SW Absorbtion Timers Awry [RE-wrenches]

Fri May 12 18:00:30 PDT 2006

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Dear Wrenches
Well, the absorption timers I reported on, on May 4, are still awry,
but at least I have a bit more data to share with you - maybe this'll
help someone come up with a new line of attack.

Today I got time to go up in the hills to the site and run a set of  
tests
suggested by fellow wrenches. Here are the results.

on May 4, Dan Rice asked a couple of good questions:

> Do the inverters exhibit this behavior after a powerdown-reset, prior
> to any programming (running factory default settings)?

Yes, darn!

> Do they exhibit this problem with both AC input and AC output
> disconnected?
> Perhaps the inverter is powering a reactive load that somehow
> fools the inverter into acting like there's an AC input (like the  
> old SW
> islanding issue) -which, with battery voltage above absorbtion  
> voltage,
> might cause the absorbtion timer to count down.

And again Yes, Darn!

I disconnected the generator AC input, the AC output, the solar array,
the batteries, and the RightHand Engineering SWMA memory backup
batteries. All at the closest points, right near or at the inverters.
I let the system rest for at least 15 minutes. I reconnected only the
batteries, and checked several settings on each inverter, to be certain
that they had fully reverted to factory default. The absorption timers
had also reset to 00:00. So far, so good. I watched for a half hour, the
timers didn't change. This further confirmed our near-certainty that the
timers do not move unless the voltage is up near the inverter bulk
charge level.

I connected the solar array, but left the AC inputs and outputs
disconnected. The battery voltage went up around the inverter bulk
setting, and the timers both started timing: 00:10, 00:20…

So resetting to factory default has no effect.
The AC inputs and outputs have no effect.
But thanks, Dan!

Phil Undercuffler had a good idea asking:

> Does the problem persist if you unplug the battery temp sensors?
>
> Since the charge set points are based on the temperature
> compensated values, glitches in the data sent from the temp sensors
> might be playing havoc with the charge times.

So, at this point, I reset the timers and disconnected the temp sensors,
leaving the AC ins and out disconnected.
(And by the way, while I was doing these tests, the equipment room
felt comfortably warm, I'd guess about in the mid-70s. The room is
insulated, and the batteries are in an insulated compartment, so
I think the temp in the batteries was pretty steady, and not far from
room temperature. Meantime, the actual voltage readings and the
temp corrected voltage readings on the SWs were all the same, as
we'd expect at that temperature. I'd recently tested the temp sensors
by putting them in ice water, and they'd both shown almost exactly the
same, significant changes as their temps changed, so I think the
sensors are functioning essentially correctly.
Accordingly, it seemed that the temp sensors were not causing odd
results. But still we need to test by disconnecting them… and…)

No help, the timers kept marching along. Darn!
But thanks Phil!

Aaron Wellendorf contacted me with an insightful suggestion:

> From info that has come out in the Wrenches list, the anomaly I see
> is that the PV can charge the battery above the inverter’s bulk
> setpoint.

I'd stated that the C-40s were set at 60v bulk,
The SWs at 58v bulk.

> I have never done an installation of SW’s where this was
> the case. PV charge controller was always set less than the
> inverter’s bulk voltage. It was mentioned in the thread that this
> problem only happens when the PV has charged the battery above
> the inverter’s bulk setpoint. Probably the battery reaching the bulk
> voltage is what triggers the absorption timer to start.
> A possible easy fix is to set the generator bulk voltage above the
> C40 setpoints. I would reverse the settings and have the C40 at 58V
> and the inverter bulk at 59V. The battery’s more efficient when being
> charged between 20-80%, so on my off-grid jobs, I usually set the
> SW’s absorption timer to 0 minutes so that the battery charges as
> fast as possible to 80%. When it reaches 59V I know the battery is
> probably above 80% and then the generator shuts off automatically.
> In your customer’s case the generator start is manual, but it won’t
> matter if the owners forget to shut it off and the battery gets a full
> charge once in a while.
> You are a very thorough person so you must have a reason for why
> the C40’s bulk is set higher than the SW’s? Maybe you could share
> your wisdom. Since the charging rate from the PV is generally slower,
> it seems like you cook the batteries with the PV equalizing to 60V
> each time they charge full. This is not necessary nor desired. Having
> higher PV setpoints won’t make the PV charge the battery much
> faster.

Hmm, well, I may have had a reason, but on consideration, I'm not
sure exactly what all the factors were.

I recall that at an advanced class for installers a couple of years ago,
Glenn Minyard said that the default settings for a lot of equipment
are quite low, and can be safely and usefully set a fair bit higher.
Looking at my notes, he seems to have suggested as high as 60v
for C-40 charge controllers charging flooded lead acid batteries in
most solar applications - but whether I've got that exactly right, and
just what the details of the rationale are, I can't remember. And I'm
also uncertain just why I'd ended up at 58 volts for the SWs, and
not a setting closer to, or the same as, the C-40s.

In any event this seemed like a very likely fix. It wouldn't solve the
underlying puzzle, but at least it might alleviate the symptom.

So, I checked the C-40s and…
The joke seems to be on me!:  They were both set very precisely at
57.6v,   .4v below the inverters!

I'm not sure if I'd tried 60v and then changed my mind, or maybe just
intended to try it, or maybe I was confusing this system with one of
the other SW systems I've installed, on some of which I know I've put
the C-40 bulk setting at least in the high 58v or low 59v range…
In any event, this diminished the likelihood that an inappropriate
differential between the C-40 and SW bulk settings was contributing
to the problem.
But it didn't completely remove that possibility, a difference in
calibration or temp sensor function might make the C-40s' bulk
settings effectively higher than that on the SWs.
So, I left the C-40s at 57.6, and bumped the SWs up several
times, until I got to 60v. Each time I reset the timers and
watched, and each time the timers marched along!

Darn. But thanks Aaron!

So, I put the SW bulk settings back to 58v, left the C-40s at 57.6
for now, re-did my other settings, reconnected everything, left it all
working just fine - until overcast makes it necessary to use the
generator (it might not happen until October, here in California).
And came home to report my results.

I'm still stuck. It'll still be a problem when the rains return. My  
clients
are still not satisfied (though they have been admirably patient and
sympathetic).

What do you all think? Any feedback about optimal bulk charge
settings (a perennial favorite) would be welcome, but most important,
What to test next to fix the run amok absorption timers?

Thanks for your good ideas so far,
and thanks in advance for the next set,

Robert Nuese

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