Float setting a lithium battery?

Extensive load shedding and the likelihood that this is the new normal have required a review of my current Axpert inverter and Shoto battery settings.  

Questions:

Should a float voltage be set for my 2 x 4.8kw lithium batteries?

Is it correct that float is only used in lead acid and gel type batteries?

That a charge voltage of 52.5V = 3.5v per cell for a 15 cell battery, is equal to 95.6% of the battery’s capacity and is the correct setting level for both float and bulk charge voltages in a lithium battery.

That you don't need to set a different lower float voltage, as it will reduce the usable capacity of the battery bank.  

The safe charging voltage (bulk charging) of a lithium battery is 3.5v per cell. While this can be increase it has consequences on cell life and is prevented from exceed a pre-set BMS value.

Charge settings (CV):

Shoto has a BMS set protection value of 3.65v per cell. At this value the BMS shuts down and will then only allow charging to restart when the 3.5v per cell is level is again reached. Thus 3.5v per cell is the maximum, normal safe charge setting for the 15 cell Shoto battery. While the battery charge may vary around the 3.5v per cell setting  momentarily due to cloud, shade and inverter tolerances this will be slight and should never get anywhere near the protection value of 3.65v per cell. In my case I see values of 3.41V to 3.52V.

Float settings:

I have often wondered why the Shoto lithium battery manual never mentions a float charge, while the Axpert King expects both.  Lithium batteries have a totally different chemical composition to the lead acid and gel types.  Lead acid and gel types definitely must have a float setting.  Consequently, this would seem to have been provided for such batteries, and not necessarily for the lithium’s.  A charged lithium, with no draw off maintains its charge state, unlike the lead acid and gel types which lose charge state even with no demand.

You can set a float value for your lithium’s without damage to them.  However, the consequences are a reduction to the full charge capacity. When preforming a bulk charge from a low charge state, well below that of the float setting value, batteries will be charged to the bulk charge value and then stop charging.  Charging of the batteries, after power draw off will then recommence once the charge level has dropped below the float setting value and charge the battery back up to the pre-set float value – not to the bulk charge setting value, reducing the full charge capacity of the battery bank.

Measured values based on my system settings:

Maximum cell voltage	3.65V per cell	54.75V for 15 cells	100% capacity	BMS Shut down
Bulk charge voltage	3.50V per cell	52.50V for 15 cells	96.0% capacity	Normal operation
Float charge setting	3.50V per cell	52.50V for 15 cells	96.0% capacity	Normal operation
Lower float setting	3.45V per cell	51.80V for 15 cells	94.6% capacity	Reduced capacity

Change in battery bank capacity for a 2.5hr load shedding period:

Float charge setting	3.45V per cell	51.80V for 15 cells	Capacity left in battery 27% - DOD 73% after 2.5hrs
Float charge setting	3.50V per cell	52.50V for 15 cells	Capacity left in battery 33% - DOD 67% after 2.5hrs

Figures are based on my system usages, your demand values could differ.   Please comment and see if you have a similar outcome.

Special acknowledgment and thanks:

Based on the excellent work of Andy, on his website on You Tube, “The off-Grid Garage” Andy has done some great work on this topic and others.

@OffGridGarageAustralia

Website:  https://off-grid-garage.com

 

Well it depends on how often you're going to be running off the battery. Long term storage should be done at about 90% SOC, float would be pointless if you're barely ever running off the batteries. However, if you're getting loadshed often and the duration of your outages is all over the place, that extra little bit of SOC could be handy. 

On the other hand, if your battery stays above 60% SOC and your loadshedding schedule is good then dedicating solar resources to float charging could be better spent supporting loads....

It really is a balancing act that depends on what you want and what you need... If you have more than enough solar to support loads and charge the batteries then why not float for that extra little bit of capacity. If you are running hybrid and need grid support from time to time then why waste on a float, rather get that little extra off your power bill?

Although, that is just my opinion and maybe someone else has some feedback from another perspective... I am curious to see what others have to say on the topic

6 hours ago, nightbyte said:

Float setting a lithium battery?

Extensive load shedding and the likelihood that this is the new normal have required a review of my current Axpert inverter and Shoto battery settings.  

Questions:

Should a float voltage be set for my 2 x 4.8kw lithium batteries?

Is it correct that float is only used in lead acid and gel type batteries?

That a charge voltage of 52.5V = 3.5v per cell for a 15 cell battery, is equal to 95.6% of the battery’s capacity and is the correct setting level for both float and bulk charge voltages in a lithium battery.

