I need advice regarding charging and floating of lithium batteries.

My question is it safe to float a lithium battery I have a 5.1 KW/h LBSA the spec call for charging voltage 56 V and floating voltage at 54 V.

Once my battery is at 100% SOC it comes down to 54 V and my inverter floats the battery at that level.

Is this safe, I'm afraid it's not good for the battery, please advise.

6 minutes ago, Antonio de Sa said:

I need advice regarding charging and floating of lithium batteries.

My question is it safe to float a lithium battery I have a 5.1 KW/h LBSA the spec call for charging voltage 56 V and floating voltage at 54 V.

Once my battery is at 100% SOC it comes down to 54 V and my inverter floats the battery at that level.

Is this safe, I'm afraid it's not good for the battery, please advise.

You need to go with the battery/manufacture spec because not all Lithium batteries have the same spec. If your battery spec says floating voltage is 54v, then you set your inverter according to that

@hoohloc Thanks for the reply, I asked the question because a while ago someone told me that one should never float lithium batteries.

And this is what my inverters are doing. I have setting 20 to float at 54 V and the growatt inverters are doing exactly is per parameter settings,

Edited December 23, 20213 yr by Antonio de Sa

I think they meant not to float as in UPS where batteries will stay in float for days, weeks or in some cases months, without being cycled. If you cycle your batteries everyday, I doubt floating them for few hours will do any harm. Otherwise that would have been stated in the installation/user manuals that you should never float your batteries. People have opinions, just take them with a pinch of salt and follow the recommendations from the manufacture. If they say float at 54v, then it means you can allow the battery to float. 😀

5 hours ago, Antonio de Sa said:

@hoohloc Thanks for the reply, I asked the question because a while ago someone told me that one should never float lithium batteries.

That someone doesn't know what they are talking about.  Once a lithium battery reaches a certain state of charge at a given voltage it will no longer accept charge.  You can keep it at that voltage and it will simply draw self consumption current at that point (nearly no current whatsoever).

The only thing to avoid is high "float" voltages.  For example, with LiFePO₄ to charge the battery fully you would technically charge it at 3.65v per cell and then keep the float somewhat lower than that because at 3.65v the cells degrade much faster.  Thus you would typically float it at a more reasonable level like 3.42v per cell.  For LiFePO₄ there are pretty compelling evidence to show even charging up to 3.48v per cell will get you to 99% SoC.  That last 1% dramatically reduces the life of the cell and is thus best avoided.  But you can easily test that for yourself if you have a BMS (ie. with LiFePO₄ charge up to 3.65v per cell, drain battery down, then charge back up to 3.48v per cell, then drain down.  Compare Wh you attained and thus be informed of what that last bit buys you.  Then adjust accordingly to meet your expectations).

Lots of these yahoos around who try to apply Lead Acid logic to Lithium batteries.  It just doesn't work that way.

Assuming LBSA uses LiFePO₄ in a 16s configuration, their 56v "charge" voltage is 3.5v per cell and 54v = 3.375v per cell.  I personally went a bit higher on my "float" but they have obviously tested this and determined that you will still get a guaranteed capacity at that float voltage.  At the float they propose LiFePO₄ is pretty far from its maximum voltage, I wouldn't be concerned at that voltage about premature aging.  The reason they went up to 56v in the first place is to ensure all the cells are fully charged.  Because the cells are in series if the "float" was 3.375 some cells may not be fully charged.  So you bring them all near enough to 3.5v and they'll all self consume down to 3.375 and be 98% or whatever full at that point.

Edited December 23, 20213 yr by Gnome

@Gnome Thanks for the clear explanation, Front the information I get from reading directly from the battery BMS I'm floating at Max 3,38 V see graph 

So I'm I'm convinced now that I'm not reducing the life of the 16 cells, from what I can see they voltages are pretty well balanced 

I see I get to 100% SOC when my cells are at 3.51 V

Once more thank you for putting my mind at ease.

