[RE-wrenches] Odd Outback FNDC/Mate issue

[Howie Michaelson]

Hi All,

Caution: this is a longish tale about a problem with Outback's FNDC
monitor, as well as a request for any other related stories.  Sorry for
the long post - sharing this here is somewhat therapeutic for me, so if
you want to cut to the chase, it is toward the bottom:

About a year ago I installed my second or third FNDC meter into a single
off grid VFX3524 OB power panel.  I had originally built this panel with
an FM80 for the eventual addition of a small array.  I removed the FM80
after about 2 months of problem free operation because the client changed
their mind about the solar and wanted to stick with just the generator
charging the 12 Surrette KS33 batteries (long boring story).  The day
after removing the FM80 I received a call from he client saying the mate
was turning off and on randomly.

When I arrived, the Mate, the Hub, and the FNDC were randomly powering
off, then back on after maybe 5 seconds, sometimes once, sometimes many
times in a row.  I checked everything obvious (wiring, cabling, voltages,
etc.) and then replaced various, then all comm cables.  This was to be the
first of many (I mean many) service calls over the next *10 months* trying
to solve the problem.  Because the symptom was erratic, it appeared at
various times that something I changed made the difference.  After an hour
or 2 of testing with no repeat of the symptom, I conjured up some rational
to explain why my particular "fix" du jour worked, congratulated myself
that I had solved the problem, and went home (which fortunately is only 50
minutes away).

An hour, day, week or two later, I would get another call informing me
that my "fix" was insufficient.  Each time, I would return with more
equipment in hand, over time replacing multiple times the Mate, the Hub,
and the FNDC, then all 3 internal VFX boards, checked grounding and wiring
for shorts, for surges... I can't remember what.  And again each time, I
would get the system to settle down, would feel satisfied enough to
convince myself to go home, and the cycle would repeat.

I had of course included OB support on this journey from the start (which
was a bit of a challenge since originally there was no land line, and cell
service was a couple miles away - after about 4 months the clients did
manage to get the phone company to come through with their land line).  OB
sent along various replacement parts as we thought new thoughts to
experiment with.  I think I talked with just about everyone at OB about
this issue, and while everyone was fascinated, helpful, and sympathetic,
in the end they were not successful at pinpointing the cause.

Of everything tried, it turned out that the only reliable action I could
take to make the symptom stop would be to unplugged the FNDC comm cable
from the hub, although by the time I stumbled across that repeatable
experiment, I was so befuddled that it took months for me to realize the
consistency and primacy of that behavior.  In the end, this was the clue
that I used to finally solve the problem (I hope), about a week ago.

So last week, after Matt James sent a complete inverter replacement unit
under the belief by the engineers that somehow the 22 volt communication
power was weak and cutting out upon surges, I replaced the inverter.
(Power surges, either charging or loads, seemed associated with the
symptom, though not consistently - as well as hitting keys on the Mate.) 
All seemed well, and I went home, only to get the dreaded call, this time
before I even made it home.  So I returned the next day with another hub,
then replaced the FNDC (again) because it was reporting 2.7 vDC though the
inverter was reporting proper voltage, and still got the flashing!

I finally surmised (I believe correctly this time) that the flashing
happened when there was a large current flow in or out of the batteries. 
If the batteries were getting charged, the symptom would only appear
during the early parts of the charge cycle, unless there was an unusually
large load surge as well.  And if the system was "on edge" do to medium
size current flows in or out of the batteries, any hitting of Mate buttons
seemed to be enough to start the cycling as well.


THE CHASE:

In a flash of inspiration/desperation, I removed the FNDC's voltage
sensing cables (the twisted pair supplied with the FNDC by OB) from the
conduit which contains the battery cables and ran them to the DC plus and
minus busses in the FW500 instead. I had run the wires directly to the
battery terminals on the advise of someone(?) thinking this would provide
the most accurate operation.  The symptom stopped and has not returned to
date.

Outback has now determined that somehow this induced current from the
battery cables into the voltage sensing wires for the FNDC can cause the
communication system to get overwhelmed, but they are not quite sure how. 
None-the-less, as of this week, they are recommending to run the FNDC
voltage sensing wires (plus and minus) *not* along side the inverter
cables.

Hopefully, this is the end of this story...

Has anyone experienced any similar symptoms?  This seems a first for OB,
but I thought I'd ask here.

Howie
-- 
Howie Michaelson
NABCEP Certified Solar PV Installer™

Sun Catcher, LLC
Renewable Energy Systems Sales and Service
VT Solar & Wind Incentive Program Partner
http://www.SunCatcherVT.com
(cell) 802-272-0004
(home) 802-439-6096