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danlist(at)syz.com
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 Posted: Sun Mar 10, 2013 3:00 am Post subject: Cross feed contactor current |
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Hi everyone.
I'm working on an RV-10 and am considering something like a Z14 configuration with one battery (PC680) behind the baggage compartment in the rear as per plans and another PC680 firewall forward (As a fairly electric-heavy plane intended for light IFR, I plan to have two alternators anyway, and I like the idea of being able to use the rear battery to power up avionics to plan flight plans, listen to radios, etc. while not worrying about draining the battery in the front for starting the engine, not having issues with EFIS reboots while starting, and having the option of closing the cross feed for more power during cold engine starts - a reality sometimes of living in northern Canada).
I have a question though. Say in a worst case scenario I drain the rear battery pretty much dead and then close the cross feed contactor. What kind of current would be trying to flow from the good battery to the depleted one? I'm suspecting that if the charge difference between the two batteries is pretty large, that current could end up being significant. Is the 4 AWG specified in Z14 between the batteries and respective contactors, not to mention the current rating on the contactors themselves, sufficient in such a scenario, considering that as far as I understand it, the current would only be limited by the batteries' internal resistance? Or if the charge difference is large would it instead be advisable to charge the batteries independently from their respective alternators until the differential is smaller before closing the cross feed contactor?
Thanks for any insight!
Dan
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Syzygy Research & Technology
Box 83, Legal, AB T0G 1L0 Canada
Phone: 780-961-2213
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klehman(at)albedo.net
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| Posted: Sun Mar 10, 2013 4:16 am Post subject: Cross feed contactor current |
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Engine off, the voltage on the good battery is not high enough to charge
the depleted battery so the highest current possible is what that
alternator can output after startup minus other loads.
Closing the crossfeed prior to start should not be a problem regardless
of charge state and it will be helpful even with a partially discharged
second battery.
If the second alternator is a small unit I'd leave the crossfeed open
after startup as a means of limiting the current into a discharged
second battery and extending the battery life. I don't like really fast
charging. In fact even in your scenario I'd wire for automatic crossfeed
closure during cranking which works well for me. One less thing to think
about as it closes and then opens automatically and it opens before the
alternator is up to speed.
Ken
On 10/03/2013 6:59 AM, Dan Charrois wrote:
| Quote: |
<danlist(at)syz.com>
Hi everyone.
I'm working on an RV-10 and am considering something like a Z14
configuration with one battery (PC680) behind the baggage compartment
in the rear as per plans and another PC680 firewall forward (As a
fairly electric-heavy plane intended for light IFR, I plan to have
two alternators anyway, and I like the idea of being able to use the
rear battery to power up avionics to plan flight plans, listen to
radios, etc. while not worrying about draining the battery in the
front for starting the engine, not having issues with EFIS reboots
while starting, and having the option of closing the cross feed for
more power during cold engine starts - a reality sometimes of living
in northern Canada).
I have a question though. Say in a worst case scenario I drain the
rear battery pretty much dead and then close the cross feed
contactor. What kind of current would be trying to flow from the
good battery to the depleted one? I'm suspecting that if the charge
difference between the two batteries is pretty large, that current
could end up being significant. Is the 4 AWG specified in Z14
between the batteries and respective contactors, not to mention the
current rating on the contactors themselves, sufficient in such a
scenario, considering that as far as I understand it, the current
would only be limited by the batteries' internal resistance? Or if
the charge difference is large would it instead be advisable to
charge the batteries independently from their respective alternators
until the differential is smaller before closing the cross feed
contactor?
Thanks for any insight!
Dan -- Syzygy Research & Technology Box 83, Legal, AB T0G 1L0
Canada Phone: 780-961-2213
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nuckolls.bob(at)aeroelect
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| Posted: Sun Mar 10, 2013 6:41 am Post subject: Cross feed contactor current |
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| Quote: | Closing the crossfeed prior to start should not be a problem
regardless of charge state and it will be helpful even with a
partially discharged second battery.
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Good answer. Old timers on the List may recall some
discussion we had on this topic many moons back. A
List reader opined that energy was exchanged in
significant amounts when one connected a charged
battery to a dead one. He bolstered the argument
with anecdotal observations of the sparks that
happen when one hooks up jumper cables between
vehicles. He also claimed to have measured significant
current flows in a charged-to-dead battery
connection.
I went to the bench and repeated the experiment.
Yes, there WAS a spark when I connected a fully
charged battery to a dead battery. Yes, the initial
current flow was substantial . . . about 40A as
I recall . . . but it fell off very quickly as
the dead-battery's chemistry quickly adjusted to
a new terminal voltage based on a source voltage
too low to significantly charge the dead battery.
Later on I offered a revision to the ol' mechanic's
tale that a battery was one of the 'best filters of
noise' on the bus. I used to subscribe to that
notion myself. My colleagues believed it.
But consider that a battery has three states of
existence in the properly functioning system.
