nuckolls.bob(at)cox.net
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 Posted: Wed Apr 23, 2008 10:27 am Post subject: Subject: Re: Re: Brownout Battery System P.S. |
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At 08:14 AM 4/23/2008 -0700, you wrote:
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<fraziernv(at)earthlink.net>
I'm building a 7A with a somewhat modified z19 system. I'll probably use
both 16ah batteries to start, so I am concerned about EFIS (GRT) brownout
too. I've been thinking for some time that the brownout situation might be
solved with a large capacitor wired to feed the EFIS for a short time. I
see very large capacitors (1 Farad for $40) advertised for the car stereo
crowd. I think they use them to limit voltage variations while they are
trying to blow out everyone's eardrums...
What do you think?
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A 1F capacitor impressed with 1A of current flow changes
voltage at 1 volt / second. Okay, supporting an EFIS system
that draws say 5A from brownout works out to this kind
of scenario:
Capacitor charge start point 12.5V
EFIS barfs at say 9V so assume .5 volt of headroom
and we don't want voltage to fall below 9.5 volts
for an allowable delta of 3 volts.
With a 5 volt per second decay on the support capacitor
(5A and 1F) we have 3/5 or 0.6 seconds to get the
supply voltage back above 9.5v + 0.7v (diode
isolation) = 10.2 volts. Probably not an unreasonable
thing to do electrically.
The other consideration is weight and volume of
the installed capacitor and associated switchgear.
A battery is probably smaller and lighter but has
a cost of ownership in maintenance.
Consider this. What's the real downside for cranking
the engine before you start up the EFIS system? The
corollary to that question is what's the hazard to
hardware or operation of the system by having the
EFIS system reboot?
This is an excellent illustration of the gyrations
that system designers go through in sifting all
the combinations of simple-ideas in search of the
elegant solution. You define "elegant" any way
you like.
Now, here's another combination of simple-ideas:
http://www.aeroelectric.com/Pictures/Schematics/Brown-Out_Protection_1.pdf
I built this device to protect a Unisom electronic
ignition system about 10 years ago. It's a simple boost
supply that you put in series with the protected system
that operates only while the engine is cranking. It's
boosts battery-only supply voltage by approximately 3
volts and becomes a passive 0.7 volt constant drop
when the engine is not cranking.
Let's say you're protecting a 5A load again. This
means the power supply is tailored for 15 watts
of load. It can be fabricated in less than 3 cubic
inches of volume and under 5 ounces. Since it's
active only while the engine is cranking, all the
DO-160 issues with respect to EMC go away.
Perhaps this fits the definition of elegant solution
for a variety of projects.
Bob . . .
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