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robster avatar image
robster asked

24v Multiplus II & Electrodacus SBMS0

Hi All,

I have the above setup, with the Electrodacus monitoring a bank of LiFePo4 cells and configured to the Multiplus via ESSassistant, as a two wire BMS.

My question is, in normal operation, should the Multiplus cease charging when the battery is full or does it rely on the Electrodacus to disconnect it?

I thought the BMS was supposed to cut in as a fail safe if the Multiplus didn’t detect a charged battery but my setup always stops charging via the input from the Electrodacus, with the associated #3 ESS code.

Thanks in advance.

Rob

Multiplus-IIBMSess discharging
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3 Answers
Matthias Lange - DE avatar image
Matthias Lange - DE answered ·

Is it the first charging? Maybe the cells are unbalanced and the BMS stops charging because one cell goes to high. Reduce the charing current and let it slowly charge over a long time.

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robster avatar image
robster answered ·

Hi Matthias,

Thanks for taking the time to respond

There is a slight imbalance, with one cell about 0.07V higher than the lowest one, and I suspect that’s why the BMS is cutting in.

I had already reduced the max charging power to 500W, but didn’t make much difference - #3 still appeared.

I’m reluctant to strip down the whole battery pack to balance it, as it’ll be a huge job, so I was hoping to tweak the settings on the Multiplus so it ends charging sooner (just before the BMS detects the high cell)

…any idea which parameters on the Victron side I can adjust to temporarily lower when the charge cycle ends - at least until the BMS had time to balance the battery?

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wkirby avatar image wkirby ♦♦ commented ·
The only thing you can do on the Victron side is to reduce the charge Voltage so that it reduces its charge current sooner. The BMS' stop charge signal is supposed to be more of an alarm signal to stop charging in an emergency.

Ideally the BMS' high cell condition should be a slightly higher Voltage than the cell balance Voltage.
For example, set the MultiPlus' Absorb Voltage at 28V, cell balance Voltage at 3.5V and cell high alarm at 3.55V. This way the MultiPlus will go into Absorb, reduce charging current and hold that Voltage with each cell ideally at 3.5V with headroom for balancing before an alarm is triggered at 3.55V which forces the MultiPlus to stop charging via BMS signal.

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Matthias Lange - DE avatar image Matthias Lange - DE ♦ commented ·

0,07V is not much and shouldn't cause the BMS to stop charging.
If possible check the settings of the BMS.

On the Victron site you can only play with the settings for charge voltage and current.

What settings do you currently have?

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Louis van der Walt avatar image
Louis van der Walt answered ·

You want the Multi/MPPTs to stop charging before your BMS has to protect your cells. If the BMS disconnects then you have already pushed it too far.

The best would be to lower your battery charge voltage. By default I suggest 3.45V per cell (27.3V for 8 cell LifePO4), but if you have issues (imballance) then 3.39V per cell until you have that solved.
See point 2 under https://github.com/Louisvdw/dbus-serialbattery/wiki/How-to-install#settings-for-your-gx-device of where to do that in the GX.

You have to look for cell imbalance then the cells/battery is fully charged. Litiums have a very flat voltage curve until the cell is depleted of full, so from 30% to 80% the cells might show only 0.07V difference, but then one will shoot out quickly above 3.50 volt while the rest are still around 3.3V You need to check for the imbalance when the disconnect happens.


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robster avatar image robster commented ·

I was wondering whether to use DVCC, but was confused by point 3 of the link regarding linking of the BMS to the Cerbo using a serial lead.

As far as I’m aware there is no facility to do this with the Electrodacus SBMS0.


The rest of what you suggested makes complete sense - I’m just reluctant to enable DVCC until someone reassures me that it’ll be ok. :0)

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