Dan Ballin wrote:
Wanted to see if anyone on the list had thoughts about batteries for a
24v system (dual alt/dual bat, dual lightspeed EI) legacy.  I have been
thinking about putting 2 12v odyssey batteries in series as the "main"
with 70Amp alt and a B&C 7.2 as the "backup" with SD20S alt .
Other thoughts?

I have the same questions and not a lot of answers, however I talked to
an electrical engineer at Cessna. He suggested the following. Definitely
use two batteries as the backup system needs the 2nd battery as a filter
if nothing else. He also suggested using a 24v battery rather than two
12v's in series. This was only for the slight improvement in
reliability. (Less parts and less connections) I still think two 12's in
series may be the way to go for packaging reasons. The use of RG type
batteries was a must. The use of an over voltage protector was a must.
They are paranoid of OV's with glass panels.  The part that got fuzzy
was whether or not to make the two charging systems separate on the
busses or not.  If they are separate, then charging both batteries is
straight forward.  If you have both batteries and both alternators
available to both busses, then charging both batteries gets a little
tricky.  I don't know how to guarantee it. If they are separate, then
some pilot action is required if one fails.  I think a good warning
system and pilot action is the solution.
I think two 20 amp-hr RG 12v batteries will crank a TSIO 550.  Not sure
about a 10 to 1 CR IO 550. I also would like to hear others thoughts.

Craig and Dan,

 

2 12VDC SLA batteries in series expose the system to complete failure if one of the batteries fails - just like if you used a single 24V battery.  An all electric system looks good if, and only if, dual alternators, dual batteries and dual busses with appropriate cross feeding and isolation along with load stripping (pullable CBs) are used.  Some feel even more comfortable if the engine electronics (FADEC, ignition, ?) have access to a separate isolated backup battery and/or another alternator, solely for the engine.

 

In flight, one must design the system for how long continued flight is desired.  Complete the trip?  Stop at a major repair field "soon"?  Provide for a minimum of 45 minutes of flight with an attitude of land as soon as possible?  For one of these schemes you might ask how much juice is needed - gear, flaps, trim, AP, transponder, radios, instruments, fuel pumps, lights, etc. and for how long?

 

My simple 320 has limitations (one battery, alternator, vacuum pump and pilot), but it does use the "Essential" bus logic.  It has these considerations:

 

1. It's an insulator made of glass without ice protection - thus no flights into icing or heavy storms - i.e. "light" IFR or land.

2. If a total electrical failure should occur, try to keep some things going for about 45 minutes to match the fuel that would be available to the engine via the header tank that is automatically kept above 8 gallons if there is main bus electricity.

3. Maintain an attitude of land ASAP.  Do not plan long water crossings or flight in the Rockies at night and do not cross ground fog enshrouded areas larger than 45 minutes of flight. 

4. The backup for the vacuum instruments is the electric TC/GPS managed AP.

5. There is a heated pitot and 2 static ports.

6. Any electronic ignition requires the use of a dedicated backup battery for at least 1/2 of the ignition for more than 45 minutes.

7. Alternator failure is indicated immediately and the essential bus can be activated - it supplies power to a comm, gps, transponder, certain lights, TC and a few other load strippable items.  The master relay, fuel pumps, gear, flaps and other hi draw items are not on the essential bus since they could be the cause of some major main electrical system failure.

8. All trims, electric or not, are spring biased and can be easily overridden by hand through all flight regimes, if necessary.

9. There is only one backup system for each major flight management component, i.e. there is no plan C (I do not rely on myself as the backup to anything).... so if a backup or primary fails, land and get it fixed (now, before both have failed).

10.  There is no backup for the engine or prop, listen to them and give them TLC - good oil, clean gas, a strong ignition and plenty of fresh air.  If it doesn't sound right, find out why, now!

11. The plane uses funny wheels, carry a spare tire and tube.

12.  Carry three flashlights.

14.  No checklist has an item numbered 13.