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I agree with Jim, except I'll take that one step further on battery
sizing. I subscribe to the Bob N school of thought that an alternator
failure shouldn't ruin your day or prevent you from going to your
intended destination.
With that said, your battery should be sized to be able to power your
essesential equipment in a worst case scenario situation. In my case,
that's hard IFR, so that includes my ESS EFIS in front of me, AHRS, efis
GPS, audio panel, garmin 430 gps/nav/com, transponder, JET 2" attitude
indicator, and autopilot.....these are all on my ESS avionics buss fed
through a diode and solid state relay by an AV master with an ESS ALT
FEED directly from the battery as a further backup.
So, say 30 minutes into my flight my primary AND ess alternator fails
(honestly, whats the chance of that..thats why the OBAM dual bat dual
alt is such a great system). If all fails to get either one going, then
I need to have enough battery reserve to keep things going for the
remainder of my trip. In my case with the supercharged IO-550, I figure
20 gallons per hour, with an average of 36 for takeoff and climb.
30 minutes into the flight, I've burned 18 gallons. I have 78 gallons
of fuel in my Legacy, so that leaves 60 gallons, or 3 hours, or just
2.25 hours with 45 minute reserves, and during this flight, I should be
able to cover just about 1000nm, so that should cover any trip I would
want to make. So, I need 3 hours of battery. My calculations show I
need about 10A continuous during IFR cruise, as long as the autopilot
isn't working overboard.
(I will have more hard numbers on this as I get more hours and will post
the actual amp draws of all my components for everyones future use)
So, 10A for 3 hours is 30Ah. I have 2 17Ah batteries in my system, so I
should be ok....thats at least the way I planned it anyway.
Fortunately, we have the opportunity as OBAM builders to build our
system to far outperform the systems that spam can fliers are stuck
with. Designing your system where very little can ruin your day only
makes sense, as it doesn't cost much more, and it really forces you to
understand your system and be a safe pilot when you are forced to use
it.
---
Shannon Knoepflein <---> kycshann@kyol.net
Jim wrote:
BTW, the battery should be able to power the essential
instruments for 30 minutes.
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