Jk BMS Vs Victron

Hi!
This has been frustrating me for a while now ... The 48v system is all Victron, with the exception of battery cells and the BMS.
The BMS SOC is always off compared to the Victron Smart Shunt.

The thing is, that for my settings in Victron Smart Shunt, I have set as fully charged voltage to be 55.2V, for which the JK BMS used to show about 97% SoC. I didn't like it, but I was ready to leave it like that.
Today though, the JK BMS shows 87% SoC for 55.2 V... This was after the batteries' SoC went to about 10%.

For my MPPTs, the following settings are in place:
55.2v absorption
53.6 float

I haven't yet connected the BMS to the Cerbo via the driver kindly developed by Louis, for this particular reason and also since I'm thinking of adding a second battery at some point.

I've tried finding a way to calibrate the SoC of JK BMS, but no success so far...

Any ideas or suggestions for this situation are warmly welcomed.

Thank you!

The BMS SOC is based on the amount of each charge and discharge. It recalibrates each cycle(I believe) . Mine are never the same. For this reason, I just ignore them. I only go by the smart shunt.

timselectric said:

The BMS SOC is based on the amount of each charge and discharge. It recalibrates each cycle(I believe) . Mine are never the same. For this reason, I just ignore them. I only go by the smart shunt.

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Can I then ask how did you set up your inverter? Is it based in SoC or voltage: i.e.: cutout, grid coming in, etc... (If you have such a set-up).

No more grid connection. But, I used to be setup on battery voltage.

Actually, I changed to SOC, right before I disconnected from the grid. I used Solar Assistant to handle the transfer.(via the smart shunt SOC)

timselectric said:

No more grid connection. But, I used to be setup on battery voltage.

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That's brilliant, very good! Glad you can do that!

do you also manually calibrate the SoC of the SmartShunt from time to time?

I've setup mine such that when it is at 55.2V for 5 minutes it'll be 100%

timselectric said:

Actually, I changed to SOC, right before I disconnected from the grid. I used Solar Assistant to handle the transfer.(via the smart shunt SOC)

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I haven't heard of solar assistant before, I'll look into that!
Thanks for sharing!

tonyg said:

I've setup mine such that when it is at 55.2V for 5 minutes it'll be 100%

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Same

Per Victron shunt "charged" voltage should be 0.2V below absorption.

You likely need to push the battery to 3.5V+/cell to get the BMS to re-sync to 100%.

I won't bother pushing them over 3.5v/cell, there is in essence no need for that. I'll have to make sure that the Smart Shunt is well calibrated with its SoC and leave it to do its job.

sunshine_eggo said:

Per Victron shunt "charged" voltage should be 0.2V below absorption.

You likely need to push the battery to 3.5V+/cell to get the BMS to re-sync to 100%.

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I just had a look in the manual and to my surprise it says 0.2 or 0.3 below float voltage. I would think below absorption is more suitable as well...

tonyg said:

I won't bother pushing them over 3.5v/cell, there is in essence no need for that. I'll have to make sure that the Smart Shunt is well calibrated with its SoC and leave it to do its job.

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There actually is:
1) LFP benefits from periodic recharges to full by "resetting" the chemistry.
2) Allows for more time for cell balance and to assess cell balance.
3) All battery monitors/BMS require periodic recharging to full to maintain SoC accuracy.

tonyg said:

I just had a look in the manual and to my surprise it says 0.2 or 0.3 below float voltage. I would think below absorption is more suitable as well...

Click to expand...

That's for AC charging. For solar, it's absorption to prevent false syncs.

Yes, with the discharge curve voltage being so flat in the normal operating range with lithium you have to go past the flat area to be able to really see the SoC.