system.
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Gross battery failures in a maintained battery
are exceedingly rare. I cannot say "zero" but
it's so close to zero that you and I are unlikely
to hear of anyone who had the experience and even
less likely to experience it ourselves.
| Quote: |
b. Include a pullable breaker (or fuse) in series with the battery
contactor side of the s700-2-10 master switch to allow the alternator to
be on-line with the battery off-line (by pulling that breaker/fuse) and
leaving the switch in the both-on position. Of course this essentially
overrides the inherent alternator only prevention and implies the user
knows the cross-tie must therefore also be on.
c. Create a more complex logic-based circuit with separate bat and
alt switches that only energizes the alternator switches when a
appropriate battery is also connected (either the associated battery or
the other battery and the cross-tie)
d. Create fully separate bat and alt switches and leave it up to the
pilot to get it right (the simplest, but perhaps not the safest).
e. Other???
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Z-14 has simmered on the stove for over 10 years
with considerable praying over the list of failure
modes by lots of folks. As published, it meets the
design goals for minimum parts count and simple
operating procedures that avoid becoming a
diagnostician/mechanic in flight. If it were my
airplane, I'd install it as-published.
Bob . . .
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