Fw: Battery Bussing [RE-wrenches]

[Joel Davidson]

Size the cables for ampacity first and then for resistance. For the SW4024
inverter cables, the inverter's rated output power is 4000 watts and the lowest
battery voltage is 22 volts and the inverter efficiency is 85%. 4000 divided by
22 divided by 0.85 = 214 amps x 1.25 = 267 amps. Since the terminals are 75°C,
use 75°C insulated 300 kcmil cable with a 75°C ampacity of 285 amps. For the
battery interconnect cables, assume that any one battery string will carry at
least half the total current and use at least 2/0 cable.


"Allan Sindelar, Positive Energy, Inc." wrote:

> 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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