[RE-wrenches] State of Charge Meter for Sol-Ark

[Kent]

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 at 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
>
>
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