MX60 question [RE-wrenches]

[Allan Sindelar]

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Wait a minute, here...
Earlier this year I did an Outback MX60 installation with 6 12V SX110s in
each of two series strings, for a nominal 72VDC input to the MX60, using a
24VDC battery bank. It was a short through-the-roof input run, maybe 20' of
#12 THHN. Two controllers failed, each after a few months. Both were
replaced under warranty and accepted as defective, but Bob Gudgel at Outback
told me that the high/low input regimen appears not to work well on short
array runs. After the second failure we rewired the array to 36V, and the
problem has not recurred. (Why did we wire it to 72V? Because we could, and
we had not been warned of any significant reasons not to, other than a very
minor efficiency loss in conversion.)

Chris, what's the story here? I expected to hear some mention of this
potential in your post on the issue, as Bob considered it to be a genuine
limitation that had been discovered for the product, and recommended
high/low only with longer input runs.

Allan at Positive Energy

----- Original Message ----- 
From: "Christopher Freitas --- OutBack Power" <cfreitas at outbackpower.com>

If the VOC on the modules you are using is 30 VDC and the location
doesn't get too cold (below freezing) than I would say your fine.  It is
likely that the open circuit voltage will exceed our current ETL listing
of 120VDC - we are planning on retesting soon to increase the approved
limit to the factory max VOC rating of 140VDC.

As far as the choice of going with a one, two, three or four in series -
it depends on numerous factors such as the distance of the PV array to
the controller and if any partial shading is present. From my
observations so far - I think wiring panels up for high voltages when
short distances are involved compared to putting them in parallel is a
wash - yes the efficiency of the MX60 goes down slightly with the high
PV array voltages charging into a low battery voltage (about 1 or 2
percent less) but the losses in the wiring are less and the PV array
will start charging slightly earlier and will work better under
partially shaded conditions and when subjected to high ambient
temperatures.

We have recently installed two identical PV arrays which will allow us
run side-by-side tests of both a low voltage system and high voltage
system charging into the same battery with the voltage being sold back
into the grid.  The results of this will be published on our website and
I will announce it once available.

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