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  • My simple LiFePo4 battery setup

    Over the years I've simplified things even further to enjoy my lfp prismatic cells. My intent is to KISS the information, and allow the reader to expand if they like from other threads. Ie, the bms / non-bms issue is covered extensively *in other threads*, as well as balance.

    This is a formula that works for *ME*. Maybe not for you. I'm just documenting what I'm really happy with.

    In OUR APPLICATION, that is a so-called "Sub-C" or low current environment, a DIY'er with a modicum of common-sense might be able to benefit.

    It goes like this:

    1) Charge each cell *individually* to 3.6v with up to a max of 0.5C, (I usually use about 0.2C) until .05C (C/100) end absorb current is reached. This can be accomplished with a variable power supply, or commercial "single cell" LFP chargers. If those single-cell chargers go up to 3.8v, then that's ok too, as GBS and Winston have no problem with that. This is [B]NOT FOR DAILY[/B] recharge, but for an [B]initial[/B] charge. This initial charge to full is critical to do before doing any cycling with the bank.

    NOTE: by "single cell lfp" charger, I mean the ones that are used for 3.2v nominal batteries, and NOT other chemistries that use 3.7v nominal! Newbies to lfp be aware of this distinction!)

    My cells are quality prismatic GBS, (Winston and CALB are similar) not used or abused beforehand, and seem to have very similar capacity and internal resistance values - although they are not lab-perfect! In addition, terminals were cleaned with non-metallic / non soapy scotchbrite, and a small amount of anti-oxidant, like No-Alox was applied. Do not use sandpaper, as this can embed other foreign materials into the aluminum.

    2) [B]For daily charging, I set my solar controller or ac charger to no more than 3.45v per cell.[/B] In this way, it takes a while to actually absorb to 100%, but I don't go there much! 3.45v leaves a bit of headroom for cell variances, which after a single-cell charge, is pretty tight anyway.

    3) I run PSOC - that is between the knees. I don't run under 3.1v for any cell at the bottom, nor do I do a total absorb to zero amps at the top - but if I DO, at 3.45v per cell, again that takes a while and is very conservative. If the cells actually DO absorb down to near zero current, the sun goes away pretty soon anyway, so no real harm is done.

    Essentially, I do not even run as high as 3.5v per cell. I could, but I just don't NEED to, since my bank is sized appropriately to allow me to stay in psoc nearly all the time.

    I've also found that with my conservative sub-c application, I can do two-terminal (ie, "pack level") charging and low-voltage disconnect protection.

    Am I taking a chance? Sure - much like taking the chance that one of the cells inside an agm that I can't reach might die. For me, If I have to go much beyond just normal maintenance as one might do with a flooded cell and a hydrometer once in awhile, then LFP is not for me.

    If I feel the need to rebalance, I just charge [I]each cell individually to full[/I] again with the single-cell charger, or use my variable lab supply. To tell the truth, the only reason I have done it more than twice in a few months was to play around.

    However, using the single-cell charger or variable bench supply is a great way to baseline your SOH, or state of health. Wait 12 hours, and note the resting OCV voltage. Do this at regular intervals say at least once a year to keep an eye on your baseline.

    The OCV should rest after 12 hours between 3.4 and 3.6v. Below 3.4v, and your cells are getting tired, or were not allowed to absorb enough. Higher than 3.5v - 3.6v, and you are overcharging! Discharge immediately.

    Surprisingly, this conservative KISS method seems to be working for a low-current application. You EV'ers, RC modelers and the like will want to use other tactics.

    Wait, what? No bleeder / vampire boards or a rat's nest of wires all over it? No thanks - for ME that is. Perhaps not for you either.

    YMMV.

  • #2
    Thanks for the write up. What is the voltage & Ah of your pack? What size solar ? What charge controller do you use ? Do you use thermal comp ?
    Powerfab top of pole PV mount (2) | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
    || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
    || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

    solar: http://tinyurl.com/LMR-Solar
    gen: http://tinyurl.com/LMR-Lister

    Comment


    • #3
      Currently the system is a 4S / 12v nominal / 40ah bank of GBS cells. Solar varies from about 80 to 250 watts - depending on how fast I want to charge it. The controller varies too, but usually a self-modified Morningstar Prostar 15 pwm (removing the temp sensor to disable temp-comp). Or a Xantrex C12 for smaller stuff - also with temp comp disabled, and custom voltages set.

      The charge has been levied against me before that unless I invest in a 4kw / 48v bank, there is nothing to learn.

