Victron Multiplus and REC BMS only charging at batteries @ 13.5v

[Jokani]

New install of a Victron Multiplus and REC BMS with 16 x CALB SE200FI.

The max charging current I have seen is 13.5v, so the cells are never reaching a voltage high enough to balance.

Please could someone take a look at my settings and see if they can spot the problem, or maybe something else could be the cause?

Any suggestions to improve the settings, also appreciated.

I am pretty sure that the Multi and REC are communicating correctly over Can.

TIA

 

[sunshine_eggo]

First, 13.5V is a voltage, not a current.

Second, did you conduct a top balance of these cells less than 7 days before the battery was built and deployed?

As I understand it, the REC BMS modulates charge current based on the highest cell voltage. If at 13.5V, ONE of the cells is at the peak cell voltage, the REC will signal the MP to reduce charge current to maintain that voltage.

What do you observe at the cell level when you've hit 13.5V?

 

[Jokani]

Hi Sunshine, thanks for trying to help me out.

My mistake, I should have said "max charging voltage I have seen is 13.5v", and my question is about voltage rather than current.

I did perform a top balance, but it may have been 10-14 days before I took them to the boat to install.

To test the charge voltage, I switched off the AC supply and used the inverter to run the heating system until the batteries were depleted by about 25%, and then switched the AC supply/charger back on, the max voltage seen was 13.5v, whereas I was expecting to see something closer to the 14.6v set as the absorption voltage on the Multiplus.

The default REC BMS Balance start voltage is 3.45v/13.8v, so I am concerned that not getting above this will mean the balance never starts, and the batteries will take longer to charge

 

[sunshine_eggo]

10-14 days is enough time for "grade A" Ali-whoever cells to drift by a tiny amount of charge and "undo" the top balance. Even 0.1% deviation in SoC can make it seem like it's going to heck in a handbasket.

The bulk of charging of LFP cells occurs below 3.4V. Unlike lead-acid, which needs a couple hours or more at absorption, the LFP absorption phase is usually VERY short especially if you are charging at < 0.5C.

Cells can be fully charged at 3.45V/cell at lower current, but do take longer to charge depending on your start current.

You need to assess your individual cell voltages and the charge current. If you're still pushing the expected max charge current at 13.5V, and the cells are balanced, nothing is wrong.

 

[Jokani]

Good point, yep it is a 130 charger so I was getting full current, and charging at full speed, so I can stop worrying about that.

I'm still worried about balancing, these are the default settings:



My cell voltages are not getting over 3.45 even when fully charged, if I cannot increase the charge voltage, maybe I need to reduce the balance start setting?

 

[sunshine_eggo]

I think you should stop worrying until you've collected more data to provide evidence that you have any problem at all.

Personally, I would lower voltage to 14.4V and see how the cells behave as they actually approach full. There really is no reason to push all cells to 3.65V. Fast charging to full can be achieved at 3.55V and above.

Lastly, for non-generator charging such as solar (lower current), I would reduce absorption voltage to 13.8V and allow them to slowly charge with a 2-3 hour absorption phase provided it meets your needs. There is good evidence to suggest that keeping your charge voltage at this level and thus reducing the current as the cells fill can significantly improve cycle life.

Honestly, you're actually charging at a somewhat low rate @130A. If you have a 4P4S setup resulting in 800Ah, 130A is only 0.16C where most charge specifications are intended for 0.5C. At this level, the voltage rise from 13.X to 14.4V will occur very late in the charge and will seem violent compared to the long period of time where it seems the voltage is barely moving.

 

[Jokani]

Thanks for taking the time to put that info together. I'll do exactly as you say, monitor over the next few weekends, and report back.

Thanks again.

 

[Multiplex]

Hi Jokani
How was the system doing with that settings?