LiFePO4 charging observation/question - Page 1
Forum section: electronics / general

author: valley001
author_rank: Frequent Contributor
timestamp: March 06, 2021, 06:06:55 pm
content: Hello all,I am top balancing a bank of lithium batteries, 200ah x8 in parallel with an HP 6033a at 3.4v 30amps and monitoring with my HP3456a. Initially I am going to 3.4 volts as I evaluate my setup and how the batteries behave, monitor temps everywhere etc. I have the sense connections hooked up.The batteries draw the expected full 30 amps, and initially the voltage rose quickly (from 3.29 to 3.1ish) and then began to tapered off. Checking on them after 5 hours I notice the voltage goingdown10 microvolts every 10-15 seconds or so at the 3.34v range. I do not understand this phenomenon I am hoping someone can explain it to me. I assume as the batteries fill I will start to see the voltage rise again.

author: Siwastaja
author_rank: Super Contributor
timestamp: March 06, 2021, 06:28:27 pm
content: If I understood correctly, the cells are connected to the power supply, the supply is on, and works in CV mode?Then, this small voltage drop would have nothing to do with the cells, but is an instability of the supply; for example due to it being cooling down after operating with higher current.

author: valley001
author_rank: Frequent Contributor
timestamp: March 06, 2021, 08:16:46 pm
content: The PS will switch automatically from CC to CV but it is in CC mode, supplying 30 amps. The voltage is set to 3.4 volts and cell voltage is 3.32ish volts drawing 30 amps. I am monitoring the parallel cells voltage with the 3456A.

author: Siwastaja
author_rank: Super Contributor
timestamp: March 07, 2021, 01:42:22 pm
content: OK, so let me reiterate,Charging at CC=30A, at approx. 3.34V you see 10µV drop each 10..15 seconds.The question is, how long does this happen, i.e., how far down the voltage goes?Since you are measuring the voltage with such accuracy, there could be some minor chemical reaction causing such small drop. I can't find that in any LFP charging curve I try to look at online, though. But some have very flat sections.Another possibility is that your supply current 30A is not perfectly stable and changes at that point, for example due to the supply heating up. If the supply current goes down, the cell terminal voltage goes down as well (but the equivalent open-circuit voltage does not). If you want to verify the phenomenon, you should add a very accurate current measurement. Its accuracy should match the high accuracy of your voltage monitoring, so it won't be cheap.

author: valley001
author_rank: Frequent Contributor
timestamp: March 07, 2021, 06:24:46 pm
content: Monitoring this now (I don't run it unattended, even with the 6033a recapped etc). Same parameters as before CC 30A target voltage 3.40v.Roughly 45min intervals:3.345793.345123.344273.343653.343563.34413I did find this paper: "DC internal resistance during charge: Analysis and study on LiFePO 4 batteries." This seems to show that internal resistance can change slightly during the charge cycle. I suppose this would account for the slight voltage change no?Quote from: Siwastaja on March 07, 2021, 01:42:22 pm

author: Siwastaja
author_rank: Super Contributor
timestamp: March 07, 2021, 06:46:45 pm
content: During 1.5hours, 45Ah went in. This is just 2.8% SoC change in your 1600Ah pack. So SoC is almost constant, so will be OCV. This means, voltage differences you see result in from differences in either current, internal resistance, or both:dV = R * dI if R is constant,ordV = dR * I, if I is constant.Note, the R of cell is dependent on the cell temperature. Lower temperature increases R.I of power supply can drift as well, also as a function of the supply. The specsheet said 0.3% accuracy if I recall correctly (looking it this morning).It's likely there's nothing special going on. Keep charging.

author: valley001
author_rank: Frequent Contributor
timestamp: March 07, 2021, 07:15:07 pm
content: Excellent. Thank you for your insight.Side note: The 6033a is proving excellent in this application. Chugging along @30 amps and cool as a cucumber.Edit: It appears after 4 hours of charging the voltage is now rising slowly.

author: Pinpi
author_rank: Newbie
timestamp: March 09, 2021, 05:01:26 pm
content: I second the internal resistance as a possibility.As the batteries get closer to their maximum charge the internal resistance gets higher from what I understand. Further the temperature rises but in your case probably only slightly. Did you measure the temperature increase of the battery too?

author: valley001
author_rank: Frequent Contributor
timestamp: March 09, 2021, 07:04:02 pm
content: Quote from: Pinpi on March 09, 2021, 05:01:26 pmI did, and it only rose 2-3 degrees Fahrenheit above ambient.I finished charging them to 3.6 volts, current tapered off to 7-8 amps. The spec sheet says to go to 3.65 and stop charging when current is .05c which would be 10 amps, I guess I am close enough. Here is the graph of my voltage measurements.

author: Siwastaja
author_rank: Super Contributor
timestamp: March 10, 2021, 07:14:14 am
content: Quote from: valley001 on March 09, 2021, 07:04:02 pmI understood your pack was 1600Ah (8 x 200Ah in parallel). 0.05C would be 80A, no? I.e., 10Aper cellassuming current shares equally which it usually does.If so, you have very very slightly overcharged the cells by using lower cutoff current at specified cutoff voltage but this doesn't matter, just sayin'.

