See below:

 

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Shannon Knoepflein   <--->   kycshann@kyol.net

 

Equipment                Current Draw @ 28V

---------------                 ------------------------------

Chelton EFIS                 3 - 5 amps (we'll use 4.0)

Tru Trac Autopilot           2.5 amps (2 servos)

Garmin Audio Panel       2.2 amps

Garmin 430 GPS/Com    1.1 amps

Garmin XPNDR              1.0 amps

Electric AI                      0.6 amps

----------------                    ---------------

Total                             11.4 amps

 

This is all close.  I show 10A, but close enough.

 

If we look at the selection of Valve Regulated Lead Acid batteries offered

by Concorde as an indication of what is available in the market, their

largest 28v battery weighs 42 lbs and has a 19 amp-hour capacity at 23c. If

we assume an end of life condition of 80% capacity compared to new, we are

down to 15.2 AH capacity. Now if we consider a cold soaked battery at

altitude, say -18c per the Concorde spec sheet, capacity has fallen to 68%

of our room temperature value or 10.3 AH.

 

I actually have a pair of batteries, each 14V 17Ah, so 34Ah total.  Per BobN recommendation, I plan to change one battery every year, so the older battery is 2 years.  I use a floating batter tender when not in use, so the batteries are maintained quite well.  With this considered, I’m confident in the batteries providing their full 17Ah.  Also, the cabin of the aircraft rarely gets below 50 degrees F, probably more like 60 F, so I don’t see that as an issue.   

 

Given the above figures, you would not have enough battery capacity to fly

even 1 hour, let alone to fuel exhaustion. Pitot heat could easily double

the current requirements in IMC, and could leave you with little time to get

back into VMC and land.

 

I feel I can easily maintain 15Ah per batter, or 30Ah total.  With a 10A draw, that gives me 3 hours.  This was stated as 2.25 hours of flight, and 0.75 hour of reserve.  If I’m down to only battery in IMC, I can assure you I’m heading to pavement.

 

The other aspect of your system that struck me as a potential failure mode

that could leave you in dire straits is the ALT FEED system that is fed

directly from the battery via a 30 amp fuse. What happens if one of your

essential items, let's say an autopilot servo, should develop a short while

you are completing your trip after the rubber coupler for your gear driven

alternator has sheared and you are in the limp-home mode with your ALT FEED

activated? What would happen if the 30 amp fuse blew first, rather than the

circuit breaker for your auto pilot (it may only take a millisecond

difference between the two reaction times to cause this to happen)? What if

your limp-home trip took more than an hour and the main contactor opened

because there wasn't enough voltage across the coil to keep the main

contactor closed?

 

This is in fact a good question.  Maybe a larger fuse is in order, or a current limiter.  I need to research the trip time of a typical CB versus that of the fuse.  I know the fuse will go first typically, but need to do a comparison.  I’ll have to get back to you.

 

The main contactor is totally bypassed with the ALT FEED on, so it doesn’t matter.

 

While I agree that your electrical system is likely better than 99% of the

GA aircraft in the sky, there is still a need for sound decision making and

judgment as Jeff suggests. Most accidents are the result of a sequence of

small events that, individually, are seemingly innocuous, but when stacked

together, lead to an emergency situation.

 

I totally agree.  I hope you understand my comments were about designing the system for safety...they were not about judgement.   I feel the system should be designed to be as safe as possible, but still doesn’t replace sound decision making.  Like Jeff said, all hardware will eventually fail.  My point is let minimize those failure modes that will ruin our day, and at the same time make sound decisions.  Now, my decisions might not always agree with everyone elses, but I’m comfortable with that, and with my system.

 

For example, to continue in IMC

after an isolated charging system failure would probably be imprudent,

whereas the same failure in VMC may be manageable.

 

Perfect example, and I agree totally.

 

It's not the first

failure that usually causes the accident, but how the pilot manages the

situation and decreases his risk and or workload such that he can handle a

second failure and avoid an emergency situation (OK, I'm off my sop box

now).

 

And with a well designed system that the pilot has seriously thought about, I hold this decision making process will be easier and safer.  I hope you agree?  I hope everyone agrees and vows to make systems that they fully understand and are comfortable with and are as safe as possible.  Having OBAM aircraft, we have that ability....ain’t life grand J

 

Sometimes I wish someone could just design a failure proof electrical system

and publish the schematics so we could simply implement the same design in

our own aircraft and not have to go through all of this brain damage. I

guess there is still plenty of room for innovation in the real world. Keep

up the good work.

 

Until that happens, Z14 with an alt feed is probably as good as it gets J