SW Absorbtion Timers Awry [RE-wrenches]

[Robert Nuese]

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Hello Wrenches

I have a problem which will hopefully be a bit of an interesting  
puzzle to share,
but not TOO interesting…
And of course, I've searched the archive pretty carefully, but it's  
still possible
this has been hashed out, in which case, my apologies and please  
guide me
to the appropriate thread.


I have some clients with a medium sized off-grid system.
I didn't build the system, but I added on the new half of the solar  
array, and
am now the main maintenance person for all but the generator.

(2) SW 5548 inverters stacked in 240v
(4) 1190 ah  industrial batteries
(24) SR100 Modules
(24) older (I forget which) 75w modules
On 4 tracking arrays
(2) C-40 Charge controllers
A 10kw propane generator



The problem is that one or the other, or both, of the inverters will  
sometimes
randomly drop out of bulk charge mode before a full absorption cycle  
(currently
set for 2:00 hours) is completed.

At first we supposed it must be a problem with the generator. An  
inverter will
normally stop charging if the output from the generator goes outside  
of the
allowed voltage or frequency range. Indeed, the generator was running  
rough,
and was sometimes difficult to start, and occasionally my measurements
showed its AC output to be near or beyond the allowed ranges.

However, we had the generator worked on, it seems to work fine now,  
and still
one or the other inverter continues to go to float mode while my  
fluke 87 says
the generator output is fine, and while the other inverter continues  
to accept
that output with no problem. Further, if an inverter drops out of  
float mode as
a result of out of spec AC supply, usually we should see a blinking  
green AC-in
light to indicate that issue, and it should go back into bulk mode  
after the AC
has stabilized. In this case the inverter goes straight into float  
mode, and stays
there for good, while the other inverter continues to bulk charge  
without a hiccup
for most of 2 hours. Sometimes one inverter does this, sometimes the  
other,
occasionally both will do it sometime before 2:00 hours is over, with  
the
generator still putting out apparently good quality AC.

I talked to several of the Technicians at Xantrex on a number of  
occasions.
They've been very helpful every step of the way.
The first suggestion was that if the generator had been turned off  
before one
or both inverters had completed the absorption cycle, then when the  
generator
was next turned on, it would take up where it had left off. If it had  
been turned off
when the absorption timer was at 1:50 minutes completed, then on re- 
connection it
would go ahead and finish the last 10 minutes of the cycle it was  
already in and
then go to float - even if a month had passed in the meantime.  
Resetting the
absorption setting to 00:00, and then back to 2:00 hours on each  
inverter should
reset the timers, and then they should be fine as long we never turn  
off the
generator without first checking that both inverters have gone into  
float mode.

(And here I'll mention that other installers have said that they're  
not sure that's
exactly how the timers normally work. Some suggest that it's not  
uncommon for
off-grid folks to run their generators for short periods for one  
reason or another
- almost out of gas, running a big motor for a while, you name it -  
and that they
expect the inverters to initiate a full cycle the next time the  
generator goes on, and
that that's how the inverters usually do work, so does anyone know  
just what the
protocol is, and if there are exceptions?)

…Anyway it didn't work. We'd always reset the timers and they'd still  
sometimes
have the same problem. We'd reset the absorption time twice in  
succession, to
make sure it took, no help.

So, the Xantrex folks told me how to view the absorption timer so I  
could see
where it actually stood at any given time. AhHa! We'd run all the way  
through
a cycle, or we'd reset the timers by resetting the absorption  
setting, and then
we'd watch the timers and sometimes see them changing, even when no AC
input is present (and even when the generator is completely  
disconnected - by
a circuit breaker at the inverters), and even when the inverters are  
not in charge
mode. Xantrex tells us this is not supposed to happen. The timers are  
only
supposed to progress when in bulk mode, with AC coming in. It seems  
that they
only DO progress when the batteries ARE around or above bulk charge  
level, BUT
they do it when the source of charge is the solar array.

Neither I nor the Xantrex folks could think of any external element  
of the system
that was likely to cause such behavior, and especially not without  
there being
other, more severe symptoms, but everything else is behaving quite well…
so it seems that it must be an internal glitch in the inverters.

Xantrex sent us a couple of slightly updated chipsets, bringing these  
inverters,
made in 2000, up to version 4.16, the highest possible on that hardware.
No help.

Then Xantrex sent us two new, mid-2005 inverters. Thank you Xantrex!
I and my clients are certainly grateful to have two nice new inverters,
that presumably will have 5 more years of service life.
However, the problem remains.

Xantrex also suggested that since this seems to be a problem of the  
internal
logic circuitry, if it is caused by an external connection, it most  
likely would be
an element that most directly connects to that logic circuitry. So  
conceivably
a defect in the stacking cable, or interference picked up on that cable?

We completely disconnected the cable, making sure all 240v loads were  
off.
Everything worked fine, except the absorption timers kept,  
occasionally, timing.

Ah, but we also, on this unit, have a pair of Righthand Engineering  
battery packs
to save programming if the real batteries are disconnected. We  
disconnected
both of those. No help.

So, Wrenches, what do you think?  And thanks, Robert.


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