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<p>William</p>
<p>This comment <br>
</p>
<blockquote>
<div dir="ltr" class="gmail_signature" data-smartmail="gmail_signature">PS: I am curious as to what
you meant by: “<span style="background-color:rgba(0,0,0,0);border-color:rgb(0,0,0);color:rgb(0,0,0)">At
least with lithium batteries the SOC meter doesn't need to
account for the return current dropping down as is required
with lead-acid batteries.” <br>
</span></div>
</blockquote>
<div dir="ltr" class="gmail_signature" data-smartmail="gmail_signature">is regarding the two plus hours
lead-acid batteries need for absorption time with constant voltage
and decreasing charge current. Most lithium batteries need little
or no absorption time. Get them to 54.4 volts (or whatever is
specified) and in a few minutes they are done. That takes one
variable out of the "full" calculation that coulomb counting SOC
meters use for re-calibration to 100%.</div>
<div dir="ltr" class="gmail_signature" data-smartmail="gmail_signature"><br>
</div>
<div dir="ltr" class="gmail_signature" data-smartmail="gmail_signature">Battery state of charge meters
that are counting amphours are over simplifying things. A
battery's full charge capacity, lithium or lead-acid, changes with
temperature and (particularly with lead acid-batteries) with load.
The full charge capacity is changing with age. It's complicated so
we can't expect them to be perfect. </div>
<div dir="ltr" class="gmail_signature" data-smartmail="gmail_signature"><br>
</div>
<div dir="ltr" class="gmail_signature" data-smartmail="gmail_signature">I also stopped using the Outback
FNDC because it would reset to say 100% at inappropriate times.
Spent lots of time collecting data to demonstrate the flaw to
Outback; the FNDC is still operating with firmware 1.0. They don't
get my business.<br>
</div>
<div dir="ltr" class="gmail_signature" data-smartmail="gmail_signature"><br>
Without a SOC meter I've struggled with lithium batteries trying
to find the right voltage to start the generator. Don't want
things shutting down. Do want to use a large fraction of battery
capacity. But the voltage is so load dependent that it is hard to
find the spot to start the generator.</div>
<div dir="ltr" class="gmail_signature" data-smartmail="gmail_signature"><br>
</div>
<div dir="ltr" class="gmail_signature" data-smartmail="gmail_signature">Spring is about here!<br>
</div>
<div dir="ltr" class="gmail_signature" data-smartmail="gmail_signature"><br>
</div>
<div dir="ltr" class="gmail_signature" data-smartmail="gmail_signature">Kent Osterberg</div>
<div dir="ltr" class="gmail_signature" data-smartmail="gmail_signature">Blue Mountain Solar<br>
</div>
<p><br>
</p>
<div class="moz-cite-prefix">On 2/10/2024 10:09 AM, William Miller
via RE-wrenches wrote:<br>
</div>
<blockquote type="cite" cite="mid:CAAgpqUNdRd7pH_sZBK83isShe_rLjvx5K1NXEakwoN=TfRtPMw@mail.gmail.com">
<div dir="auto">Kent: </div>
<div dir="auto"><br>
</div>
<div dir="auto">You raise a good point. I am now trying to
correlate voltage versus SOC versus battery current. </div>
<div dir="auto"><br>
</div>
<div dir="auto">I do know that after the system shut down and with
zero battery current (at rest condition) the battery voltage was
48 and the SOC was in the mid 50s. The event code was a low
battery shutdown. 48 volts at rest should not be 50% SOC. It
should read 0% or 10%, at most. </div>
<div dir="auto"><br>
</div>
<div dir="auto">Fortress tech support opined that the SOC was out
of calibration and I needed to charge to 54.4. I did so and the
readings now make more sense and the generator will now auto
start. </div>
<div dir="auto"><br>
</div>
<div dir="auto">I’m pretty confident we had an error in SOC
calibration but, per your point, it did not necessarily occur
suddenly or at the moment in time I suspected. </div>
<div dir="auto"><br>
</div>
<div dir="auto">Per the general subject of this thread I still
