LFP full charge @ 3.4V/cell

[robby]

I think a lot of this is lost in translation.
I posted a graph above showing how I am charging 32Kwh of 51.2V LFP batteries with 3.4V per cell and they are fully charged by 2:30pm even at this time of the year.
Only difference is that my bulk charging continues until a bit over 80% capacity.

 

[HighDesertOffgrid]

I think a lot of this is lost in translation.
I posted a graph above showing how I am charging 32Kwh of 51.2V LFP batteries with 3.4V per cell and they are fully charged by 2:30pm even at this time of the year.
Only difference is that my bulk charging continues until a bit over 80% capacity.

Do you have balancing enabled at that voltage via bms?

 

[robby]

Do you have balancing enabled at that voltage via bms?

Yes.

 

[McKravitts]

Andy from the Off-grid Garage did a similar test and came to the same conclusion: 3.4V gets you the vast majority of capacity, it just takes longer in the absorption phase to get there.
YouTube video

Charging at lower voltage also extends cycle life of the batteries.

What Causes Lithium-ion to Age?​

 

[HighDesertOffgrid]

Yes.

That would be my concern with a battery's internal bms. I had 6 12v batteries in 2s 3p charging to 27.6v. I suspect I was getting cell drift after 700 +/- cycles after noticing some loss in capacity. Bumped charging up to 28.4v and it seems to have corrected. I suspect bms wasn't reaching balancing voltage.

 

[McKravitts]

That would be my concern with a battery's internal bms. I had 6 12v batteries in 2s 3p charging to 27.6v. I suspect I was getting cell drift after 700 +/- cycles after noticing some loss in capacity. Bumped charging up to 28.4v and it seems to have corrected. I suspect bms wasn't reaching balancing voltage.

Some less expensive BMS's don't even try balancing until the float voltage is reached. If you don't keep a float voltage on them they never balance.
Although the bms specs of any battery are a very important factor in battery charging and maintenance, very few manufacturers publish them.

 

[LakeHouse]

I think 5 hours at an average of 3.40V/cell is probably giving more time to balance than a rapid charge to 3.55V.

More time, certainly. But I'm not sure I like the idea of balancing at 3.4V though, it seems a little too low.
I detail in a post here that for my cells, balancing at 3.4V was causing imbalance. Since switching to 3.45V to start balancing, the active balancer in my JK BMS spends far less time doing anything because it isn't both creating and then solving a balance problem.
But that's just an anecdote, not data. YMMV.

 

[AshleyL]

Some less expensive BMS's don't even try balancing until the float voltage is reached. If you don't keep a float voltage on them they never balance.
Although the bms specs of any battery are a very important factor in battery charging and maintenance, very few manufacturers publish them.

Sometime it is not the BMS fault, but rather the inverter itself.

For instant, despite using expensive JKBMS with inverter communication with Growatt, the moment the BMS reports 100% SOC to the Growatt inverter, Growatt with its infinite wisdom will stop charging and disregard JKBMS request to float charge.

Growatt with its "infinite wisdom" decided to stop charging when its AIO inverter received SOC 100% notification from BMS and only start to recharge when the SOC drops to 95%.

The funny part is, the inverter is drawing its idle power from the battery and not solar during these 100% - 95%.

In this 5% stage, the inverter will initiate solar production "zero-export" between the battery and load. Yes, you read it right. Growatt "unlimited wisdom" decides to perform "zero export" to the battery for that 5% range while the inverter still draining 50-70watt from the battery. If the load is more than solar production, then it will drain the battery power. Once the load is lower than solar production, back to "zero export" again.

 

[sunshine_eggo]

Sometime it is not the BMS fault, but rather the inverter itself.

Nope. Still the BMS. That's how comms works. It surrenders control of the system to the BMS.

 

[AshleyL]

Nope. Still the BMS. That's how comms works. It surrenders control of the system to the BMS.

Unfortunately, it is really Growatt issue. The inverter decided to take over the control of the charging after SOC 100% instead.
Have 5000ES and 6000ES, both react the same. 100% - 95%. With the latest firmware too.

Sent email to Growatt asking about it.....after few weeks, I got a reply and they said it is as designed. The purpose is "The algorithm will protect the charging relay from damage faster due to use" <-----Chinese-English translation obviously.

I watched Andy from off-grid-garage on the JKBMS "controlled charge mode". Amazingly, even after the JKBMS sent SOC 100% to his Victron system, his Victron continues to charge the battery as per his "RCV Time" and "RFV time". My Growatt ---> instantly stop charging with no current flowing after SOC 100%. No float.......nothing. Charging simply stopped and the battery voltage settles down to 53.3V.

I am not certain if this Growatt charging mechanism/algorithm is same with other Axpert OEM inverters because no one replies when I asked about it on few threads.

 

[RCinFLA]

Be aware many chargers and power supplies have a soft transition between constant current mode and constant voltage mode. They don't hold their constant current setting as voltage cap limit setting is approached.

Constant current may start to drop half a volt before absorb or float setting voltage is reached.

LFP cell will have 10-25 mV of overpotential voltage at very low cell current. In order to reach full charge at 3.40v per cell you have to have current taper down to very low level to reduce cell overpotential voltage to low level allowing cell to reach 3.40v.

The required minimal overpotential at low absorb voltage setting with a charger/power supply transition between constant current and constant voltage can result in a long time extension for achieving full charging. The results are highly variable depending on particular charger/power supply transition characteristics between constant current and constant voltage regulation.

 

[McKravitts]

Sometime it is not the BMS fault, but rather the inverter itself.

For instant, despite using expensive JKBMS with inverter communication with Growatt, the moment the BMS reports 100% SOC to the Growatt inverter, Growatt with its infinite wisdom will stop charging and disregard JKBMS request to float charge.

Growatt with its "infinite wisdom" decided to stop charging when its AIO inverter received SOC 100% notification from BMS and only start to recharge when the SOC drops to 95%.

How do you know that the BMS is reporting 100 % SOC to the growwatt?
And how do you know that the JKBMS is requesting a float voltage?

 

[AshleyL]

How do you know that the BMS is reporting 100 % SOC to the growwatt?
And how do you know that the JKBMS is requesting a float voltage?

There are many methods to trigger 100%SOC. For example, one can set a very low SOC 100%Vol to trigger it. Or can set a very low capacity, eg, 10ah capacity for 200ah cell to trigger the 100% as soon as possible?. Or hijack the CAN signal and manually push a 100%SOC notification, thanks the canbus analyser tool and related software? Which one you prefer?

After "RCV Time" is up, JKBMS sent an instruction to change the float voltage to the one I set 3.35v. Main problem --> Growatt LI BMS communication mode, Growatt engineer "cleverly" set/force in the inverter BulkChargeVolt = FloatChargeVolt. Thereby ignoring the float voltage signal from JKBMS........JKBMS never sent signal for the inverter to stop charging.

Edit: I never see JKBMS send "Bit 7 Charge enable 1: yes；0: request stop charge" to the inverter when 100% SOC is sent to the inverter. The inverter itself decided to stop charging based on the SOC 100%.

Let say if I set the JKBMS cell balance start at 3.45v, SOC100% 3.45V and Vol Cell RCV to 3.451V.
The moment if any cell runner that I wish to balance hit 3.45v, JKBMS will send 100% signal and inverter stopped charging and ignoring the "RCV Time" and "RFV time". Remember, there is no floating in Growatt LI BMS communication. Thus, JKBMS Controlled Float Mode is useless with Growatt inverter. I will never be able to balance all cells.