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jgswartout(at)earthlink.n
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 Posted: Mon Oct 16, 2006 9:48 pm Post subject: E-bus Operations With SD-8 |
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Gents:
How much current should I expect a nearly-fully-charged 20 amp-hour RG battery to draw in the event the main alternator gives up the ghost and the SD-8 carries the E-bus load? I’m planning a Heavy-Duty Endurance/Avionics bus which would draw a continuous 6 amps, however I could shed an up-to-2-amp load (the PDA running a moving map) and let it run on its own battery for a couple hours, and/or turn off the COM radio and transponder (~2 amps) if battery charging required more than 2 amps or so. But how would I know? By the E-bus Alternate Feed fuse blowing? I’m not planning to have an ammeter.
[quote][b]
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nuckollsr(at)cox.net
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| Posted: Tue Oct 17, 2006 7:04 am Post subject: E-bus Operations With SD-8 |
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At 01:46 AM 10/17/2006 -0400, you wrote:
| Quote: | Gents:
How much current should I expect a nearly-fully-charged 20 amp-hour RG
battery to draw in the event the main alternator gives up the ghost and
the SD-8 carries the E-bus load?
|
"Nearly fully charged" is un-quantified . . . so a quantified
answer is not possible.
| Quote: | I m planning a Heavy-Duty Endurance/Avionics bus which would draw a
continuous 6 amps, however I could shed an up-to-2-amp load (the PDA
running a moving map) and let it run on its own battery for a couple
hours, and/or turn off the COM radio and transponder (~2 amps) if battery
charging required more than 2 amps or so. But how would I know?
|
Consider this about alternator and battery behavior:
"Overloaded" alternators will not put out power at a
regulated voltage . . . as the current demands rise to
exceed the alternator's rating, the output voltage will
sag.
The current that a battery draws is strongly influenced
by bus voltage while the current that appliances draw
is roughly proportional to bus voltage. This means that
if you "overload" the alternator with a combination of
running and battery-recharge loads, the voltage will sag
and transfer of power will favor of the running loads.
Starting an engine takes but a few percent of a battery's
capacity. Likelihood that you're going to have a seriously
discharged battery battery by the time you launch is low.
If the battery is still wanting to take in energy from a
14.2 volt bus and your main alternator quits, a 6A running
load will still leave 2A to top off the battery but in no
case is the operation of running loads compromised because
the battery is not topped off.
| Quote: | By the E-bus Alternate Feed fuse blowing?
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The e-bus fuse protects that wire ONLY. If your system
is properly designed, that fuse will NEVER open based
on the combination of battery/alternator/running loads.
| Quote: | I m not planning to have an ammeter.
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That's what load analysis is all about. KNOW what the
loads are for Plan-A and Plan-B and in some cases
Plan-C. Electrical system failure management in flight
based on readings taken from panel instruments is
not good design, planning or maintenance.
Bob . . .
---------------------------------------------------------
< What is so wonderful about scientific truth...is that >
< the authority which determines whether there can be >
< debate or not does not reside in some fraternity of >
< scientists; nor is it divine. The authority rests >
< with experiment. >
< --Lawrence M. Krauss >
---------------------------------------------------------
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mprather(at)spro.net
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| Posted: Tue Oct 17, 2006 7:31 am Post subject: E-bus Operations With SD-8 |
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I don't see this as a big concern.. I think the system behavior will be
good with the loads and supplies you are talking about. If the main
alternator fails, the SD8 will supply enough power to run your e-bus plus
2A.
If the battery is really flat, the bus voltage may sag from the regulated
voltage (14.5V) to no lower than about 13.5V. It won't go lower than that
because the battery charge current will drop off steeply as the bus
voltage sags below 14.5V, and would probably be zero if the bus sags to
13V.
The battery will take whatever excess charging energy is available from
the SD-8, but no more. And, if the non-battery-charging bus loads rise
above the output of the SD-8, any charge left in the battery will keep the
bus voltage from sagging further.
As Bob has suggested in the past, a voltmeter is a great tool in this kind
of system.. If your goal is to be able to run until you're out of gas,
reduce electrical loads until the bus voltage rises above 12.5 (indicating
that you aren't depleting the battery). You could then be confident that
the SD-8 was keeping up.
Regards,
Matt-
| Quote: | Gents:
How much current should I expect a nearly-fully-charged 20 amp-hour RG
battery to draw in the event the main alternator gives up the ghost and
the
SD-8 carries the E-bus load? I'm planning a Heavy-Duty Endurance/Avionics
bus which would draw a continuous 6 amps, however I could shed an
up-to-2-amp load (the PDA running a moving map) and let it run on its own
battery for a couple hours, and/or turn off the COM radio and transponder
(~2 amps) if battery charging required more than 2 amps or so. But how
would I know? By the E-bus Alternate Feed fuse blowing? I'm not planning
to have an ammeter.
|
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jgswartout(at)earthlink.n
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| Posted: Tue Oct 17, 2006 9:14 am Post subject: E-bus Operations With SD-8 |
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jgswartout(at)earthlink.n
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| Posted: Tue Oct 17, 2006 9:28 am Post subject: E-bus Operations With SD-8 |
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Thanks, Matt. I didn't comprehend that the battery would be a kind of
"spongy" load that would only draw whatever surplus current is available. I
get it now.
--
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frank.hinde(at)hp.com
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| Posted: Tue Oct 17, 2006 10:16 am Post subject: E-bus Operations With SD-8 |
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While we are on the subject....What voltage is the SD8 normally regulated to? I have yet to test it for real (and I have electric fuel pumps so its hard to get more than 12.2V at normal ground running RPMs...)But I do see the volts drop from 12.2V to 12 at idle so I assume its functioning.
I think I might take off with just the SD8 running (after taxiing out to the runway with the 60A altrunning to make sure I have a fully charged battery) and cirle over the airport to make sure the SD8 keeps 12.1V or above during my min power draw emergency scenario.
Sound reasonable?
Thanks
Frank
[quote][b]
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mprather(at)spro.net
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| Posted: Tue Oct 17, 2006 11:53 am Post subject: E-bus Operations With SD-8 |
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Noodling around on the aeroelectric website, I saw this pdf:
http://www.aeroelectric.com/Pictures/Alternators/SD8-Performance.pdf
It shows the output current at 13.8V and 12.5V. I don't know what the
voltage would be at zero load... Depending on what "normal ground running
RPMs" actually are, you could extrapolate what the bus load is, or if you
know what the bus load is, you can determine wether the alternator is
actually functioning. 12.2V, depending one where you are measuring it,
seems kind of low..
Matt-
| Quote: | While we are on the subject....What voltage is the SD8 normally
regulated to? I have yet to test it for real (and I have electric fuel
pumps so its hard to get more than 12.2V at normal ground running
RPMs...)But I do see the volts drop from 12.2V to 12 at idle so I assume
its functioning.
I think I might take off with just the SD8 running (after taxiing out to
the runway with the 60A altrunning to make sure I have a fully charged
battery) and cirle over the airport to make sure the SD8 keeps 12.1V or
above during my min power draw emergency scenario.
Sound reasonable?
Thanks
Frank
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