think that SOC calibration errors are a real scenario. For SI
systems this can have greater consequences than for systems not
so dependent on SOC. I think we agree that any SOC reading
should be treated with some skepticism. </div>
<div dir="auto"><br clear="all">
<div dir="auto">
<div dir="ltr" class="gmail_signature" data-smartmail="gmail_signature">Thanks for pointing out
something I missed. </div>
<div dir="ltr" class="gmail_signature" data-smartmail="gmail_signature"><br>
William Miller</div>
<div dir="ltr" class="gmail_signature" data-smartmail="gmail_signature"><br>
</div>
<div dir="ltr" class="gmail_signature" data-smartmail="gmail_signature">PS: I am curious as to
what you meant by: “<span style="background-color:rgba(0,0,0,0);border-color:rgb(0,0,0);color:rgb(0,0,0)">At
least with lithium batteries the SOC meter doesn't need to
account for the return current dropping down as is
required with lead-acid batteries.” I want to learn as
much as I can about all available battery technology. </span></div>
<div dir="ltr" class="gmail_signature" data-smartmail="gmail_signature"><br>
</div>
<div dir="ltr" class="gmail_signature" data-smartmail="gmail_signature">WM</div>
<div dir="ltr" class="gmail_signature" data-smartmail="gmail_signature"> </div>
<div dir="ltr" class="gmail_signature" data-smartmail="gmail_signature">Miller Solar.com<br>
805-438-5600<br>
<a href="http://www.millersolar.com" moz-do-not-send="true">www.millersolar.com</a><br>
</div>
</div>
</div>
<div><br>
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<div><br>
<div class="gmail_quote">
<div dir="ltr" class="gmail_attr">On Fri, Feb 9, 2024 at 3:19
PM Kent via RE-wrenches <<a href="mailto:re-wrenches@lists.re-wrenches.org" moz-do-not-send="true" class="moz-txt-link-freetext">re-wrenches@lists.re-wrenches.org</a>>
wrote:<br>
</div>
<blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left-width:1px;border-left-style:solid;padding-left:1ex;border-left-color:rgb(204,204,204)">
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<p>William,</p>
<p>I think that the SOC determined by lithium BMSs come
with similar accuracy issues that occur with the
Trimetric, FNDC, Magnum BMK,... - small measurement
errors integrated over a long time become big errors.
That's why Fortress (and every SOC meter) is saying that
the battery needs to reach 54.4 volts once a week for
recalibration. At least with lithium batteries the SOC
meter doesn't need to account for the return current
dropping down as is required with lead-acid batteries.
The internal BMSs probably do a better job of estimating
the SOC than these external devices, but I do not expect
them to be perfect. Same goes with everyone's electric
car, while we put a lot of faith in the displayed SOC it
probably isn't much better than ±5% and if it were off
by 10% you probably wouldn't know. <br>
</p>
<p>In regards to your graph showing a big voltage
difference between two 55% SOC occurrences over a 12
hour time frame. I question your assumption that the
voltage should be the same. The data show different
voltages for the same SOC, it seems unlikely that the
BMS measurement drifted by enough to make that happen,
so I think the data shown may both be correct within
reason. The voltage of lithium cells is highly load
dependent (probably somewhat temperature dependent too)
so perhaps the Fortress battery is actually close to
right at both times.<br>
</p>
<p>Since using the SI SOC meter for starting the generator
is problematic, an external device for starting based on
voltage is a good idea. If you want a ready to go
product to do that, I think the Morningstar relay driver
will work well. A little difficult to program or
reprogram but very reliable. One issue you will observe
is that the generator won't start at a consistent SOC as
indicated by the battery or the SI.<br>
</p>
Kent Osterberg<br>
Blue Mountain Solar
<p><br>
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