Fw: Battery Bussing [RE-wrenches]

Mon Sep 10 13:29:26 PDT 2001

If you are concerned about this (and maybe you should be, I'm not sure) 
simply make all of the wires that go from buss to battery the same length. 
Yes, some will be longer than they need to be, but we're talking dollars 
here, not even tens of dollars. At this point, you'll be as perfectly 
balanced in resistances as possible.

Of course, batteries vary internally (I assume) and different batteries 
could well have different internal resistances. At high loads, the strings 
with higher internal resistance will contribute less. But since resistance 
drops linearly with decreasing current draw (linearly), and amperage drops 
with the square of decreasing power draw, it seems to me that small 
differences in resistance (of the order we seem to be talking about) only 
have a significant effect during high amperage / high power draw periods. 
Most system we put in are designed to make the inverter happy at a 4000W 
draw. I personally, due perhaps to limited field watching time, have not 
seen an inverter sit anywhere near it's peak power draw for any significant 
period of time. If a house system did sit near peak, the battery bank would 
need to be enormous.

So unless the battery internal resistances are significantly different from 
string to string (someone want to do the math to define "significant"), 
then seems that we've done enough. (I guess you could test the individual 
strings and mix and match until the strings had equal resistances under 
some load level...)

This is all assuming that all the cables are crimped equally and equally 
and adequately terminated to the batteries...

Enough for now.

Jeff

On Monday, September 10, 2001 1:32 PM, jay peltz [SMTP:jay at asis.com] wrote:
> Hi Joel, and Tom,
>
> My question is about wire lengths.  In the plumbing world, to truly 
parallel
> something, the lengths of pipe,fittings etc must be the same length. 
 Seems to me
> that the shorter wires will pull more power.  Yes I realize that we are 
talking
> really small differences here, but it is going to take the path of least 
resistance.
>
> What say you?
>
> jay
>
> peltz power
>
> Joel Davidson wrote:
>
> > Tom,
> > Thank you very much for the pictures and story on your web page. Using 
a common
> > bus makes a lot of sense. I wish I had more time and money to do 
research and
> > testing. It would be good to put ammeters on the series and parallel 
 cables to
> > confirm lower current coming from inner batteries. Perhaps your battery 
supplier
> > has some test data.
> > Best regards,
> > Joel Davidson
> >
> > Tom Elliot wrote:
> >
> > > Allan,
> > >
> > > I've been following this thread with some interest as I've just gone 
through
> > > my own changeover due to (what I consider) premature cell failure in 
an L-16
> > > in my original bank.  I've done a write-up on my web page (the 
write-up is
> > > at http://www.wagonmaker.com/newbatt.html ).
> > >
> > > As I say on the webpage, the battery people I deal with who do large 
telco
> > > installation literally cringed when I said I had a series/parallel
> > > installation.  The cell in my system that failed first was the one 
closest
> > > to the positive inverter takeoff.  I didn't have cross-connection on 
the
> > > bank, I took the power from one end (I know not the best) but they 
told me
> > > it only made a little difference to take power from opposite ends of 
the
> > > bank.
> > >
> > > According to what I was told, virtually *any* paralleling of 
batteries is a
> > > bad idea.  The center strings add voltage but not much current to the
> > > inverter draw which means that the outside series strings take more 
of a hit
> > > than the center strings.  (I'm getting clarification and more 
technical
> > > explanation of this from them but it may take time).
> > >
> > > Anyway, when I saw the "battery bussing" subject line I thought folks 
would
> > > be talking about the way I've installed my "new" (to me) bank of 
AGMs.  All
> > > my series strings (pairs, each is 12 volt) have their own connection 
to a
> > > main set of buss bars so that each series strings contributes equally 
and is
> > > charged equally.  This makes for more cable but an ultimately easier 
to
> > > maintain installation.  It also means none of that rotating batteries 
in a
> > > bank stuff.  I have a friend who does that also and it seems like a
> > > hellacious waste of time.
> > >
> > > Another great thing about using a main buss bar setup is that you 
have no
> > > limits on the number of series strings you can use and it makes it 
easier to
> > > use more, smaller, batteries in large systems.  If you get a chance 
check
> > > out a large telco installation and see how they do it.  They are 
often
> > > connecting over 100 batteries to main busses.
> > >
> > > My original set of L-16s failed (at least one cell did) after only 7 
years
> > > of use and an excellent maintenance and charge regime.  I expected a 
lot
> > > more out of them and if I had done it this way originally they'd 
still be
> > > chugging along just fine.
> > >
> > > Telco installations change out their banks much more frequently than 
we in
> > > the industry do and they were surprised at my 7 year battery life but 
in our
> > > kind of installations and usage patterns longer life should be a bit 
issue
> > > and I think eliminating parallel strings would extend battery bank 
life.
> > >
> > > Tom Elliot
> > > Guffey Energy Works
> > > http://www.wagonmaker.com
> > >
> > > ----- Original Message -----
> > > From: "Allan Sindelar" <allan at positiveenergysolar.com>
> > > To: <RE-wrenches at topica.com>
> > > Sent: Thursday, September 06, 2001 2:22 PM
> > > Subject: Re: Fw: Battery Bussing [RE-wrenches]
> > >
> > > >
> > > > Re this part of my earlier post on battery interconnects:
> > > > > The point of diagonal tapping is to minimize
> > > > > the effect of cumulative interconnect and terminal resistance 
from
> > > causing
> > > > > one string to charge and discharge "sooner" than the others; you 
could
> > > > also
> > > > > say "more" or "less" than the other strings. The additional 
resistance
> > > is
> > > > > very slight with clean connections, but as the amperage can be 
very
> > > high,
> > > > > the resultant voltage drop from parallel string to parallel 
string can
> > > be
> > > > > significant. A consistent variation in voltage over time leads to
> > > > > incremental premature failure of some strings--and I admit I'm 
not sure
> > > if
> > > > > it's the harder-working strings or the less-cycled strings that 
would
> > > fail
> > > > > first.
> > > > Can anyone answer my question? How will an unevenly-matched battery 
bank
> > > > deteriorate? Which cells will fail first, and why?
> > > > Thank you...
> > > > Allan at Pos E
> > > >
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