SW Absorption Timers Awry [RE-wrenches]

[Robert Nuese]

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Hello Wrenches

I have some developments regarding a problem that I'd originally
posted on May 3.

To briefly recap:

On a system consisting of…

(2) SW 5548 inverters stacked in 240v
(4) 1190 ah  industrial batteries
(48) Assorted modules on 4 tracking arrays
(2) C-40 Charge controllers
(1) 18kw Kohler propane generator - turned on manually (pretty
much just during cloudy spells in the rainy season, based primarily
on trimetric readings),

… we have the problem that one or the other, or both, of the
inverters will sometimes randomly drop into float charge mode
before a full absorption cycle (set at 2:00 hours) is completed.
Sometimes it'll go to float after only 10 minutes.


In the earlier post, I gave more details, several wrenches replied
with questions and suggested things to check, and I posted
detailed answers
- please refer to the earlier posts if you'd like to know all, I think
the last was mine, answering some questions on May 12…

But the dilemma remained that the timers moved forward
whenever the batteries were up near the inverter bulk voltage
setting, even when the source of power was the photovoltaic
arrays and no AC supply was present.

Xantrex had no answer, but said that the timers shouldn't work like
that.


Since then, Xantrex has had an inverter on the test bench,
and they recently contacted me with some initial results and we've
agreed on a temporary workaround.

The Xantrex engineer told me that after playing around with the
inverter a fair amount, he found that not only does the test
inverter sometimes do the same thing, but it generally does it, and
in fact, experiments on other SW inverters, including ones 10
years old, all seem to behave the same way, much of the time.
It's surprising that this hasn't come to light before.

Aaron Wellendorf suggested a work around - set the float level at
the same voltage as the bulk level. Then it won't matter if the
inverters go into float prematurely, they'll still be charging at an
appropriate voltage. All that is lost in our case, where the
generator is turned on and off manually, is the indication on the
inverters that the batteries have passed through a set period of
charging, plus whatever time it took to get up to bulk voltage.
Since my clients are normally not lingering near the inverters
watching the indicator lights, but are instead watching their wrist
watches, or listening for a timer to ring, it doesn't make much
difference.

The folks at Xantrex agreed that this would probably work.
Eventually, reluctantly, they said that we may as well use it on
our system for the moment. However, they said that they're still
working on the problem, and in the mean time they do not
recommend this workaround for anyone else. One thing that has
made it difficult for them to come to a definite conclusion is that
the test inverter doesn't seem to always behave exactly the same,
so they are playing with the test conditions to try to find out why
that would be.

The above workaround leaves a potential problem for those who
have off-grid systems with solar and generator back-up, who want
to have the generator controlled automatically: using the above
system, the inverters would never go to float, and the generator
would never automatically turn off - Not good.

Talking this over with Aaron, he suggested another work around
that would 'solve' the problem with automatically controlled
generators:
1) Set the bulk setting higher than usual - he suggested that about
59.5 volts might work for many lead acid battery setups.
2) Set the absorption time to Zero. In this case, it will take longer
for the inverters to reach the bulk voltage level, and by the time
they do, they'll be mostly charged. Then the inverters will start
their absorption timers, instantly complete the absorption phase,
and go straight to float mode. Just how completely the batteries
will becharged can be adjusted by adjusting the bulk charge a bit
up or down.

Xantrex was not enthusiastic about this, and do not recommend it.
Hopefully they'll come up with something better. I'll let you all know
when and if they do. But in the meantime, anybody else have any
ideas about all this?

Thanks, Robert Nuese


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