Growatt SPH SOC settings

[solarmaker1973]

Hi everyone, I need your expert opinion. I have a Growatt SPH4000 inverter: from the settings I can set the minimum SOC value (25%) while it is not possible to set the maximum SOC value and therefore it always manages to charge up to 100 %. if I wanted to stop charging the Pylontech batteries at say 90%, what should I do? I can't find it in the settings a way to stop charging at 90% in LOAD first mode (the same is instead possible while in battery first mode..)… Thank you very much!!

 

[sunshine_eggo]

You don't. Charging only to 90% is a horrible idea. Batteries must be fully charged to ensure the cell stay in balance.

Pylontech tells the inverter how to charge it. You would need to get the cable and software and reprogram the batteries to charge to a lower voltage like 3.45V/cell. You will struggle to get to 100%, and it will happen slower causing less stress to the cells and optimizing cycle life.

 

[solarmaker1973]

You don't. Charging only to 90% is a horrible idea. Batteries must be fully charged to ensure the cell stay in balance.

Pylontech tells the inverter how to charge it. You would need to get the cable and software and reprogram the batteries to charge to a lower voltage like 3.45V/cell. You will struggle to get to 100%, and it will happen slower causing less stress to the cells and optimizing cycle life.

thanks for kind reply, so you are suggesting to fully charge the pylontech to 100% all the times (yes, I know balancing happens at 100%, but also knew that charging all the times to such level is bad for life battery.. isnt it?). and what about minimum SOC. 25% is a good /conservative level?

 

[sunshine_eggo]

thanks for kind reply, so you are suggesting to fully charge the pylontech to 100% all the times (yes, I know balancing happens at 100%, but also knew that charging all the times to such level is bad for life battery.. isnt it?).

Depends.

Rapid charging to 3.65V/cell maximizes stress. The most damage occurs while charging. By charging to 3.45V/cell, you charge at a lower current as you approach full. This minimizes the charging stress. From a life cycle perspective, it is analogous to charging to a lower peak SoC, but you get the benefit of extended time for balancing.

and what about minimum SOC. 25% is a good /conservative level?

20% is generally regarded as the standard for LFP. Other manufacturers dictate other levels. The general consensus is that the calendar effects of aging will degrade cells faster that deep discharges. I set safeties VERY low. I really don't want the system telling me I have nothing left when I have something left. I take comfort in knowing that my typical usage pattern means I'm not consuming my entire battery capacity daily.

 

[solarmaker1973]

sorry, dont understand your reply fully.
when you say "Rapid charging to 3.65V/cell maximizes stress. The most damage occurs while charging. By charging to 3.45V/cell, you charge at a lower current as you approach full. This minimizes the charging stress." it means that charging to SOC=100% is not a good idea for life of the battery. but on the first answer you said "You don't. Charging only to 90% is a horrible idea" which made me think that charging to 100% was a good idea.. which is true? Should I try to charge (during everyday use..) to 100% or limit to 90%? thank you very much, I appreciate time you are spending to reply

 

[Supervstech]

It is the rate of charge above 3.45V that causes heating degredation.
Instead of limiting the soc, you want the voltage maxing at 3.45v per cell, so the rate of charge drops off as the peak is approached, then the 100% soc isnt stressing the cells, and balancing is maintained.

 

[sunshine_eggo]

@Supervstech filled in the blanks, but to address each bit for clarity.

sorry, dont understand your reply fully.
when you say "Rapid charging to 3.65V/cell maximizes stress.

Referenced a voltage, not a SoC. You don't HAVE to charge to 3.65V/cell to get to 100%.

The most damage occurs while charging. By charging to 3.45V/cell, you charge at a lower current as you approach full. This minimizes the charging stress."

Again, I referenced a voltage. Can get to 100% or very nearly so @ 3.45V/cell.

It's how you get to 100%, not whether you do or not.

it means that charging to SOC=100% is not a good idea for life of the battery. but on the first answer you said "You don't. Charging only to 90% is a horrible idea"

What I didn't state is that charging to 90% SoC is very difficult to do. It can't be done reliably based on voltage, and only charging to 90% SoC means you never get balance time meaning your cell SoC's drift and you lose capacity.

which made me think that charging to 100% was a good idea.. which is true? Should I try to charge (during everyday use..) to 100% or limit to 90%? thank you very much, I appreciate time you are spending to reply

Charging to 100% is a good idea.

 

[solarmaker1973]

perfect, now it is crystal clear, thank you
You said"
Again, I referenced a voltage. Can get to 100% or very nearly so @ 3.45V/cell.
It's how you get to 100%, not whether you do or not."

Unfortunately on my growatt inverter I can not see voltages, but only % of SOC, but fully understand your point.
And I noticed that charging is fast until 90%, then it becomes very slow so assuming this is what the BMS protocol mandates not to stress batteries. So, I understand when you said that it is BMS task to manage the re-charge, and the way it happens (fast at beginning, slow later near to 100%)
Thank a lot again, you were very useful and kind!
Fabio