Battery Question

[WHIP23 at aol.com]

In a message dated 1/13/03 7:05:42 AM Pacific Standard Time, 
BUDDYonRC at aol.com writes:

> 
> Pros and Cons regarding dual battery use

Hi

You will probably get more help here than you want, but I have been doing 
pretty much what you describe for ten years or more, no problems.  In some 
cases I isolate the batteries from each other through diodes, in this case 
you must use 5 cells,  I have also run 4 or 5 cells in parallel with on 
isolation.  There has been a lot of discussion about what will happen if a 
cell shorts, with no isolation between packs.  The answer is nothing, you 
will wind up with a well charged 3 cell pack and adequately charged 4 cell 
pack in parallel.  The only real advantage of isolation is that if a pack 
really shorted (terminal to terminal) the diode would protect the other pack. 
 Odds are, in a situation like this either the wiring would burn open or the 
airplane would catch fire and go down in flames.  I think with two packs in 
parallel through two switches and careful monitoring/maintance you have about 
as reliable a system as you can reasonably design, beyond this you start 
adding complication with no additional redundancy, unless the system is 
designed VERY carefully.  We could go on all day, but adding failure points 
in the critical path, that are not redundant does not improve reliability and 
in fact usually degrades reliability.  Case in point are the many "battery 
backup" systems on the market, which while adding dual batteries also add a 
switching circuit in the critical path.  Since this switching circuit is not 
redundant you now have a situation where you have redundant batteries, but a 
switching circuit which can take the system down.  Which is more reliable, I 
don't know, but a sure answer is no switching circuit required is better than 
the best switching circuit?  I see guys rewire DPDT switches such that one 
switch turns on both packs, sound great, but you loose the ability to check 
the switches independently.  And it goes on and on.  If you sit and think it 
through carefully you will find that the simplest is generally the best and 
simple with redundancy is nearly failure proof, add in careful 
monitoring/maintance and you have a very reliable system, beyond that you 
pays yer' money and takes yer' chances.  

Bob
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