SoC vrs Voltage Charts ..and recommended Victron settings with SBMS0

[PeterBC]

After many months deep diving into this world, my head still spins at times.   I know its unwise to try and use voltage to determine SoC but many charts do exist. 
I understand that charging voltage vrs SoC (at various rates and temperatures) have one spread of answers. Then there are discharging rates vrs Voltage and SoC again at various rates and temps with their spread. And even at rest voltages vrs SoC in between one hopes !

https://diysolarforum.com/resources/general-lifepo4-lfp-voltage-to-soc-charts-tables-12-24-48v.109/  has one one set of tables, but doesnt say if for charging/discharging or rest ?
These agree very closely with my observations of some test batteries (just 6AH worth) , slowly discharging via SMBS0 load of 14ma or so. Currently after 2394 mAh  from 6000 mAH stated capacity that puts me at 60% SoC and pack voltage is 26.144 for average of 3.268v (13mv spread). On the DIY chart referred above thats between 50% 3.263v and 60% 3.275v ..  Temp is 20degC. 

I noticed a chart Dacian posted showing a typical spread of charging voltage vrs SOC vrs discharging voltage vrs SOC having about a 0.1 v spread. (dont know what rates the charging or discharging were). 

So just to be clear the default SBMS0 charging profile is  on up to 3.55V, when it will signal the charger to stop. It will remain stopped till it drop to  recovery voltage of 3.35v then charge again. 
on discharging it will stop at 2.80 and only allow again when at 3.20v. I presume we are calling the top point 100%. The bottom point I'm guessing is approx 5%  Correct ? 

What are the recommended settings for say a Victron Quattro and MPPT charger ?  Is it just to be higher than 3.55V per cell and charge till the SBMS0 says stop ? 
Likewise I presume a low voltage setting on the Quattro should be below the 2.80v/cell that the SBMS0 will stop loads ?
I've read some discussions of the possibility of 'overcharging' LFP if held at 3.45V or so for a long time. Could one do this if accidentally set a Float voltage on a Victron of 27.6V (3.45v/cell) and left it at that for days at a time with no loads ? The SBMS0 would not disconnect I gather until the V hit 3.55 on once cell ? I gather if we set a higher voltage, there is less chance of this happening as it would get to the 3.55v on a cell and disconenct. 

I presume Bulk/Absorbtion/Float/Storage are not meaningful here on the Victron... should all V settings be the same at say  3.60V/cell or higher and let the SBMS0 control things ?

My remaining question is re winter storage on my boat. If I'm on shore power, I'm nervous about maintaining my LiFePO4 at 100% all the time, despite some reassurances from Dacian. Can I run my Quattro with the battery disconnected ? so it will supply the few AC loads still from the shore power input ? I'd rather just isolate the battery , including SBMS0, at some 50% SoC or so for the non activity period. Would it even still supply the DC loads still connected to it ? Or will it have a hissy fit when it sees 0 battery voltage etc ? 

Thanks.. Peter
 

[Dacian Todea]

Peter,

Sorry but you can not use voltage as an indication for SOC with LiFePO4 not even a rough estimation. The only thing voltage can be used is to say if battery is full or empty and nothing in between.

I think you refer to the graph below that I shown a few times on the forum. You can have the battery at 90% or at 30% SOC and see the same battery voltage (so it will be completely useless to try and guess the SOC).

Answering the question of the charge/discharge rate in that graph will be fairly mining less unless you also know the capacity and internal resistance of the cells (cell resistance also fluctuates significantly with temperature).

The top point on SBMS0 is 3.55V default and SOC will be reset/corrected to 100% on the 2.8V all loads will be stopped so that will be the 0% SOC point but SBMS0 will not make any SOC corrections at 0% SOC so if you did not set the capacity correctly the SOC indication may be 0% before the battery is actually at 2.8V or if you set the capacity higher than it actually is then it will show some higher than zero SOC value even if a cell got to 2.8V and all loads where disconnected so then you can see that value say 10% and correct your real capacity setting as SBMS0 will not do anything to SOC when battery is fully discharged (not to mention that battery will likely never be fully discharged but will almost every day be fully charged).

