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1. 05:43 AM - Re: Re: Power Generation without Battery? (Robert L. Nuckolls, III)
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| Subject: | Re: Power Generation without Battery? |
At 14:00 2015-01-21, you wrote:
Bob,
I think the most likely battery failure modes would be temperature
related due mild overcharging/cell imbalance/high ambient temperatures.
I think I'll be pretty protected from a runaway thermal overload due
to frank over voltage as I will use your Z figures with overload
protection/notification. An internal short may also be more likely
with lithium than with lead acid.
I think you're over-worrying the design decisions before you.
Keep in mind that the aviation industry has a rich history
of what works and lessons learned for what doesn't.
If you sort through the NTSB accident reports, you will be
hard pressed to find one that has root cause in failure of
an electrical system component to perform as advertised. Even when
such stories are found, we read about accidents that just
didn't need to happen based on poor architecture, craftsmanship
and/or operational decisions . . . NOT borne out of any
failure-to-perform to specifications.
http://tinyurl.com/mwo3f4x
http://tinyurl.com/kv7eugm
http://tinyurl.com/msfmldj
A well maintained battery (meaning you don't simply
run it until it doesn't crank the engine any more)
is the single most reliable power source on the airplane.
LIMITED in amount of energy stored but a reliable
resource when that energy is skillfully managed.
Risk of shorted cells in a well managed SVLA based
system is essentially zero. Even the lithium incident
recently discussed had root cause in severe over-charging
from a condition that went unnoticed by the pilot for
a LONG period of time.
Regarding alternator #2: I request your advice. Running the engine
with the EFII requires 10 amps - mostly for electric fuel pump.
Are these published, MEASURED numbers? That's a LOT
of power for a task that takes less than 5A in my
1987 pickup. Are there examples of your proposed system
flying? Have any of those operators shared any current
draw observations?
I think I need perhaps 15 amps for a comfort margin and to run the
radio. Output curve vs rpm is significant considering fuel and
ignition function are at stake. But this is a vfr aircraft and I can
navigate and fly with nothing but the engine running.
So the SD8 does not have adequate output.
The SD-8 is a 10A max machine but it does have
adequate power for the majority of elegantly
crafted architectures
Perhaps adapting one of B&C's larger units intended for
Continentals? Wind my own? Hopefully something off the shelf though!
Wind one? If the over-riding concern driving this thread
is system reliability for an airplane that clearly isn't
going to spend hours at rarified heights . . . any
performance advantages for 'going hi-tech' are not
going to be realized. The most proven recipes for success
in system reliability used independent ignitions either
self-contained (magnetos, p-mags) or very low current
demand (light speed, et. als.)
Electronic fuel injection for a back-country, puddle
jumping pick-em-up truck only adds complexity with limited
return on investment.
If it were my airplane: Lyc with p-mags or light speed
ignition, L-40 belt driven alternator, SD-8 pad driven
alternator, well maintained SLVA battery (or EarthX
if you're ready to $spend$ just for weight reduction.)
This combination has evolved with a track record of
ancestral architectures going back 100 years. Clearly,
your project has the potential for being a great
fun-machine. But time, talent and resources being contemplated
to make it look more like a Lancair IVP only drives up complexity,
adds to your burdens for understanding all the nuances of
its architecture, drives up worry-pressures and adds
almost nothing to observable performance.
I contacted B&C on the self/persistent excitation issue and was
advised that the SD8 requires a battery in the system to operate the
regulator. He referred me to you. Also called Plane Power as it
seemed their internally regulated design should be able to continue
to run following removal of external battery power, but was advised
not. Answers may have been tainted by my emphasis about actually keep
the engine running!
Actually, self-excitation on the SD-8 was discussed
here on the List some time back. Figure Z21 in the
'Connection illustrates the concept for adding a few
components to keep PM alternator's rectifier/regulators
'awake' sans battery.
I don't think I have any large loads in the plane which would tend to
stun the self-excited alternator. Think I may make a small placard to
remind about being gentle with manipulations if running under such
circumstances.
I believe you drew Z12 and Z13 prior to predominance of 100%
electrically dependent engines. They both show a magneto in the
system for example.
Do you feel these Z figures are appropriate for adapting to
electrically dependent engines using a lead acid battery?
Do you think Z12/13 are reasonable for use with a current technology
(eg EarthX) Li battery?
Which Z do you favor and why?
If yes, is there some means to enhance the alternators propensity to
continue to produce power following battery failure?
If you spend as much time taking care of a battery as
you do worrying about it failing, then it's not going
to fail. Taking care of it means, "operate it within
well established limits and preventative maintenance
to verify integrity."
What do you recommend for a first and second alternator for my
system? Minimum output needed is 15 amps at perhaps 1800 engine rpm.
Weight is a factor for my setup.
Z-13/8, simple ignition/fuel delivery, battery optional
but DEMANDING of your attention no matter what alternators
or battery is installed. We tend to worry more about changing
oil in the engine or watching tire tread wear than to
track and verify battery performance. Yet more unhappy
days in the cockpit are rooted in poorly archtiectured/
maintained electrical systems than for dirty oil or worn
tires.
Bob . . .
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