Fw: Battery Bussing [RE-wrenches]

[Allan Sindelar, Positive Energy, Inc.]

Allan here, again,
Kurt is right on, so is Windy.
1. Coat your terminals before assembly. According to Windy, no problem with
conduction; coats best for air barrier.
2. Joel, your original problem allowed two SW4024s. At 267A ea (+/-) that's
534 amps. I would use way more than #4 AWG interconnects. While I've never
seen this addressed, in either tech articles or the NEC, I'd use 2/0 or even
4/0 on the parallel interconnects. Series interconnects could, by common
sense, be sized to max load amps /number of parallel strings. In your
example, 534A / two strings = 267A. Whoa, better use 4/0 again. And note
that if one string builds up resistance from terminal corrosion right before
you make that 1-year service call, then the other string will carry a
greater amperage, so better go 250 or 300 MCM...do we see a pattern here?
3. Your original question: Will current flowing through any interconnect
differ from current flowing through any other interconnect? I say yes, per
Ohm's law. If one parallel string has more interconnects than the one next
to it, it will have slightly more resistance because of those interconnects.
Thus it will carry/absorb/supply slightly less current than other strings
without the added resistance. 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. The effect, though, is that the entire string eventually
underperforms if part of the bank underperforms, and then the whole bank
must be replaced.
Allan @+E

 ----- Original Message -----
 From: "Joel Davidson" <joeldavidson at earthlink.net>
 To: <RE-wrenches at topica.com>
 Sent: Friday, August 31, 2001 1:52 PM
 Subject: Re: Battery Bussing [RE-wrenches]

>
> > 1. If you have a battery with one series string of cells, you are
drawing
> > current from one end. No problem.
> > 2. If you have a battery with 2 or more series strings in parallel and
the
> > paralleling cables are sized properly, it should not matter what point
is
> > connected the load because the paralleling cable(s) is a common bus.
> > 3. More cells are more chances for cell problems. So true.
> > 4. More parallel strings are more chances for resistance problems. So
> true.
> > 5. Most battery designers limit to 4 or 5 parallel strings by
convention.
> > 6. Most battery installers limit to cells or batteries that can be
handled
> > (in)conveniently.
> > Nothing new here. Any other other "rules"?
>
>

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