Re two batteries, was Miracle switch failures

[s.vannostrand at kodak.com]

Thanks Bob.  Your advice appears sound to me.  I'm printing this one.
--Lance





WHIP23 at aol.com
Sent by: discussion-request at nsrca.org
03/28/2003 05:00 PM
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        Subject:        Re two batteries, was Miracle switch failures



Ok third try, I must be e-mail challenged!!


Hi Guys

All the "battery backup systems" I have seen put something of unknown 
reliability/untested in the critical path.  The best setup is two 
batteries diode orred (sp?) together (diode in each path from switch to 
receiver) through two switches (must be 5 cell to allow for the diode 
drop) fully redundant, you would have to have a failure in both paths at 
the same time to loose control due to battery.  Assuming you check things 
every flight the odds of a failure in both paths is too remote to 
consider.  There is little weight penalty, run two batteries 1/2 the size 
you would normally run.  Being a worrier I also make a point of never 
running either pack down too far (if I happen to be running on one pack I 
want enough charge in that one pack to last a long flight).  The diodes 
must be in the path from the switch to the receiver (if you put them 
between the battery and the switch, as once got printed incorrectly, with 
MY name attached, you won't be able to charge in this configuration)  I've 
been doing this for about 10 years with no problems and it has saved me an 
airplane, both batteries good on take off one was bad (open) on landing, 
if only one battery, bye bye airplane.  Guess you could argue that the bad 
battery might have been in someone else's airplane VBG.  It is a little 
more complex, two switches (one of the biggest failure items) two charge 
jacks, two batteries.  Assuming you have an unused channel you can plug 
the battery into any unused channel (preferred) if not then I would wye it 
to a non mission critical channel, last choice would be to wye the two 
batteries together (the wye is then mission critical, I told you I was a 
worrier)  There is also good evidence that the diodes are not necessary, 
that a 5 cell pack won't charge a 4 cell pack (one shorted cell), sounds 
right and I've used this scheme, but I have never tested it with a cell 
shorted so I have no first had experience.  I'm not sure I could sleep at 
night, anymore, with only one battery pack and switch, just seems like 
unnecessary risk.

Flame suit on, Bye

Bob






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