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Sorry Bill, you've skipped a decimal point in your figuring the Ampere
Hour
(AH) rating af a battery. Figuring your way, would make it a 120 Ampere
Hour
battery instead of 12 AH!
The bottom line is: A 12 AH battery will deliver 1.2 amps for ~10+ hours,
12
amps for maybe 45 minutes and it goes down from there.
Dan Schaefer
Dan -
I should have gone to Oshkosh and stayed out of trouble...I had
thought that the diode would be the only component in my design open to
question, but my misunderstanding of battery capacity created another. Well,
this has turned into a good, worthwhile discussion. You're absolutely
correct, I miscalculated battery capacity!
Manufacturer's most commonly design their lead-acid batteries for a
ten hour discharge, but some are designed for shorter or longer discharge
times. I again spoke with Concorde and was surprised to learn that their
aircraft batteries are optimized (and rated) for one hour discharge, which
is pretty nearly matched to the time we might need to fly on backup power (a
battery having a discharge time optimized for one to three hours would be
most efficient for our purposes, but this does not mean that a battery
designed for a longer discharge time could not be used for backup). Another consideration is that battery terminal voltage decays as the
battery is discharged and the question arises at what level the voltage will
be insufficient to spin the gyros and they "flag". Manufacturer's supply a
chart to show voltage decay as the battery discharges; however, activating
the backup battery while on the ground and noting the time and voltage when
the gyros "flag" will be the only accurate measure of your battery's
endurance.
To choose a backup battery, we would of course have to first
determine the required current for critical devices (i.e. attitude and turn
coordinator gyros, electronic ignition). 5 to 10 amps is a good, liberal
estimate (electric gyros are labeled for running current). Two to three
hours should be the minimum time they must operate. So, your demand would be
10 to 30 A-H and that would be the minimum battery capacity.
I checked and found a Johnson Controls "Dynasty" 30 A-H battery that
should be an ideal backup battery. In fact, it's designed for that purpose
in the telecommunications industry. It's a sealed RG battery and will
supply 8 amps for 3 hours to a minimum of 11.4 volts. It's 7-3/4" L x 5-1/4"
W x 6-3/4" H, weighs 27 lbs and should fit the area forward of the right
side rudder pedals on the 320. Bill Rumburg
N403WR (Sonic bOOm)
LML website: http://www.olsusa.com/Users/Mkaye/maillist.html
LML Builders' Bookstore: http://www.buildersbooks.com/lancair
Please send your photos and drawings to marvkaye@olsusa.com.
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