Edited December 24, 20213 yr by Antonio de Sa

@Gnome Sorry to bother you with silly questions, what would you say is the safest lower voltage on a Lithium battery cell.

When I use the battery I switch over to mains when the cells are at 3,17 V, in your opinion is this OK or should I keep them at a higher value?

10 hours ago, Antonio de Sa said:

@Gnome Sorry to bother you with silly questions, what would you say is the safest lower voltage on a Lithium battery cell.

When I use the battery I switch over to mains when the cells are at 3,17 V, in your opinion is this OK or should I keep them at a higher value?

No bother.  Lithium cells don't need to be charged.  They age slower at lower voltage.  Again from what I've read online from 3v per cell down to 2.5v per cell is less than 1% of capacity.  So you want your low voltage cut-off to be around 3v per cell average and your low voltage minimum cut-off around 2.8v.  (so if any one cell reaches 2.8 then cut-off power).

Keeping your "float" voltage at 3.375 as per LBSAs recommendation is totally acceptable.  But if you can't bring them back up to charge due to not enough solar that is fine too.

LiFePO₄ has a very narrow voltage range, it will spend most of its time at 3.2->3.3v

Lastly Lead Acid logic doesn't apply.  Lead Acid must be kept at full charge or it will sulfate and damage the battery.  Lithium is the opposite, the higher the voltage the faster it ages.  Lead Acid has a pretty wide voltage range and if you put a lot of load on it the voltage will drop a LOT (Peukert's law).  This is not applicable to lead acid.

When you put a heavy load on a LiFePO₄ the battery voltage will recover a bit when you remove that load but it'll be something like 3.35v -> 3.6v per cell.  Whereas Lead Acid will easily drop from something like 12.8v to 12v on a heavy load very quickly, then rapidly recover once the load is removed.

Just a  thought , my charge/float is 55v cut off 47v.

 

 

2 hours ago, Raj35 said:

Just a  thought , my charge/float is 55v cut off 47v.

 

 

@Raj35 I have the following, Charge 55.2 V Float 54 V and cut out at 48 V and so far it looks fine. I'm running my system without BMS.

see attached.

@Antonio de Sa without bms, just becareful, something I tried after 55.8v the cell become a runaway train. I have tried it some cells can reach 3.7v

 

1 hour ago, Raj35 said:

@Antonio de Sa without bms, just becareful, something I tried after 55.8v the cell become a runaway train. I have tried it some cells can reach 3.7v

 

Hi my understanding is that bms should still function even if not connected to the inverter. Battery management system is there to protect the battery and being able to communicate with the inverter is a plus 1 or bonus, as it will make it easy for the bms to achieve that protection by taking over the settings of the inverter. There is a BMS card on the batteries, which gets all the info about the cells or batter as a whole and that control card is independent of the inverter, meaning it should work whether there is comms between the battery or not, I stand to be corrected  

@hoohloc and @Raj35 I've been monitoring my system like a hawk and so far I can see that some how the battery internal BMS is doing it's job.

I have never seen my battery voltage going over the values I've set on the inverter, also I monitor and archive every single cell voltage and the higher I've seen so far is 3.45 well within safety levels.

See attached screen dump of the last 12 hours

Once the battery is fully charged they slowly balance themselves to 3.2 V

 

 

@Antonio de Sa your BMS is doing what it is meant to do, even if for some reason you exceed the safe operating limits, the BMS will trip/switch off your batteries. Communication with the inverter is cherry on top for those inverters that can communicate with bms, so that you don't have to worry about setting up some parameters for your batteries. If you have the spec for the batteries, you do not really need that comms because you can input the settings manually. I personally find bms comms with my growatts, to be nuisance and have decided not to use it anymore. My system works perfectly and I'm now more than happy with it 

5 hours ago, Antonio de Sa said:

@hoohloc and @Raj35 I've been monitoring my system like a hawk and so far I can see that some how the battery internal BMS is doing it's job.

