Jk BMS jumping charging discharging

Q&A and troubleshooting
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1

Dear community

I have built and installed quite a few eve mb31 batteries ( 48V, 314 Ah) .

I have an issue that I have never seen and would like to see if anybody else had the same…

Installation : 12 X 605 watt bifaial, multiplus ii 8000, mppt 250/100 and 150/45 , automatic generator start stop through Lovato interface, 16 X eve mb31.

My BMS , the jk BMS with latest firmware, is in control of charging / discharging. It is programmed to float at 53.6V. I dont have a a shunt because I have been fine relying on the jk in other installations for somewhat reliable soc calculations.

The issue is that after absorbing, when it goes to float , it will be charging/discharging the whole day, slowly lowering the soc. Lille literally every few seconds the system starts to discharge, then charge. See screenshot:

Screenshot_20241222_141853_VRM
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Have anybody had this? Should I recalibrate the battery? What is causing this?

I would like to understand what’s making this behaviour happen before installing a shunt and letting the Mppts do the charging…

I found one other topic that reported the same problem. So there must be other people out there with this issue?

Thanks

Brechr

2

I found this thread: Multiplus jumps from charge to discharge - VictronEnergy

Is there a way to PM this user or is this user on this new forum site?

3

There is no way to PM on this site. Not sure if the user is even on here. Accounts weren’t transfered.

4

How is your system programmed otherwise?
Since you are offgrid the charging and discharging is likely related to pv not covering loads and system consumption/losses.

The thread you linked was an ESS system. So not likely related to your use case.

The vrm can be a bit laggy so not the best way to see what is happening. Use the remote console.

5

it is definitely not the case of PV not covering loads. I have plenty of PV. there is something else happening that I’m not understanding. It is as if the battery gets slightly overcharged, then has to discharge itself. but none of the math of the remote console makes sense.

System is programmed like this otherwise:

  • dvcc is on, with shared voltage and current sense as well as temperature sense.
  • The jk bms is in charge of charging and discharging
  • generator auto start when soc drops below 15%
  • float voltage: 53.6, absorb voltage: 55.2 ; absorb time : 1 hr (set in JK BMS)

JK BMS settings and parameters:

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Thanks for your interest in this. I would love to figure it out. I am hoping it is a voltage calibration issue, will test that tomorrow.

It is as if the BMS is asking for 53.6V, but when the MPPT’s give it, it considers it too much and starts discharging. but not a little, really a lot. today it discharged 6 hours long around 350 Watt, basically constantly. (with occasionally charging happening, but overall the soc dropping, even though there is non stop sun) (location: portugal)

6

I have a 2 x JK systems at home. So it is interesting.

Am abs and float are 56.2v and 55.5 though.
The voltages you are using are more like the ones i see on second life cells.

Take of STS to start though.

And how is the inverter (and mppts) programmed?