I few notes considering charge processes.
The higher the amperage during charging, the higher likelihood of cell deviations relative to each cells IR.
The higher the Discharge Amperage again will cause cell deviations during the Voltage Sag. Again relative to cell IR.
TYPICALLY, cell deviations will occur @ the Working Voltage Limits (High or Low) where they become more pronounced. LFP standard default working voltage is 3.000-3.400 with Nominal Voltage being 3.200 (this is the accepted industry metric)
-- NB -- Grades, A+, A & A- will provide full "Working Voltage Range" and these cells will A-Typically test out with Gross Capacities exceeding "Label Rating". Example 280AH cells A+ > A- will test no lower than 282Ah but usually closer to 290AH.
-- Grades B and below to Bulk Commodity will have varying IR's and as cells are almost always NOT Properly Matched or batched (very hard to do with commodity cells) they may provide the Full Working Voltage but typically test out at Lower Capacity than Labelled Value. Example: 280AH cells testing between 270-280AH gross capacity.
? Gross Capacity ?
This is calculated from 2.500-3.650 Volts per cell which is the Full Voltage Range but not the "Working Voltage Range" often referred to as "the flat curve" which is 3.000-3.400. "A+, A & A-" test out above rated capacity by a few Percent and will provide a functional working voltage from 2.875-3.475 "usually.
NOTE: This can ONLY be determined using a proper Matching & Batching Test process. This process cycles cells from 100% SOC to 0% SOC and back under different Amperage Rates with sample readings taken across the cycle. The point is to MATCH the Cell IR at different voltages so the cells within a pack once assembled act identically. MOST VENDORS DO NOT DO THIS ! They Voltage & IR Match at Static Storage Voltage and provide no Cycle Reports with Cell Serial # & QR Code. IT IS AN EXPENSIVE PROCESS and always results in cells failing grade and becoming Bulk Commodity cells for cheap.
Solutions:
As previously noted, Top Balancing and fully saturating cells is very important. Full Saturation means charging to 3.650 and waiting till amps taken for charge reaches <2A.
A Surface charge would go to 3.650V and stop regardless of amps taken, not "topped off"
-!- All LFP settle post charge, this is perfectly normal, It is not uncommon for cells to settle from 3.650 100% to 3.500 +/- a bit within 60 Minutes of charge stopping. Again this relates to Working Voltage Curve.
- If possible test the IR of your cells at Lower Voltage (below 2.800) and again at higher voltage starting at 3.450. Then place your two Strongest Cells as you B1 & B4 with the two weakest as B2 & B3. B1 & B4 (in a 4S config) are the ones who take the hit first or who pushes first, the guys in the middle just struggle with the ends.
(I know that doesn't sound good, but it is how it works)
-- Tip IN CASE: Sometimes cells have been stored for a while (New Old Stock), sometimes these will be "sluggish" charging/discharging at first, this can create issues. IF IN DOUBT, one way to make sure they are Fully Active Internally is to cycle them from 10% to 90% and back down up to 3 Times with "Medium Charge/Discharge Rate" IE: 0.5C Charge Rate Max and 0.25C discharge rate
Couple of things that can really muff up BMS Readings and Cell activities... Even a BURR or Ridge on a Bussbar can toss readings off
(people are always surprised by that, but do realize, we are dealing with Millivolt & Milliamp Sensitivity). When cells are manufactured & packed, they often have machine oil on the terminals (from taping screw holes) and waxes as well (stop corrosion on Aluminium Surfaces) which need to be wiped off, acetone & rubbing alcohol are your friends. Oxguard / NoAlox is suggested (in very very tiny amounts) if using busbars which
are not Tinned Copper or Aluminium to prevent any corrosion.
Balancers can help level up cells in tired packs or slightly mismatched packs BUT they cannot fix/correct defects or cell deficiencies. I myself use QNBBM Active Balancers on my 5 Production Packs and Heltec Capacitive Balancers on my Utility packs. Links Below for your interest. I use the Active Balancers in concert with the Passive Balancing afforded by the Chargery BMS' on my Production Packs.
Type: Lead-Acid Batteries Usage: Car, Bus, UPS, Electric Power, Lighting, Electric Bicycle, Boat Nominal Voltage: 12V Discharge Rate: Medium Discharge Rate Shape: Flat Battery Electrolyte: Acid
deligreen.en.made-in-china.com
This product is no longer available.
cncdheltec.en.alibaba.com
Bottom Line: These cells will work fine but you need to top balance them - Placing the Strongest cells as B1 & B4 and by possibly using an Active Balancer suitable for the pack. BTW: These is a fair bit of misinformation on this tech, it is not helped by the fact that there are several different types and their behaviours & what they do is different. Also not ALL are compatible with various chemistries, also some will not work in conjunction with a BMS.
Below is my SCC Profile (I run 24V but that's ok) I have Bulk Commodity cells in packs (2x 280AH), as well as Grade A+(1x 280AH) and two packs of used EV LFP (the 175's) in my primary bank... So a good cross section of variations and was tricky to work out... But with the hard Thrash Testing (seriously abusive "to the edge tests") got it sorted nicely.
Settings for \Midnite Classic-200 Charge Controller settings
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%
Hope it helps & Good Luck. Link #2 in my signature is an LFP Battery Assembly Guide which has some other info which may be of help to you, please download that PDF from the resources. Maybe the Voltage Chart in my Sig as well (better one in the LFP Guide though).