      Frankly, I'm prepared to go 10 times this size after a LOT of hands-on, and higher nominal voltages too if I need to. But that's me, not for the guy next door. Sunking, Doug Ingraham, WB9K, T1 Terry, MaineSail, Burgerman and numerous others got me up to speed fast.

      The great thing about starting with a small prismatic learner bank is that you can test all the various disciplines, methadologies, and so forth in a relatively short amount of time with commonly available gear, and choose what is right for *you* and your routine. You can also vet what what is read in the forums to see who is doing armchair engineering, and those who are actually involved.

      Comment


      • #4
        KISS method for determining the SOH, or State Of Health:

        This has nothing to do with balance, although the procedure of using a single-cell charger, or variable power supply for your very first charge to full helps!

        Baseline your SOH performance when new:

        Do the initial charge to 3.65v dropping to .05C absorb and stopping there. Or use a single-cell charger of at least .2C current capability. Let cells rest for at LEAST 12 hours. Measure.

        Note: none of this is valid unless you trust your meter! Don't spend $$ on LFP, and cheap-out on your meter!!

        You should see about 3.38 - 3.41v OCV. If you are much lower than that, then you have a problem, poor quality cells to begin with or horrid cell interconnect resistance wiring.

        If you charge up to 3.8v, and absorb down to about .05C, rest for 12 hours, you should see about 3.45-3.5v OCV.

        Under NO circumstance should you EVER be above 3.6v rested ocv!! That is total overcharge - discharge immediately.

        You may want to record these resting OCV values, and perhaps do the initialization charge regularly, like yearly or so to keep track of the measurements.

        Tip: When reading forums or other specs, be sure to note whether one is talking about "cell voltage", which is what it is while rested, and "terminal voltage", which is what is measured during charge. The values during charge are not always the same for every person, since battery capacity, internal resistance, charge current and other things like speed of intercolation and diffusion are at play.

        Yeah, so the KISS method of measuring a resting cell-voltage after a full charge is a good way to keep track of the SOH if you don't have a lot of other lab gear and can cut down the variables that tend to make one's head spin.

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        • #5
          Well al I can say is you had better know exactly what you are doing with a lot of experience to run Lithium batteries without the proper equipment. Otherwise use the expensive BMS and special equipment made for lithium or else you have a fire or destroyed battery in your near future. PN I know you have gained the knowledge and experience. But this is a DIY site, not professional.
          MSEE, PE

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          • #6
            Well, that's why it is a write-up for my system. Each reader must know his own limits and not try to bite off more than they can chew.

            But know that I wouldn't even do a write-up like this if I didn't think it was safe and sane.

            One thing about KISS, is that it can reduce the accident-prone fear-factor, and enhance respect, like any battery system needs. Nothing here is going into the danger-zone, at least from an LFP cell stanpoint in our sub-c application.

            In fact, for those that can read between the lines, we can obtain professional results - something that even the best commercial bms can't understand - cell degradation. In an attempt to balance based solely on voltage, they may well be babysitting trash cells, which are a hazard in the first place regardless.

            A single-cell charger is the easiest way to make sure that each cell is "top charged" to full. This can be done before connecting them into a battery, or even after all the cell links are established if one is running single-cells in series. This is critical to do even IF one wants to do a bottom balance - but we'll leave that to other threads.

            After a 12 hour rest period, measuring the unloaded open-circuit voltage can make it very easy to spot a problem cell before placing it into service if that resting voltage is well above or below the others. Bad quality cell, or poor wiring infrastructure? Now is the time to find out and document it up front for later analysis as the years go by.

            In our application, "balance" does not mean lining up "terminal voltages" to the exact same value under charge, since each cell has it's own characteristic small variance in capacity and internal resistance. Hence we are relying on the manufacturer's own QC, like any other chemistry. CALB, Winston, GBS gotten from reliable sources is the best bet. There are others, but there are also "private label" trash too. Same as in the Pb world.

            If one has purchased quality cells, they are likely to be close enough so that after an initialization charge, balance is reasonable enough for our purposes without having to put a rat's nest over it. But yes, one will need an LVD like they would use with any other expensive battery system.

            Example: In my case, the 4 GBS cells after an initialization charge, measured no more than .025 v delta between highest and lowest ocv at both the top and AND bottom end of 80% DOD (roughly 3.19v per cell) OCV (which means 12 hours rest). And the cells that had the .025v spread were different from each other depending on whether they were measured at top-charge ocv, or 80% DOD discharge / rest ocv.

            Thus, I'm really seeing these things in their "native" format from the factory, which are real-world slightly different from each other. I want to know the characteristics of each one, and fortunately, they are close enough to work well.

