I've been doing some research regarding LI batteries, some say it's no good to float LI batteries, what are your opinion regarding this subject.

 

1 hour ago, Antonio de Sa said:

I've been doing some research regarding LI batteries, some say it's no good to float LI batteries, what are your opinion regarding this subject.

 

I queried this exact issue with the Growatt rep in China and basically the float charge value according to his explanation doesn't even apply.
The Growatt goes into discharge the moment it's charged to 100%.
The discharge is the power usage of the inverter and fluctuates between 0,3 and 0,8 Amps.
Once the SOC drops below 95% it goes into full charge mode again. 
His explanation was that this is their algorithm. In essence, my opinion, the so called float charge voltage is meaningless in the 5000ES as it never float charges to keep the battery at 100% SOC once fully charged.
Here you can see, once the battery is fully charged it starts dropping 1% SOC every hour and 20mins or so, until it hits below 95% and then applies bulk charge back to 100%.
Who's scientific explanation is correct would depend on which one we wish to believe I suppose. 

Strange because my two growatt inverter do float once the battery is fully charged. see dashboard

My question is is it OK to float LI batteries? 

2 hours ago, Antonio de Sa said:

I've been doing some research regarding LI batteries, some say it's no good to float LI batteries, what are your opinion regarding this subject.

 

I think the issue is more "at what voltage do you keep the batteries once charging is complete" than "is it OK to float LI batteries" - floating implies that it is a a lower voltage than the initial (bulk/absorb) charge.  With lead acid that is necessary - with LFP it is not.

One thing that everybody seem to agree on is that it is bad to store LFP cells at high voltages and high temperature.  One of the worst things that you can do is charge your LFP batteries very quickly and then leave them hot and full for the rest of the day  Fast charging produces far more heat in the cells than slow charging does - the security of having you batteries full by 11am comes at quite a cost....

So, I think the answer is that you should keep them at the lowest voltage that keeps them at a SOC that is acceptable to you.  My Pylons (US3000B on firmware 2.8) need to be charged to about 53V in order to reset the SOC counter to 100%, but I do not keep them there - after 60 seconds at 100% I set the charge voltage down to 50.5V.  That causes the batteries to quickly discharge to about 99.5% which is where they stay whilst there is sufficient sun.  Once a week I give them an additional 10 minutes at 100% for more complete balancing.  (theses voltages are for Pylons which have 15 cells)

 

Edited February 7, 20223 yr by Calvin

There has been allot of arguments back and forth about "floating" LiFePO4 batteries on this forum, the facts are as follows: You need to charge your batteries to a high voltage (as per the cell configuration) to get the pack to balance, then as soon as the balancing is within acceptable levels, the charge current to the battery needs to stop - now some do that by dropping the "charge" voltage so that the batteries are not forced to absorb or dissipate energy as heat anymore, but a properly designed system will be instructed to cut charging to the battery by the BMS. I have learned the hard way that if you keep your lithium batteries at a high cell voltage, they will not last. 

Lithium batteries are pretty close to behaving like an ideal battery.  They'll take charge until they reach a given state of charge at a given voltage and then it will simply stop taking current.  If you have a bench power supply you can easily see this by setting your lab power supply to say 2.4v.  Once you reach 2.4v the lab supply will be in the mA range (self discharge).  So float just means charging them to a voltage at which you have a good compromise over life expectancy and the amount of energy you are storing.  Higher voltage = lower life.  But if you remove the voltage from the battery completely it'll self discharge over time.  If you cycle your battery often that isn't a problem.  But if only use it say once a year, it'll lose a fair amount of capacity by the time you need it.

Of course if you don't "float" the battery it'll last longer because you aren't keeping it at a higher voltage and thus it'll age slower.  But you can't "overcharge" a Lithium battery by keeping it at a certain voltage. (assuming that voltage is below 3.65v per cell).  It simply stops accepting current, the end.

Lead acid terminology confuses people because they don't behave anything like an ideal battery.  You need a "bulk" stage with Lead Acid to get the battery to 100% SoC (Lead Acid must be kept at 100% or its life is reduced).  The reason the bulk stage has higher voltage is to overcome the significant resistance of a lead acid battery and it also has a serious inefficiency problem (This is linked to Puekart's law).  But if you keep the lead acid at a higher voltage, due to its inefficiency it'll start boiling the electrolyte away.  So you drop the voltage down to a point where it will prevent the battery from discharging but not boil the electrolyte.  Notice that with a Lead Acid, unlike Lithium, at no point will the current stop and drop to nearly 0.  It'll just keep wasting energy as heat (due to inefficiency).

So in both chemistries you increase the voltage in a "bulk" stage to equalise and get them "100%".  But the reasons for dropping down the voltage is very different.  You would kill a Lead Acid very quickly if you kept it at "bulk" voltage.  But on your Lithium cell you could run that way for years. 

On 2022/02/07 at 3:10 PM, Johan-Henry said:

You need to charge your batteries to a high voltage (as per the cell configuration) to get the pack to balance, then as soon as the balancing is within acceptable levels, the charge current to the battery needs to stop

You don't *need* to but it is a good idea if you want to get them all to a similar state of charge.  You also don't need to lower the voltage if you don't want to.  The charge current will drop to very nearly 0.  The only reason you want to drop the voltage slightly is, higher voltage = shorter life.  So once you drop the voltage the battery will self discharge a bit so that each cell is roughly at the same state of charge.

Edited February 8, 20223 yr by Gnome

Also good video demonstrating what I posted above