Growatt 3500TL LVM-US and SOK 48v 100ah questions

[Bhilton]

Hello,

I don't have my PV array with me, I only have my Growatt and SOK 48V 100ah battery connected to grid power.

I have these options set:
001 - Sbu
005 - Us2
012 - 48.0v
013 - 54.8v
019 - 57.6v
020 - 55.2v
021 - 46.0v

When I connected to grid power my SOK cells were all at 2900, they were not quite balanced, all within 100mV.

The grid power charged for a while and then stopped.

The SOK BMS says the battery is at 51.5v right now and the Growatt display shows the battery at 52.2v.

The cell voltages are all balanced very well at 3220mV.

Question 1, Is that normal? Or why didn't it charge up to the 57.6v or even the 55.2v float setting?

Question2, when I try to change the Growatt option 021 to from 46.0v to 48.0v for low DC cut off, it reverts to 46.0v, why can't I set it to 48.v?

 

[sunshine_eggo]

Cell voltages below 3.4V don't count towards "balance"

Batteries are "balanced" when their cell voltages are nearly identical when charged to full. Even imbalanced batteries will show near identical cell voltages in the 3.0-3.4V-ish operating range.

Q1 read the description for 12 and 13. When SOL or SBu set, 12 is when it starts charging from AC. 13 is when it stops charging.

You need to set 001 to ut1. It should observe settings 19 and 20 in that mode. Recommend 55.2V bulk and 54.0V float

Q2 Is that allowed in the manual? The manual typically indicates minimum and maximum values when applicable.

Discrepancies between BMS and inverter reported voltage are either due to current (inverter will read higher voltage than BMS when charging and lower voltage when discharging), poor connections or insufficient wire gauge. When current is 0A, any difference is simply measurement error.

 

[Bhilton]

Cell voltages below 3.4V don't count towards "balance"

Batteries are "balanced" when their cell voltages are nearly identical when charged to full. Even imbalanced batteries will show near identical cell voltages in the 3.0-3.4V-ish operating range.

Q1 read the description for 12 and 13. When SOL or SBu set, 12 is when it starts charging from AC. 13 is when it stops charging.

You need to set 001 to ut1. It should observe settings 19 and 20 in that mode. Recommend 55.2V bulk and 54.0V float

Q2 Is that allowed in the manual? The manual typically indicates minimum and maximum values when applicable.

Discrepancies between BMS and inverter reported voltage are either due to current (inverter will read higher voltage than BMS when charging and lower voltage when discharging), poor connections or insufficient wire gauge. When current is 0A, any difference is simply measurement error.

Regarding balance, thank you for that insight. Now I know.

Q1, Ok, right now I have no loads, other than the Growatt itself which uses 0.88Amps quite constantly. I changed it to Ut1 for now, since its only power source is utility grid power. It immediately changed on screen to show grid power is feeding the growatt now, and the battery is charging more now! At the cabin I am primarily relying only on Solar. I have a backup generator that I would manually start only in (hopefully) extreme scenarios.

Q1a, Should I change option 001 from Ut1 to SOL when I bring the Growatt and battery back to the cabin?
Q1b, The SOK battery specifications say to set absorption to 57.6 (recommended) and float 55.2. Should I follow that or stick what your recommendation?

Q2, The manual says option 021 low DC cut off "Default 42.0v, 40.0v to 48.0v settable"

The battery voltage reading discrepancy might have been caused because the Growatt was drawing 0.88A when I observed the battery bms saying 51.5v and the growatt screen saying 52.2v.

 

[sunshine_eggo]

Q1a, SBu is the more common choice. I struggle to understand the difference, but it makes more intuitive sense.
Q1b, The Growatt tends to have a long absorption cycle - longer than preferred for LFP. A long absorption cycle @ 3.45V/cell is more "gentle" to the battery. Some express concern of float of 3.40V/cell potentially overcharging cells at a very low rate causing Lithium plating, which removes it from active use and results in battery capacity. Their settings are fine. My settings are more conservative.

Q2, No idea. Maybe it's because you had 12 set to 48?

0.7V difference at 0.88A is almost certainly measurement error. Grab a voltmeter and check the battery terminals. Compare to BMS and Growatt values. Worth double checking that all connections are properly torqued.

 

[Bhilton]

After around 6 hours it has stopped charging and the SOK bms displays 53.27v, the SOC indicator lights show 4 of 4 lit up.
All the cells show 3326mv or 3327mv.

Is it fully charged I wonder?

 

[sunshine_eggo]

After around 6 hours it has stopped charging and the SOK bms displays 53.27v, the SOC indicator lights show 4 of 4 lit up.
All the cells show 3326mv or 3327mv.

Is it fully charged I wonder?

It sounds like it's in float mode as that's 0.7V below the 54V float. Assuming it hit and stayed at 55.2V for at least an hour or two, it should be close. You MIGHT need to set your voltages 0.7V higher due to the measurement error, but I would want to confirm with a separate meter.

 

[Bhilton]

My battery bms display reads correctly, my voltmeter matches it. I tested on both the battery terminals and the growatt terminals where the wires bolt on, same thing. Yet the growatt display shows 0.7-0.8v higher.

 

[sunshine_eggo]

My battery bms display reads correctly, my voltmeter matches it. I tested on both the battery terminals and the growatt terminals where the wires bolt on, same thing. Yet the growatt display shows 0.7-0.8v higher.

You may want to consider adjusting your charge voltages +0.7V accordingly since the growatt is using its measurements.

 

[Bhilton]

You may want to consider adjusting your charge voltages +0.7V accordingly since the growatt is using its measurements.

Ok I have tried it and the battery bms showed cells up to as high as 3617mv.

But as soon as it stopped charging, the growatt pulls 0.8A and all the cells dropped to 3350mv very quickly.