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 Berland
IV-P