We KNOW that an engine driven power source must
be adjusted for at LEAST 13.8 volts in order to
fully charge a battery at 20C. Setting it somewhat
higher, say 14.2 to 14.6 overs a faster replenishment
of charge after cranking the engine and perhaps
a prolonged battery-only pre-flight activity.
At the same time, we know that turning the
alternator off allows system voltage to fall
quickly to 12.5v or so . . . it marches downward
from there with time.
So what happens in the range of 12.5 to 13.8
volts? Not much. The battery is incapable of
taking on significant energy below 13.5 and
doesn't start delivering significant energy
at more than 12.5 volts. I.e. the battery becomes
'unhooked'. The idea that it takes on the role
of some 'super capacitor' for the smoothing
of noise is without foundation. I've verified
this both on cars and airplanes where disconnection
of the battery while the alternator is running
produces a only a small rise in bus noise.
A 14v alternator is a low impedance source with
about 1.5v pk-pk ripple built in as an artifact
of 3-phase rectification. This is a given that
drives DO-160/Mil-STD-704 requirements that
qualified devices be designed to function
as advertised in the presence of such noise.
The noise spectrum to be tolerated is plotted
here . . .
http://tinyurl.com/b3rhjwq
This is a 28v system plot where worst case
noise is 1Vrms over the range of 1 to 5KHz
and falls off on each side. 1Vrms sine wave
is 2.8Vpk-pk. Any departure from sine wave
allows the pk-pk values to rise markedly. In
short, the battery is not an effective filter
for anything except gross brown-out events and
standing off a runaway alternator for tens
of milliseconds required for the ov protection
system to assert system shutdown.
This ol' dog didn't learn it until about half
way through a 40+ year career. Not that it
was hard to figure out . . . just didn't have
a reason to question what was proffered as
common sense.
Bob . . .
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nuckolls.bob(at)aeroelect
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| Posted: Sun Mar 10, 2013 7:28 am Post subject: Cross feed contactor current |
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At 05:59 AM 3/10/2013, you wrote:
--> AeroElectric-List message posted by: Dan Charrois <danlist(at)syz.com>
Hi everyone.
I'm working on an RV-10 and am considering something like a Z14 configuration with one battery (PC680) behind the baggage compartment in the rear as per plans and another PC680 firewall forward (As a fairly electric-heavy plane intended for light IFR, I plan to have two alternators anyway, and I like the idea of being able to use the rear battery to power up avionics to plan flight plans, listen to radios, etc. while not worrying about draining the battery in the front for starting the engine, not having issues with EFIS reboots while starting, and having the option of closing the cross feed for more power during cold engine starts - a reality sometimes of living in northern Canada).
Have you conducted an energy study on your
anticipated operational needs? Certainly
a panel full of electro-whizzies drives notions
of extra-ordinary electrical system demands
but have you worked out the numbers?
What is passed around as common sense often
misses the mark for calculated reality. For
example, at a lunch time learning session at
HBC I put this plot up on the screen . . .
[img]cid:.0[/img]
This is an exemplar voltage-current plot for getting
the first engine started on a Beechjet. As you can see,
current flows are substantial and the duration is
considerably longer that what it takes to get your
4-banger lit. The battery that produced this
performance curve was rated at 35 a.h.
My question to those in attendance was, "What percentage
of battery capacity is expended in this event?" Guesses
were all over the place but I don't think anyone guessed
less than 25%. In fact, this event consumed about 6%
of total battery capacity.
I offer this anecdote to support the notion that the
perceived magnitude of a condition is often far removed
from reality.
My question about an energy study goes to your decision
to install 2 batteries and 2 alternators of significant
capacity in an RV. . . even if it does have a lot of
electro-whizzies.
The load well crafted load analysis will drive good
decisions on sizing batteries, alternators and crafting
a pre-flight check list that puts real numbers to
battery only ops before the engine is started. It
also encourages you to craft a plan-B condition where
en route endurance loads do not exceed 8A thus saving
a fully charged battery for approach to landing where
you can turn on anything you like.
I have yet to see a study for a configuration in an
RV that truly taxes a system to the extent that Z-14
yields a good return on investment. That's a
substantial increment in initial costs, weight,
and cost of operation as opposed to a Z-13/8 system
that offers nearly equal system reliability with
a much lower cost of ownership. You might find
it more attractive to trade empty weight for
fuel and/or baggage.
I suggest you run the exercise to prove to yourself
that Z-13/8 is indeed inadequate to your design
goals based on the numbers as opposed to square
inches of glass on the panel.
There are a number of exemplar exercises posted
at http://tinyurl.com/9rt6ymn along with a blank
form that can be used to put your arms around
the answer.
Bob . . .