author: valley001
author_rank: Frequent Contributor
timestamp: March 10, 2021, 03:04:38 pm
content: Quote from: Siwastaja on March 10, 2021, 07:14:14 amWow, this is objectively correct and an oversight on my part. Indeed I should have cut off charge when current reached 80 amps. My power supply max is 30 amps so I could not have seen the current taper at 80 with them wired in parallel. The spec calls for 3.65 and current drop to .05c, I charged to 3.60 and stopped at .005c hopefully this did not cause much damage to the cells. They did not stay on long after reaching 3.6 volts.Side note: There seems to be a lot of popular advice out on the web to (for the purposes of top balancing) wire these big cells in parallel, charge with a cheap amazon 10 amp supply "at 3.65 until current goes to zero." This seems to be in direct contradiction to manufacturers specifications. I wonder why this advice is so prevalent and remains largely unchallenged? Perhaps I am missing something?

author: Siwastaja
author_rank: Super Contributor
timestamp: March 10, 2021, 03:13:43 pm
content: "Until current goes to zero" is just some random babbling, it obviously never goes to zero because it's an asymptotic decay finally reaching the self-discharge current which is significantly higher-than-normal with the overcharged cell.Even if you wait until it goes to zero within the resolution of a cheap, say 3-digit meter, it would still take days or weeks, no one will wait for that long (hopefully). And by saying hopefully, I'm not talking about cell damage as much as the wasted time when you could do something more fruitful than spending days trying to push a battery at 101.0% into 101.1%.The amount of overcharge by using 0.005C cutoff instead of 0.05C is not much. If you do this 1000 times, it may wear the cells a bit more than doing 1000 charges by-the-spec. It's hard to say if the extra wear is more from the state-of-charge itself, or the time spending at holding at higher voltage.LFP differs from most other li-ion chemistries in the fact that the charge voltage specification is actually higher than the respective open-circuit voltage, by more than just R_dc * I_termination. For example, a 2Ah 50mOhm LCO cell would have ending voltage 4.20V and ending current 100mA, and resulting OCV would be logically 4.20V - 100mA*50mOhm = 4.195V. This matches with real-world observations.But with LFP, the full-cell OCV is somewhere around 3.45V IIRC but you don't CV charge to 3.45V (well youcould!), instead you charge to 3.60V or 3.65V because the cell can take the extra voltage for quite some time. But this "surface charge" diminishes in a few hours in simple leakage and the OCV drops by far more than predicted from the stopping current times cell resistance. Also for this reason, CV phase isn't so important with LFP in my experience. For example, with a typical LCO or NCA cell, when you have tapered down to say C/5 at 4.20V, you still have some 5% to go. With LFP, when tapered at C/5 at 3.60V, I'd say you are already at 99%. In an EV conversion I just turned the charger off when ending voltage was reached, saving the hassle of controlling charge current.The chemistry-level reason for this is that the LCO or NCA cathode is never fully depleted of lithium ions when charged. So what we call 100% isn't really the physical 100% limit, you can charge the cell way more, it's just unsafe to do so. LFP cathode by contrast, if I am right, completely depletes of lithium ions so fully charged really is fully charged, and extra voltage won't put more charge into the cell. Then any extra voltage is only about whether it causes electrolyte decomposition or other ill effects. If not, higher voltage can be used. I remember some Winston Chung's LFP "products" having rating of 4.20V for no good reason, and people using 3.60V instead. But apparently the "products" survived even the 4.20V charging.You are correct you can't reach the datasheet stopping conditions with just a 30A supply. You could have used slightly lower stopping voltage, like 3.57V, instead. But this isn't important. Just don't float for weeks or months at 3.60V.

author: valley001
author_rank: Frequent Contributor
timestamp: March 10, 2021, 03:30:20 pm
content: There is a LOT of babbling about it (the current to zero or near zero) on other sites, I wonder how many people are float charging their LiFePO4 cells to 3.65 days on end?I am thankful on this site there is a lot more signal compared to noise.

author: mbartosch
author_rank: Newbie
timestamp: March 24, 2021, 08:10:36 am
content: Quote from: valley001 on March 10, 2021, 03:30:20 pmI have a 12V/400 Ah LiFeYPO4 pack (4P4S 100 Ah Winston) as a boat house bank well as two 12V/40 Ah packs as portable battery packs for powering a dinghy motor. The following is my personal summary which I extracted from reading a 7000+ posts thread over at Cruisers Forum and own research and practical use over 5 years with my own banks.In my experience it is totally sufficient to charge my 4S packs to 13.8 V or 13.9 V pack voltage (3.475 V to 3.5 V cell voltage) while still getting a nominal 100 % of cell capacity. Not charging to 3.6 V will leave some (over)capacity unused, but should also somewhat benefit in terms of longevity.Float charging LiFePO4 should be avoided completely in my opinion. Any current flowing into the cell, no matter at which cell voltage, contributes to the SoC of the cell. It is possible to overcharge a LiFePO4 cell with any voltage above about 3.4 V, it just takes a long time. Overcharging damages the cell, reducing capacity.In my opinion the best charging regime for low C charging applications is to charge to a set voltage and then just stop charging. The lower the charge current with respect to C the lower this cutoff voltage should be set, e. g. when charging with 0.5 C personally I will charge to 3.5 V (set point for my alternator), whereas at 0.05 C I limit it to 3.45 V (set point for my solar panel MPPT charger).