I upgraded to a 4S2P [12V] config but each string is not sharing power...

[Oneorangedog]

Hi, a couple of years ago I built a 3.36KWh 4S 12V power pack with a victron inverter and charge controller included, for an offgrid woodland cabin.

Everything has been fine, but recently I included another 4S string in parallel to increase it to 6.72KWh. It has its own Daly BMS exactly same spec as the original string [though a newer model].

I didn't manage to fully top balance the new 4S string before taking it all to the cabin - I tried, but after 4 days at 10A it only got to about 75% SOC.

Now, when charging it only charges the older string. It gets to 95% and moves to "float", but the other string is still only at 60% and does not increase charge

I also have both strings connected into the victron shunt - but that has stopped sending data/connecting to the victron app... maybe not a related problem.

Here are some pics... [since reading some other posts on here I am considering upgrading to Class T fuses now... The cabin is only a store area, but I can't afford it all to burn down!]

Thanks in advance,

 

[sunshine_eggo]

BMS are not accurate until they get a full charge according to the BMS parameters.

This is where you triple check each and every connection for quality and proper torque.

Verify ALL voltages (batteries and cells), MPPT absorption voltage value, shunt value, etc., with a separate meter.

Verify current is flowing through each BMS (-) from the shunt with a clamp DC ammeter under high current conditions. They should be very close.

It's kinda impossible for two batteries to be in parallel and at 0.4V difference unless one of them is in protection mode OR there are notable voltage measurement errors.

Are your BMS operating in some sort of parallel mode, or is that just a display function of the app?

 

[Oneorangedog]

Are your BMS operating in some sort of parallel mode, or is that just a display function of the app?

Hmmm, you've got me thinking now... I manually selected the "parallel" status on the Daly app assuming that it referred to my having 2 parallel Daly BMS's and strings.

Perhaps that is wrong and by doing so, it has sort of isolated each string and thus caused this charging anomaly?

 

[sunshine_eggo]

Is there any sort of physical connection between them. I wouldn't think BT is reliable enough.

 

[Oneorangedog]

Is there any sort of physical connection between them. I wouldn't think BT is reliable enough.

No physical connection between the two Daly BMS's.

I think I should be using this "Series" setting [see attached pic]. As each battery string & Daly BMS is in a series configuration... right?

Also, I will add more charge to the newer string to get it closer to the older string, before I re-connect them - I disconnected the older string as it was almost fully charged and preventing the newer string from getting charge.

 

[Texas-Mark]

I think I should be using this "Series" setting [see attached pic]. As each battery string & Daly BMS is in a series configuration... right?

Yes. Also once the battery is fully charged, you may have to manually reset the Daly SOC to 100%