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mrspudandcompany(at)veriz
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| Posted: Sun Mar 10, 2013 2:34 pm Post subject: Cross feed contactor current |
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| Quote: | We KNOW that an engine driven power source must
be adjusted for at LEAST 13.8 volts in order to
fully charge a battery at 20C. Setting it somewhat
higher, say 14.2 to 14.6 overs a faster replenishment
of charge after cranking the engine and perhaps
a prolonged battery-only pre-flight activity.
|
Bob,
Do you recall what we should expect for an
alternator output voltage when using a gereric
Ford Voltage Regulator? I believe these are
factory set and cannot be changed. However
I think I read that You can drop the sense
voltage through a diode and increase the alternator
output voltage. Is this correct and useful?
Roger
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nuckolls.bob(at)aeroelect
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| Posted: Sun Mar 10, 2013 5:27 pm Post subject: Cross feed contactor current |
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At 05:33 PM 3/10/2013, you wrote:
--> AeroElectric-List message posted by: "R. curtis" <mrspudandcompany(at)verizon.net>
Bob,
Do you recall what we should expect for an
alternator output voltage when using a generic
Ford Voltage Regulator? I believe these are
factory set and cannot be changed.
Yes. When I was working there they were set for 14.2 volts. I'm not aware of any reason to have changed it.
However, I think I read that You can drop
the sense voltage through a diode and increase
the alternator output voltage. Is this correct
and useful?
The original regulators sensed bus voltage alternator through the field source lead. This shared duty is a set-up for the famous "dancing ammeter syndrome" common to many SE aircraft including the Cessnas. Search for dancing AND ammeter on the AeroElectric-List forum.
Anything you put in series with that wire increases changes for regulation instability. I suspect that modern replacements are all solid state even if they look like the old electro-mechanical products. They might have a pot on the board if you can get he cover off.
Bob . . .
Roger
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03/10/13
Bob . . . [quote][b]
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danlist(at)syz.com
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| Posted: Tue Mar 12, 2013 2:21 pm Post subject: Cross feed contactor current |
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Hi Bob, and everyone who respond - thanks for doing so!
It's good to see that the charge differential between two batteries isn't enough to generate considerable current when they're connected in parallel. I suspected it might be so, considering how many wiring diagrams I've seen with a cross-feed contactor or equivalently, two battery contactors to the same bus, but it's nice to be able to verify the current response curve (40A peak trailing off quickly) from the experiment you did. It's always good to be able to back up assumptions with real information.
| Quote: | Have you conducted an energy study on your
anticipated operational needs? Certainly
a panel full of electro-whizzies drives notions
of extra-ordinary electrical system demands
but have you worked out the numbers?
|
The reason I was considering Z-14 instead of Z-13/8 was for a few reasons:
- I did do a load analysis, but as it was planned, my endurance bus had the potential to be drawing around 10A. I may be able to get that down a bit - I had been assuming "worst case scenario" of intermittent loads being always on, which of course isn't true... and I may have to make more critical decisions on what really is essential vs. just nice to have. I just want to make sure I won't overtaxing an 8 amp backup alternator enroute during a possible diversion due to electrical problems of the primary alternator... Maybe Z-12 might be preferable to Z-14 if the 8 amp isn't enough?
- I've heard people sometimes having difficulty starting an IO-540 in cold weather with a PC680. The battery is aft of the baggage compartment, so there are some losses due to the length of the wire run, which don't help matters either. Some put a larger PC925 back there, but I'm already expecting a fairly aft CG as it is, adding oxygen back there as well. So placing a PC680 in the firewall seems like an obvious choice to bring the CG forward, having a much shorter wire run to the starter, and allowing me to connect the two with the cross-feed contactor for more power in starting if needed.
- I've also heard of EFISes rebooting during engine start due to the voltage sagging. Since I'm displaying engine information on the EFISes, that doesn't sound like such a good situation. One possibility is to provide backup power to them with something like the TCW Integrated Backup battery (one of the options my EFIS manufacturer, Advanced Flight Systems, recommends). But it did strike me that a more efficient approach might be to use a separate PC680 battery for that purpose, if there was the use for one for other reasons anyway. Of course, that wouldn't work in situations where I have to cross-tie the two batteries for starting, but at least it would help where I didn't. I was planning on making a slight change to Z-14 so the cross-feed isn't closed necessarily automatically for starting.
- I'm still trying to decide on an ignition system. I'd like the idea of dual E-mag "P" models (sometimes called P-mags) if they have their 6 cylinder versions available soon. Failing that, I'm still leaning towards electronic ignition of some kind, but if I go that route, I'd like a completely redundant electrical system for each ignition source. For that, Z-14 seems the best choice.
Nonetheless, I'll continue to see if I can tweak things so that something like Z-13/8 will work - I agree it's a simplified, lighter, and less expensive design than Z-14, and may even provide better reliability for some types of failures. Thanks for sending the link to the sample load planning Excel files as well - I'm going through them to see if my numbers are obviously out of whack, and to get ideas to see if there is anything I may have missed.
Dan
--
Syzygy Research & Technology
Box 83, Legal, AB T0G 1L0 Canada
Phone: 780-961-2213
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