That you don't need to set a different lower float voltage, as it will reduce the usable capacity of the battery bank.  

The safe charging voltage (bulk charging) of a lithium battery is 3.5v per cell. While this can be increase it has consequences on cell life and is prevented from exceed a pre-set BMS value.

Charge settings (CV):

Shoto has a BMS set protection value of 3.65v per cell. At this value the BMS shuts down and will then only allow charging to restart when the 3.5v per cell is level is again reached. Thus 3.5v per cell is the maximum, normal safe charge setting for the 15 cell Shoto battery. While the battery charge may vary around the 3.5v per cell setting  momentarily due to cloud, shade and inverter tolerances this will be slight and should never get anywhere near the protection value of 3.65v per cell. In my case I see values of 3.41V to 3.52V.

Float settings:

I have often wondered why the Shoto lithium battery manual never mentions a float charge, while the Axpert King expects both.  Lithium batteries have a totally different chemical composition to the lead acid and gel types.  Lead acid and gel types definitely must have a float setting.  Consequently, this would seem to have been provided for such batteries, and not necessarily for the lithium’s.  A charged lithium, with no draw off maintains its charge state, unlike the lead acid and gel types which lose charge state even with no demand.

You can set a float value for your lithium’s without damage to them.  However, the consequences are a reduction to the full charge capacity. When preforming a bulk charge from a low charge state, well below that of the float setting value, batteries will be charged to the bulk charge value and then stop charging.  Charging of the batteries, after power draw off will then recommence once the charge level has dropped below the float setting value and charge the battery back up to the pre-set float value – not to the bulk charge setting value, reducing the full charge capacity of the battery bank.

Measured values based on my system settings:

Maximum cell voltage	3.65V per cell	54.75V for 15 cells	100% capacity	BMS Shut down
Bulk charge voltage	3.50V per cell	52.50V for 15 cells	96.0% capacity	Normal operation
Float charge setting	3.50V per cell	52.50V for 15 cells	96.0% capacity	Normal operation
Lower float setting	3.45V per cell	51.80V for 15 cells	94.6% capacity	Reduced capacity

Change in battery bank capacity for a 2.5hr load shedding period:

Float charge setting	3.45V per cell	51.80V for 15 cells	Capacity left in battery 27% - DOD 73% after 2.5hrs
Float charge setting	3.50V per cell	52.50V for 15 cells	Capacity left in battery 33% - DOD 67% after 2.5hrs

Figures are based on my system usages, your demand values could differ.   Please comment and see if you have a similar outcome.

Special acknowledgment and thanks:

Based on the excellent work of Andy, on his website on You Tube, “The off-Grid Garage” Andy has done some great work on this topic and others.

@OffGridGarageAustralia

Website:  https://off-grid-garage.com

 

Stricly speaking float should  not have any impact on Lithium batteries.

Likely what is happening is that the inverter is switching to float voltage to early (before the battery is fully absorbed) which then results in the constant voltage charging phase not occuring at the optimal voltage. This will cause the battery to not charge to its full capacity as there will not be enough pressure to fully charge the battery.

Once the battery is fully absorbed the float voltage should not play any role if the battery is equipt with a decent BMS. In other words, if you charge with 56v (by example) and the battery reached 100% SOC (for a battery with a full capacity of 100000mAH the available capacity is then also 100000mAH), setting a float voltage at 55v (or any value lower than 56v) should not cause any change in the SOC and the battery will remain at 100000mAH).

But if the battery is not yet 100% full (absorbed to 100000mAH in my example above) and the inverter then switches to float voltage, then I agree, you will not charge the battery full and will have less available capacity. If the float is set to low and this occurs it can actually discharge slightly in the final charging phase. By example if the battery is still charging and sitting at 55.5v and the inverter then starts to float at 55v the battery will discharge until the BMS reacts to stop this.

 

I made good experience with my settings: Bulk charge 54V, floating 52.5V. I have 4 packs LiFePo 100Ah 15 cells in parallel. When it reaches floating state the BMS reports 100% SOC and remains there. The charging current from PVs is reduced to cover the actual load. Such state never lasts long time anyway as it is only reached late afternoon, if at all.

By the way: awkward 100000mAH = 100Ah. Our electricity prof at the university accepted test results only with numerical values between 1 and 999. I know, BMSs and many reporting software do it that awkward way. That tells me how much the programmer knew what he was dealing with.