REFLECTOR: Dual battery Duel

Terry Miles terrence_miles at hotmail.com
Wed Jun 21 17:14:18 CDT 2006


Thank you, Keith.  I am headed to that shared philosopy, the more I look
at this topic of battery abnormals.  Thanks for tip on the #4 welding
cable, and being careful about the posts too.  It's my plan to swap one
out each annual as well.
Terry

-----Original Message-----
From: reflector-bounces at tvbf.org [mailto:reflector-bounces at tvbf.org] On
Behalf Of Keith Hallsten
Sent: Wednesday, June 21, 2006 10:24 AM
To: 'Velocity Aircraft Owners and Builders list'
Subject: RE: REFLECTOR: Dual battery Duel


Terry,

Modern sealed lead-acid batteries are reliable as long as: 
1.  You replace them before they get too old.  Replacing every two years
will get you a very high level of reliability.  Replacing every year
will get you an extremely high level of reliability. 2.  You connect to
the battery with a short pigtail of soft #4 welding cable.  Stiff leads
bolted to the battery can lead to failure of the battery posts. 3.  The
battery is large enough that you don't beat it absolutely flat with a
difficult start.  Batteries last longer when not cycled too deeply.
This is not generally a hard criteria to meet as long as the engine is
working properly and the battery is sized to provide some reasonable
level of alternator-out endurance. If you take care of the three factors
above, I wouldn't waste much effort worrying about battery reliability.
The modern SLA (also known as "Absorbed Glass Mat", or "Starved
Electrolyte" or "Recombinant Gas") battery is not subject to the cell
shorting that occasionally occurred with the old flooded cell type.

Multiple batteries of the same chemistry and voltage, even if at
different states of charge or of different sizes, will live very happily
on the same bus.  One will not discharge another; the more charged
battery will just provide power preferentially. 

Regards,
Keith Hallsten


-----Original Message-----
From: reflector-bounces at tvbf.org [mailto:reflector-bounces at tvbf.org] On
Behalf Of Terry Miles
Sent: Wednesday, June 21, 2006 5:32 AM
To: 'Velocity Aircraft Owners and Builders list'
Subject: REFLECTOR: Dual battery Duel

Kurt,

Thanks for the nice clear description below.  Let me paint a scenario,
best as I get all this stuff.  Please I am only trying to toss out ideas
and get a grip on this topic.   From your sentence below:  "If bus
voltage starts to drop..."  I get it about the part if V goes below say
13 that the alternator is likely at fault, and what to do about alt
failures, and how to design indicators and annunciators for alternator
issues.  

My concern is about batt abnormals.  Or perhaps let me ask this question
to those of us with two equal batteries, are you trying to design for
further fault tolerance at the "alternator failed" level of your
electrical system?  Let's say your system voltage drops from it's normal
steady 13.8 and starts to spike up and down for no reason by 2 volts or
so.  You decide to take your the alternator off line, and run a load
shed routine.  Now it reads 10.8 which is 1 volt or more lower than what
you expected, and you have both batts on line.  You start to worry that
a weak batt feeding is a stronger one.  You have no way of knowing which
one is which while they are paralleled.  And who would suggest being
brave enuf to turn off one batt contactor and see how they look
isolated...and of course the off line batt V will go back up w/ no load
on it.  

This point here...this when and why of IN FLIGHT isolating batts in a
dual batt system with a failed alternator...that I find to be a worthy
electrical design topic or to be ignored as a  "never happen, nothing
you can do" situation.  

If I understand it:  if you use that marine batt tender and depowered
your alternator, it will think you just dropped anchor and will pull
your starting battery off line.  (Is that right?)

Terry    






> Terry,
>  
> Pretty close,
>  
> For the sake of illustration, grounds are correct as in your example.
>  
> The Hot side of each battery goes to it's own contactor.. the IN side.
>  
> The switched side of the contactors (OUT) I connected with a brass
> bar.
> This bar goes over each "out" post, with a central "MAIN" coming out
of 
> the middle to the Buss.
>  
> My Alternator lead (again, switched and fused) runs to the starter 
> contactor. There it joins with the #2 cable that runs thru the center
of 
> my conduit ground, and connects to the switched side (OUT) of the
> contactors.
>  
> This way, I can run on either battery or both.
> I can also do any combination since I have the ability to turn off the

> field wire anyway. Nuckols writes that if you monitor buss voltage, 
> and it is in the "charging" area, all is well.
> If buss voltage starts to drop, then you have a charging or battery 
> problem. No need for seperate voltage readings on each battery, since
if 
> they are both on line, you will get MAIN average anyway.
> The addition of a quality Ammeter will warn of a charging problem
before 
> main line voltage would drop anyway, so that is an extra.
>  
> I also plan to follow his plan of swapping a battery every other year,

> so the odds of a battery problem are much reduced (my whole airplane
is 
> electric). I'm using (2) B&C 32AH Batteries.
>  
> My origonal idea was to use a high quality RV charging splitter. These

> charge the battery that needs charging the most, and include their own

> Diodes and sensor so that a bad battery can not bring down the good
one, 
> etc. Bob seemed to think (and I'll defer to him) that his system is
> efficient, easy, safe and light without added complexity, cost or
weight.
> I also asked about not having that big #2 cable unfused. He wasn't
> worried. Since I bought the best cable I could find, added additional 
> shielding and then encased it in my copper conduit cround, that it is 
> about as safe as it can get. If something BAD ever does happen, flip
off 
> both battery switches... and everything stops.
>  
> Hope that helps.  I can take pics or make a drawing if you want.. or 
> feel free to call.
>  
> Kurt Winker.
> 505-610-3676
>  
>  
> 
> 
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