Idea

baxter avatar image
baxter suggested

Configurable maximum SOC in DVCC/ESS controled systems

In an DVCC/ESS controled system using Lifepo4 batteries it is not good to charge the cells over 90% SOC in daily operation (you should just do it for top balancing).

As the voltage does not reflect the SOC in a wide range of lifepo4 charge curve, it would be usefull to have a configurable value in the ESS menue for the maximum SOC. As soon the SOC is reached the CVL should be lowered to a configurable value. Charging shall be stoped.

ESSSOCDVCC
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nickdb avatar image
nickdb commented

The challenge you have here is the battery vendors don't want their products used this way and I doubt victron would want to start mucking with parameters which could affect that warranty and potentially cause them support issues related to imbalance and SOC drift, as examples.

On a similar topic, I previously shared a SOC histogram for an ESS, it clearly showed that the battery didn't spend enough time in the "undesired" area. For me, at least, this seems to be a non-issue.

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baxter avatar image
baxter commented

Why should this limit any warranty? It is up to me as user to decide which SOC range I will use and I am interessted to be in the best operating range as much as possible. Charging every day to 100% makes vendor happy because the can sell a new battery very soon.

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gnagflow avatar image gnagflow commented ·
Yes, the only usage of have no max. SOC feature is that battery vendors are happy because batteries will be dead after warranty. But has nothing to do with what the customer wants. The customer wants to care about his baby the best possible way and not in the other way around.

Having a max SOC feature is the simplest change in Victron ESS settings with the highest customer value.

I cannot change every summer the BMS settings for each customer to make them happy. I am an installer an I know what the customer wants.



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Mike Dorsett avatar image
Mike Dorsett commented

If you are using Louis VanDer Walt's BMS interface code, this can be done as the Max cell voltage and Max Soc state interface to the d-bus charging commands are accessible. I configured my set-points on the maximum cell voltage (and Min cell voltage) as these are the most critical values for cell life. Max voltage was backed off 100 - 150mV from manufacturer's specified limit, min was raised similarly. Battery max charge current is also ramped down as max cell voltage is approached - this eliminates any overshoot in larger systems with slow response times. Time spent near max is usually limited to 4 hours per day, so this is not a major issue.

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nickdb avatar image nickdb ♦♦ commented ·
Battery specs vary among commercial vendors.

I can think of one whose specs are based on 3.4V per cell and others who are much higher.

Most well known brands are already taking this into account long before the usage profile of an ESS is considered.

Derating it further isn’t helpful considering the other factors affecting battery life.

Different story if you are DIY’ing.

With a 10 year warranty being fairly common these days, why muck with it?

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thomasw-1 avatar image thomasw-1 nickdb ♦♦ commented ·

@nickdb : your battery might have 10 years warranty, however according to legislation it is your part to prove that granted parameters are not delivered anymore.

Good luck by engaging an expert witness to show the restriction to the manufacturer ;-)

In the meantime there are many scientific studies showing in particular for LiFePo batteries the impact of regularily charging batteries to 100%. Even Samsung implemented in their recent smartphones a fetaure to prevent full charging.
From my point there is sufficent evidence in going into that direction!

Personally I trust these studies (had recently also personal contact a scientist for renewable energies) and I will be glad to use these batteries any time beyond 10 years.
Other people can still believe that earth is a slice and go their way ,-)

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baxter avatar image baxter thomasw-1 commented ·
@ThomasW. pls vote for this idea, many thanks
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baxter avatar image baxter commented ·

Cell voltage does not reflect SOC. BMS is in place anyway limiting max cell voltag, thats not an issue and also not the usecase. it just needs a device (cerbo) stopping charging at a specific SOC. My Tesla can do this. It would be quite easy to implement but most likely I have to do it by myself with node red

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nickdb avatar image nickdb ♦♦ baxter commented ·
Your tesla has to do this. The batteries that the majority of users have, in an ESS environment, don’t.
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baxter avatar image baxter nickdb ♦♦ commented ·

Why not? Recommended SoC Window is 10% to 90%, regardless if the cell is used in a car or ess

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nickdb avatar image nickdb ♦♦ baxter commented ·

SOC is a relative measurement of capacity based on how a battery is charged. If it has been derated to begin with, to make the guaranteed number of cycles/life, lowering it further is meaningless. An ESS battery has a different use case to an EV and the BMS/specs are configured to reflect that.

Most solar batteries want you above 10%, that low down you’re unlikely to get the performance you are expecting.

Driving it that low will have a worse effect on lifespan than not shaving 10% off the top.


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baxter avatar image baxter nickdb ♦♦ commented ·

Lifetime is based, beside other parameters, on the used soc window. Charging the battery every day to 100%, regardless if the cell voltage is 3,45 or 3,65 is not favourable. A lot of commercial vendors are interested in maximum capacity and fast charging and not a long lifetime.


But I see Victron is still to much in the lead acid worl of thinking and not reflecting all aspects of LFP usage so I have to solve it by myself

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thomasw-1 avatar image
thomasw-1 commented

Hi Baxter,
I fully see your point and agree with your requirement. I covered almost the identical requirement in this thread.

In addition to the fixed Min-SOC I implemented a forecast dependent dynamic Min-SOC approach preventing a long term discarge.

The max-SOC feature (inclusive periodic balancing) is currently in beta test, however it showed quite good results recent weeks so far.

1660994611884.png

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thomasw-1 avatar image
thomasw-1 commented

Here is another practical example why regularily loading the battery beyond 80% SOC cannot be valuable to battery life. Mentionable is the peak increase after 80% by additional ~10°C. That can't be healthy to battery chemics.

1661157645345.png

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nickdb avatar image nickdb ♦♦ commented ·

No idea what batteries you're using or how they are being charged.

Even in our warm climate, not behaviour we see on decent (and supported) batteries, some increase is obviously expected.


screenshot-2022-08-22-at-110021.png

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gnagflow avatar image
gnagflow commented

My main problem beside the need to reduce the max SOC in summer is, that i use CANBUS based BMS, which overwrite the DVCC max. Voltage setting. I would be fine with a max. voltage setting as well, but anyway anyhow, it doesnt work for me, or did i get something wrong!?


--> on the other hand, i understand that voltage related max. soc setting for lifepo doesnt work (but i use nmc)

--> if you use good BMS's which calibrate at LOW and HIGH voltage, even use other algorithm, the SOC can be very accurate, as it is when you use for example REC-BMS, as i do. I am very satisfied with the accurateness of the SOC


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