Float Lifepo4?

[solarnoob22]

Search isnt working right now and ive a MPP LV2424 with the bulk charging at 29.2 and the float set at 27.2, am I leaving any capacity on the table at 27.2? As soon as I turn the batteries on the voltage drops to 26.8, etc etc

 

[BentleyJ]

am I leaving any capacity on the table at 27.2? As soon as I turn the batteries on the voltage drops to 26.8, etc etc

26.8V corresponds to 3.35V per cell. This is the normal resting voltage after charging has stopped, or a load is placed on the batteries after a full charge. 27.2V is 3.4V per cell and is considered the proper float voltage.

As long as the Bulk or Absorption phase of the charging cycle is at least 27.6V and it lasts long enough to enter the low amperage "tail current" portion of the curve, you are not missing much capacity. Perhaps a couple of percentage points at most.

 

[MisterSandals]

bulk charging at 29.2 and the float set at 27.2

Is charging terminated by the SCC with all cells 3.65V or is there an over volt condition?

I'd be WILDLY surprised if SCC terminates with 8 perfectly charged cells.

So i will assume the BMS terminates charging (bad). What is the state of charge when this happens? Is 1 cell at 3.67V and all the rest at some significantly lower voltage?

If you can provide the 8 cell voltages at charge termination, there is a lot of information in that.

 

[Steve_S]

Here is a moderate profile which keeps LFP within the Working Voltage Ranges without stressing any cells. This also generally avoids having over volt disconnect issues & such which many encounter. Continually overdriving the cells above the Working Range is NOT doing you any favours.

Bulk / Absorb: 27.6V (Absorb for 45 minutes (3.45vpc) (some call this boost)
Equalize: OFF
Float 27.5V (3.437vpc)
MIn Volts: 21.2V (2.650vpc)
Max Volts: 28.6V (3.575vpc)
Rebulk Voltage: 25.6V (3.200vpc)
End Amps/TailCurrent: *1 (Triggers change from Absorb to Float)
(*1): End Amps is calculated: (100AH X 0.05 = 5A or 280AH X 0.05 = 14A.
EndAmps = TailCurrent
Coulumbic Efficiency / Battery Status Meter Efficiency for LFP = 99%.
This gets the bank charged to full with high amps (Constant Current Constant Voltage) and then float (Constant Voltage, Variable Current) tops off so the cells are at 3.437Vpc. I
! Do Not forget to adjust for Voltage Offsets between Actual Voltage @ Battery Terminal & at Solar Controller.

BMS With Active Balancing should be set to start Active Balancing at 3.420V or possibly lower to 3.410 so that the cells can balance out at the top and not affect the charging process.

Very Special NOTE: Floating & Saturating to 3.437vpc, accounts for the Voltage Settling post Charge of any kind which actually brings the cells to just below 3.400Vpc.

 

[hwy17]

Float 27.5V (3.437vpc)

Team 3.437!

 

[OM617YOTA]

I run the recommended voltages for 5000+ cycles in the DIY battery sticky.

Recommended Charge Profile for DIY LiFePO4 Batteries *Sticky Post*

12V 4S LiFePO4 Battery w/ BMS: Absorption: 14.5V Float: 13.6V Inverter Cut-off: 10.7V-12V (depending on size of load and voltage drop etc) 24V 8S LiFePO4 Battery w/ BMS Absorption: 29V Float: 27.2V Inverter Cut-off: 21.4V-24V 48V 16S LiFePO4 Battery w/ BMS Absorption: 58V Float: 54.4V...

diysolarforum.com

28.2 absorption, 27.2 float, and pull full capacity. It does take a bit to get the last few watts in at the top, but no troubles.

Edit: Actually I lied, I run 27.3 float because my Smartshunt is programmed to reset it's tracker to 100% SOC at 27.2, and 27.2 at the AIO made the shunt see 27.199 and never reset.

 

[Steve_S]

28.2V = 3.525 Volts per cell in 8S. That is 1.25V over Working voltage and does NOTHING other than stress the cells. 3.450 is pretty much max for long-life healthy cells. But do whatever you want with YOUR batteries, it's your wallet, your problem/troubleshooting chasing magic ghostly monkeys, and all the risks / consequences are yours alone.

 

[OM617YOTA]

28.2V = 3.525 Volts per cell in 8S. That is 1.25V over Working voltage and does NOTHING other than stress the cells. 3.450 is pretty much max for long-life healthy cells. But do whatever you want with YOUR batteries, it's your wallet, your problem/troubleshooting chasing magic ghostly monkeys, and all the risks / consequences are yours alone.

You mean 0.125v over? I'm fine trying 27.6 absorption and seeing what happens.

Despite the 28.2 setting, I've never seen 28.2 at the shunt, which means the batteries aren't either. Highest I see is ~27.29 ish.

I don't think it's a big deal, I'm quite comfy running what Victron and Will recommend. Full rated life is achieved cycling the cells between 2.5 and 3.65, and I never get anywhere close to those extremes. I expect to achieve more than acceptable cycle life.

 

[Steve_S]

Cells are factory tested to comply within the Working Voltage Range (3.000-3.400 V per cell). The allowable range that causes no harm is 2.500-3.650. From 2.900 down to 2.5 that decrease is FAST (the cliff fall)/ from 3.450 to 3.650 is the cliff climb and never stays because the Chemistry will force it to settle close to the top of the working range. "Runner Cells" usually have a different IR at the higher end and create the running case.

When you observe LFP cells charging through a BMS APP and when pushing over 3.450 to the cells, runner cells pop up and race to 3.650 and cause HVD (High Volt Disconnects) which instantly stop charging on the pack, until the cell voltage drops to within range, but the moment charge is reapplied, the runner races & stops charging. This is effectively handicapping the pack. It sounds exactly like what is happening to you and it's tripping the BMS and keeping it lower than it could go.

Good Luck

 

[OM617YOTA]

Cells are factory tested to comply within the Working Voltage Range (3.000-3.400 V per cell). The allowable range that causes no harm is 2.500-3.650. From 2.900 down to 2.5 that decrease is FAST (the cliff fall)/ from 3.450 to 3.650 is the cliff climb and never stays because the Chemistry will force it to settle close to the top of the working range. "Runner Cells" usually have a different IR at the higher end and create the running case.

When you observe LFP cells charging through a BMS APP and when pushing over 3.450 to the cells, runner cells pop up and race to 3.650 and cause HVD (High Volt Disconnects) which instantly stop charging on the pack, until the cell voltage drops to within range, but the moment charge is reapplied, the runner races & stops charging. This is effectively handicapping the pack. It sounds exactly like what is happening to you and it's tripping the BMS and keeping it lower than it could go.

Good Luck

Is this still directed at me?

I never hit high or low voltage disconnect, all cells are within 0.01v at the top and bottom SOC, and pull full capacity.

Regardless, I reset absorption to 27.6. Didn't make any difference to actual system voltage, which again, I've never seen go over 27.299 ish. Hopefully I can come back in 10 years and say thank you, my batteries are still at 85% capacity instead of being down to 65%.

 

[pollenface]

float @ 3.35v/cell is my preference in accordance to everything I've read on the net over the last few years.