Yes chargers should be set around 3.55V or slightly more and they should stop when the SBMS0 say so.

Inverter can be set at 2.8V to 2.9V per cell as one cell will always be the first to fully discharge and get to 2.8V while most other cells will still be around 3V

As mentioned the charger will be set at >3.55V for bulk charge and since there will always be a cell to get there first while others will be around 3.4 it is unlikely charger will ever get in to absorption let alone float and even if by any chance it gets in to absorption current will still be higher than cell balancing current so highest cell will continue to increase in voltage and get to 3.55V terminating the charging.

Yes you can set all values to 3.6V as they will have no meaning since SBMS0 will stop the charging.

One charge to 100% per day will not do any harm to battery as that will be considered a light normal use and battery should last 10 to 20 years in normal even heavy duty operation. After that charge battery will not be charged again for 24h if you set a low SOC for recovery. Low enough that the small DC loads you may have do not discharge the battery to in 24h period.

[PeterBC]

Thanks Dacian,

I'll try to stop pestering trying to understand SoC / voltage relationship !  

I dont quite understand how to set a lower SoC for recovery of charging (in a 24hr period) vrs the default "Over Voltage Recovery" of 3.4V ?  

I still think it might be 'safer' to just disconnect the LiFEPO4s for winter, as long as the Quattro will still allow shore power to power my minor AC loads with no battery connected ,for monitoring (and trickle charging of Engine 24v batteries and Generator 12v batteries via Victron Blue Smart chargers) ?   If I disconnect the SBMS0 I'd have to rewire the "Allow charging" signal to the SSR controlling the shore power AC, but I'd be happy (and perfer) to leave the SBMS0 connected to the isolated battery to monitor the cells over winter. Its load of 30ma or approx  0.7AH/day for my 816 AH set would not present a problem even after 200 days. (140AH so 17% of capacity).

However I'm becoming more comfortable with leaving connected to shore power and letting SBMS0 control, so once per day to a 3.55v charging disconnect, then my AC loads of approx 75W would drain approx 10% capacity in 24hrs (assuming I set a 80% SoC recovery level to prevent SBMS0 charging before the next day). I'll have the thermistor set to stop charging below 5deg C too. 

I'll set the Victron charge profiles to 28.8V for Bulk/aborb/float  (3.6v/cell) so it would never be attained and SBMS0 will control the charging. 

A separate question.. if I had a low voltage disconnect, and SBMS0 turns off my (8 yrs old) Quattro, I gather I cant charge via the Quattro then at all ? (using shore of generator). So I'd be reliant on Solar MPPT to bring the voltage back up enough to allow the quattro to turn back on ? 

reagrds.. Peter

[Dacian Todea]

Peter,

The SOC limit is set in the DMPPT Settings menu. By default is set at 99% that means it will be based on 3.4V recovery voltage but as soon as you set that SOC lower 98% or below the charging will only be started based on SOC and not voltage.

If your Quattro has no assistant (Aux i/o) for separate controls of charge and discharge the it can not be used with the SBMS0 as SBMS0 will have no way to stop the charging only the discharge by turning OFF the inverter.

Once all loads are tuned off solar panels should be able to charge the battery enough to re-enable loads (inverter) no matter how cloudy it is.

[PeterBC]

Thanks Dacian,
I hadnt look inside the DMPPT settings as I'm using a SmartSolar MPPT , so that setting is still valid in this case ? 
If my Quattro lacks AUX I/O , I'm planning to use SSRs to disable AC sources to stop the Quattro charging under SBSM0 control. 
I'll trust I can 'bootstrap' a low voltage disconnect by the Solar MPPT.. but should be able to avoid it as I'd always turn on the generator in the worst case before it goes under the LVD. 
rgds.. Peter

[Dacian Todea]

Yes that setting is valid no matter what charge source you have.