Voltage dropping of LiFePo4

[ahmed abdulnoor]

Hi.. i have 8 S ,280AH LFP bank with Daly 200A smart BMS and now it under test (no loading yet ); i set the high & low cutoff volts to 3.45 ; 3 V respectively ; balance starts at 3.4 V ; SOC sets for 100% ; my system has a balancer too . I noticed that the voltage is dropped ( after high volt cut-off ) from 3.45 V to about 3.3xx V appr. and when activate the bms again it starts charging till one cell hits 3.45 V ( charging current from solar controller is about 12 V app. and keep constant till cut-off ) then switched as usual but after a while the voltage of cells dropped again to 3.3xx V appr. .. even after changing the setting by rising the high cutoff volt to 3.55 V i got almost the same results or with few changes . I read about " absorption " but not quite sure if it's the cause or not and and Daly BMS has no such setting for absorption criteria or so and don't no why volts dropped though no loads applied . any help or explain will be appreciated .

 

[Steve_S]

LFP always settles post charging, this is perfectly normal. Usually if properly charged, they should settle to the top of the Working Voltage Range (3.425Vpc).

Here is a Working Charge profile that give you the 90% capacity without stressing anything and generally avoid triggering HVD (even with Runner Cells).

Profile for Midnite Classic-200 other SCC very similar.
All equipment MUST BE Voltage Corrected & Calibrated (VERY IMPORTANT) see link in my signature on how to do it.
Divide Values X2 for 12V. Multiply X2 for 48V.
Absorb: 28.2 for 15 minutes (3.525vpc) (some call this boost)
Equalize: OFF
Float 27.9V (3.4875vpc)
MIn Volts: 22.0 (2.750vpc)
Max Volts: 28.7 (3.5875vpc)
Rebulk Voltage: 27.7 (3.4625vpc)
End Amps: 14A (*1)

(*1): End Amps is calculated from the Highest AH Battery Pack in a Bank. IE: 200AH X 0.05 = 10A 280AH X 0.05 = 14A.
NB: Victron Forum discussion says EndAmps = TailCurrent
This get's the bank charged to full with high amps (Constant Current) and then float (Constant Voltage) tops off so the cells are on average between 3.475-3.500. I am running 7/24/365 so float is used up by the Inverter + provides whatever the packs will take to top off.

** Coulumbic Efficiency for LFP is 99%

 

[ahmed abdulnoor]

Thanks a lot for replying but I already mentioned that i had a Daly bms ..not Victron .. Daly hasn't Absorption option or controlling the charging current etc..

 

[Steve_S]

A BMS knows absolutely NOTHING about what an SCC ( Solar Charge Controller ) does.
A BMS has only ONE Job, that is to protect the Cells within a Battery Pack. THAT'S IT !
A BMS does NOT control how much current goes to a Cell, it only watches for Hi Voltage (greater than 3.650 Max High Limit) or Low Voltage (Max Low Limit of 2.500V per Cell)

Generic Basic LFP BMS Settings:

Setting	settings	NOTES
Over charge P Voltage	3.65	P protection cutoff
Over charge R Voltage	3.55	R recovery voltage to disengage Hi Volt cutoff.
Over charge Current	-->	0.5C rate example: 100AH x 0.05=5A
Over Discharge P Voltage	2.65	CUTOFF Trigger
Over Discharge R Voltage	2.75	Release @ this V. Facilitates Voltage Sag Recovery from heavy loads
Over Discharge Current	-->	1.0C Rate 100AH cell @ 1C = 100A
High Temp cutoff	70C	Celsius
Diff of Batt Temp	15C
Diff of cell Voltage	200mv	Unmatched (BULK) cells drift, Allow up to 1mv per AH capacity
Temp Unit	C
Cut off Delay Time	10S	10 Seconds to prevent Surge Voltage Sag when Load surge demand occurs.
Temp Alarm	ON
Cell Empty Voltage	2.50
Cell Full Voltage	3.65	FOR LiFePO4
Battery Capacity AH	-->	Label Value of cells ie 100AH, or 280AH
Battery Power WH	-->	FORMULA (NominalVolts * #ofCells * RatedAH) (3.2*8*280=7168)
Low Temp cut off in Charge	2C
Low Temp cut off in Discharge	-10C

 

[HRTKD]

Did you top balance the entire set of eight cells following the guide in the Resources section of the forum?

 

[MisterSandals]

I noticed that the voltage is dropped ( after high volt cut-off ) from 3.45 V to about 3.3xx V appr. and when activate the bms again it starts charging till one cell hits 3.45 V

You are using the BMS to control charging. The BMS is a safety cutoff device, NOT a charge controller.
Your BMS settings should be set near the safety limits of your battery (2.5V - 3.65V or maybe a little tighter).

Your SCC needs to control the charging. I like the 3V to 3.45V range, it is nicely conservative and pretty similar to where all my systems charge.

 

[John Frum]

This is a FAQ.
I wonder if there is some bad doco out there that is giving folks the bum steer.

 

[ahmed abdulnoor]

This is a FAQ.
I wonder if there is some bad doco out there that is giving folks the bum steer.

Sorry .. it's not .. !!

 

[John Frum]

Sorry .. it's not .. !!

Its not an FAQ?

 

[ahmed abdulnoor]

You are using the BMS to control charging. The BMS is a safety cutoff device, NOT a charge controller.
Your BMS settings should be set near the safety limits of your battery (2.5V - 3.65V or maybe a little tighter).

Your SCC needs to control the charging. I like the 3V to 3.45V range, it is nicely conservative and pretty similar to where all my systems charge.

My SSC have a pre-set cut off volts that can't be changed individually . i already used the safe limits (3-3.45 ) that prolong the pack life .

 

[MisterSandals]

My SSC have a pre-set cut off volts that can't be changed individually

Which SCC do you have?

If you continue to abuse your BMS, you should probably have a spare on hand if this is a critical system.

 

[ahmed abdulnoor]

It's a Chinese made low freq. solar inverter .. i think it stop charging for my lead acid pack now @ 28.2 V . didn't try it yet for my under test LFP . I had another solar controller but i can adjust it's parameters & i like connect it too so that both of them charging the same pack ( as i did now with LA pack for more than 2 yrs. ) .