I have never seen my battery voltage going over the values I've set on the inverter, also I monitor and archive every single cell voltage and the higher I've seen so far is 3.45 well within safety levels.

See attached screen dump of the last 12 hours

Once the battery is fully charged they slowly balance themselves to 3.2 V

 

 

@Antonio de Sa I thought you had no bms at all, so there is one internal that's fine system should operate fine

On 2021/12/23 at 11:29 PM, Gnome said:

That someone doesn't know what they are talking about.  Once a lithium battery reaches a certain state of charge at a given voltage it will no longer accept charge.  You can keep it at that voltage and it will simply draw self consumption current at that point (nearly no current whatsoever).

The only thing to avoid is high "float" voltages.  For example, with LiFePO₄ to charge the battery fully you would technically charge it at 3.65v per cell and then keep the float somewhat lower than that because at 3.65v the cells degrade much faster.  Thus you would typically float it at a more reasonable level like 3.42v per cell.  For LiFePO₄ there are pretty compelling evidence to show even charging up to 3.48v per cell will get you to 99% SoC.  That last 1% dramatically reduces the life of the cell and is thus best avoided.  But you can easily test that for yourself if you have a BMS (ie. with LiFePO₄ charge up to 3.65v per cell, drain battery down, then charge back up to 3.48v per cell, then drain down.  Compare Wh you attained and thus be informed of what that last bit buys you.  Then adjust accordingly to meet your expectations).

Lots of these yahoos around who try to apply Lead Acid logic to Lithium batteries.  It just doesn't work that way.

Assuming LBSA uses LiFePO₄ in a 16s configuration, their 56v "charge" voltage is 3.5v per cell and 54v = 3.375v per cell.  I personally went a bit higher on my "float" but they have obviously tested this and determined that you will still get a guaranteed capacity at that float voltage.  At the float they propose LiFePO₄ is pretty far from its maximum voltage, I wouldn't be concerned at that voltage about premature aging.  The reason they went up to 56v in the first place is to ensure all the cells are fully charged.  Because the cells are in series if the "float" was 3.375 some cells may not be fully charged.  So you bring them all near enough to 3.5v and they'll all self consume down to 3.375 and be 98% or whatever full at that point.

I concur with @Gnome in respect of his explanation. My other 2-cents is you can even charge to a maximum of 95% SoC and even if it floats at that level, this means it's not "force-charging" your battery. Also, ideally the BMS will help if over-charging, but ideally don't let it float at 100%SoC (State of Charge).

I am trying to get the ideal float setting on my lithium ion batteries to stop the SOH level from dropping and not rech arging to 98%,any suggestions

 

 

3 hours ago, Mdvr said:

I am trying to get the ideal float setting on my lithium ion batteries to stop the SOH level from dropping and not rech arging to 98%,any suggestions

 

 

which batteries?

On 2022/11/18 at 5:27 PM, Mdvr said:

I am trying to get the ideal float setting on my lithium ion batteries to stop the SOH level from dropping and not rech arging to 98%,any suggestions

After weeks of playing with bms and inverter settings, I found changing my float voltage to 54.5V (from 54V) ensures my 16s battery reaches 100% every day.

My bulk setting is 56.4V, but sometimes the inverter will exit the bulk stage before 100% soc in bad weather, so the increased float voltage makes sure some cells stay above 3.5V and the passive balancing remains active.

4 hours ago, jumper said:

After weeks of playing with bms and inverter settings, I found changing my float voltage to 54.5V (from 54V) ensures my 16s battery reaches 100% every day.

My bulk setting is 56.4V, but sometimes the inverter will exit the bulk stage before 100% soc in bad weather, so the increased float voltage makes sure some cells stay above 3.5V and the passive balancing remains active.

This is good info for 16s batteries but we still need to know which batteries the poster is asking about in case they are not 16s. I have 15s and so my values are quite different. Also differs if NMC.

Thanks for the post though and your experience