William
This comment
PS: I am curious as to what you meant by: “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.”
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%.
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.
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.
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.
Spring is about here!
Kent Osterberg
Blue Mountain Solar
On 2/10/2024 10:09 AM, William Miller via RE-wrenches wrote:
Kent:
You raise a good point. I am now trying to correlate voltage versus
SOC versus battery current.
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.
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.
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.
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.
Thanks for pointing out something I missed.
William Miller
PS: I am curious as to what you meant by: “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.
WM
Miller Solar.com
805-438-5600
www.millersolar.com <http://www.millersolar.com>
On Fri, Feb 9, 2024 at 3:19 PM Kent via RE-wrenches
<re-wrenches@lists.re-wrenches.org> wrote:
William,
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.
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.
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.
Kent Osterberg
Blue Mountain Solar
_______________________________________________
List sponsored by Redwood Alliance
Pay optional member dues here: http://re-wrenches.org
List Address: RE-wrenches@lists.re-wrenches.org
Change listserver email address & settings:
http://lists.re-wrenches.org/options.cgi/re-wrenches-re-wrenches.org
There are two list archives for searching. When one doesn't work, try the other:
https://www.mail-archive.com/re-wrenches@lists.re-wrenches.org/
http://lists.re-wrenches.org/pipermail/re-wrenches-re-wrenches.org
List rules & etiquette:
http://www.re-wrenches.org/etiquette.htm
Check out or update participant bios:
http://www.members.re-wrenches.org
