Confused about BMS and inverter settings for LiFePo4 battery

[Skabboz]

Hi everyone, I think I'm writing in the right section of the forum.

I'm making this post to get some tips on the configuration of my new PV system, to which I recently added a LiFePo4 battery.

These are the components of my system.
Hybrid Inverter: Easun SMG II 3.6 kW
BMS: Daly SmartBMS 8S 24V 150A
Batteries: 8x Prismatic EVE LF280K

What I would like to do is to use my batteries in the 80-20% range so that I can extend their useful life as long as possible. However, I am confused between the different voltage parameters that I can configure in both the BMS and the inverter.

For the BMS, I have configured Daly's SmartBMS app as you see in screenshot 1 and 2.
For the inverter, the manual recommends changing the parameters that I have shown in the third screenshot.

Could you help me understand what values I need to put in? I think I have some confusion in my head and could use some help from you guys. I read the beginners guide to voltages, but I'm still confused about the inverter values.

 

[stig44]

Your inverter values are almost same as mine. I have 16 cells LFP in series, 48V system. Check Your inverter settings for system voltage. With 8 cells You should have 24V system voltage.

 

[mikefitz]

Trying to limit battery SOC to 80 to 20 is not practical at the high SOC, and attempting to do so with the BMS, rather than with settings in the chargers and loads, is not the correct procedure. Don't attempt to control charging with the BMS.
You need to allow the battery to reach over 3.4 volts for effective balancing to occur. Thus set ballance start volts ar 3.4 volts, by attempting to balance at low volts, all you will do is mess up any balance that occured at high volts. Cells will only start to develop significant inbalance voltage over 3.4 volts per cell. Thus to keep the battery balanced it must be charged to a high enough voltage.
Set cell overvolts to the normal value 3.65 volts.
If you want a low stress charge voltage, try 28 volts and a float at resting volts, 26.8. Re boost at 26.4.
Charging to almost 100 SOC is OK providing you don't keep the battery at high charge volts for long periods.

The low SOC is where most accelerated ageing occurs, below 10%. Since the battery falls steeply in this region setting the inverter low volts disconnect is protection, say at 25 volts.
Keeping the battery away from the very high and very low SOC may be useful but the practical needs to achieve a high enough for effective balance must be met. There have been reported issues of considerable battery capacity loss due to consistent 'short cycling'.
I suspect the 20% to 80% SOC advice resulted from a series of graphs that exist on the Internet. These were based on research papers published some years ago on lithium ion batteries.
Note regardless of treatment there is natural aging of the battery.

Mike

 

[Skabboz]

Your inverter values are almost same as mine. I have 16 cells LFP in series, 48V system. Check Your inverter settings for system voltage. With 8 cells You should have 24V system voltage.

If you are referring to the third screenshot, that just the manual default values, they aren't mine. It's just a screenshot that I took from the manufacturer PDF to show the different settings, everything on my side is 24 V

Trying to limit battery SOC to 80 to 20 is not practical at the high SOC, and attempting to do so with the BMS, rather than with settings in the chargers and loads, is not the correct procedure. Don't attempt to control charging with the BMS.
You need to allow the battery to reach over 3.4 volts for effective balancing to occur. Thus set ballance start volts ar 3.4 volts, by attempting to balance at low volts, all you will do is mess up any balance that occured at high volts. Cells will only start to develop significant inbalance voltage over 3.4 volts per cell. Thus to keep the battery balanced it must be charged to a high enough voltage.
Set cell overvolts to the normal value 3.65 volts.
If you want a low stress charge voltage, try 28 volts and a float at resting volts, 26.8. Re boost at 26.4.
Charging to almost 100 SOC is OK providing you don't keep the battery at high charge volts for long periods.

The low SOC is where most accelerated ageing occurs, below 10%. Since the battery falls steeply in this region setting the inverter low volts disconnect is protection, say at 25 volts.
Keeping the battery away from the very high and very low SOC may be useful but the practical needs to achieve a high enough for effective balance must be met. There have been reported issues of considerable battery capacity loss due to consistent 'short cycling'.
I suspect the 20% to 80% SOC advice resulted from a series of graphs that exist on the Internet. These were based on research papers published some years ago on lithium ion batteries.
Note regardless of treatment there is natural aging of the battery.

Mike

Thank you very much for you response. As you correctly guessed, the 20-80 rule comes from several advice that I read all over the Internet, also on this forum, so I assumed that it's the best thing to do to extend the life.

Regarding on what is controlling the charge on the battery, I just changed the values as you suggested. The third screenshot is from SolarAssistant, that is pulling data from the Inverter. Are they correct now?

Thank you very much again