            As has been recently discussed for a few years, the so-called "memory affect" for LFP, which is really when one does PSOC charging all the time (and doesn't do a good absorb), is mitigated by doing an initialization charge once in awhile. This also helps bring any cells starting to go out of balance from a reasonable standpoint back within the fold. A win-win so to speak if done on a regular basis, say yearly.

            Charging to only 3.45v per cell on a normal cyclic basis, which takes a bit longer to absorb to full than say running quickly up to 3.5v or higher, allows for a bit more headroom in case a cell goes a bit rogue, and also does not keep any cells at a high voltage when absorb is basically over waiting for the sun to set. Thus we avoid daily floating at high-ish voltages for those actually lucky enough to finish absorb in the first place! But I suppose it could happen.

            At any rate, I think the world over in every forum, it is generally accepted that what is good for one guy may not be good for the other depending on circumstance,

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            • #7
              PN don't you have a Powerlab8 or Icharger? I believe I have heard you mention that several times. Anyone with LFP bateries should have one if they are not going to use a BMS
              MSEE, PE

              Comment


              • #8
                PNjunction, Yes - KISS is where it's at as long as it's effective. I agree and have also been doing this for years (with lithium to boot). Recently, another off-grid system in my local area has been installed and we have a bit of a show-down. It's all in good fun, but this is 400ah of GBS vs 400ah of CALB. Initially I am quite impressed with all the cells involved (GBS and the CALB brands). No cell voltage drift and there's no rats' nest. Each system's solar array is close in size (around 2kw) and controllers/inverter are basically the same units. I believe you use the iCharger (or Samlex) and I used the PowerLab8, and metering is very important as you say. Thanks for your contributions in this area!
                ~CrazyJerry
                [B]I'm around...[/B]

                Comment


                • #9
                  Happy New Years everyone! Thought I'd reply before I'm unable to do so.

                  I have a variety of chargers, and lately invested in a 40A variable bench supply for testing. This allows me finer control over the absorb end-current when I want to do the 100% SOC capacity testing on each cell. I believe the PL8 allows for end-current customization, whereas the iCharger only has one or two presets.

                  Ie, if I do 3.6v, I take it down to .05C and stop.
                  If I do 3.5v, I take it down to .005C and stop.
                  At 3.45v, I take it down to nearly zero and stop.

                  So I needed finer control than what the iCharger or other chargers could do. Might have pulled the trigger for a PL8, but the KISS of the variable bench supply was a big draw. Of course my Fluke resting directly across the terminals is the standard for voltage.

                  Still, the overall goal of two-terminal pack charging with reasonable balance has been achieved. This is not for everyone obviously. For normal charging, I use the Samlex set to 14v (3.5v per cell) and stop when I feel like it. Had I built a 24v system, I'd have no qualms about their 24v models either.

                  With the supply, I was able to fine-tune some testing, and the importance of doing an "initialization" charge at higher voltages to each cell individually has really been beneficial to ensure full charge saturation. I kind of poo-poo'ed that before.

                  NOW I fully understand why Thundersky used 4.2v, Winston 4.0v and GBS 3.8v - this is for INITIALIZATION, and not for normal cyclic use. But you would never guess that from the quickie spec sheet that doesn't spell this out! Early users burned up their cell life not knowing the difference. Please - don't walk away or float at these higher voltages - stop at .05C or less absorb!

                  Previously, no matter what charger I used, I NEVER exceeded 3.65v like you are supposed to heed normally, and my resting ocv values have always been in the 3.38 to 3.4v range after a 12 hour rest.

                  NOW, when I do my NORMAL lower-voltage charging, my resting voltages of at least 12 hours are higher than before! I'm not trying to achieve any more capacity, but I believe this reflects a better state of even charge saturation inside.

                  In other words, had I handed Sunking my 40ah GBS cells for use in his racing golf-cart without this initialization charge being performed even once in their lifetime, I'd bet bottom dollar they would barf half-way down the track due to the uneven surface charge and never really thought about.

                  In my sub-c application, this unevenness of charge across the internal surfaces may never have been an issue. T1 Terry tipped me off to this in other forums, and now that I have given it a go, I am a firm believer in an end-user doing a *controlled* higher-voltage initialization charge to each cell *individually* prior to putting the cells into service - even if one is doing a sub-c application. My bench supply did fine for that, but perhaps even a PL8 if it had fine absorb-current control could do that too?

                  Comment


                  • #10
                    Note for GBS users:

                    The "single cell" voltage you can charge to listed on the spec sheet is 3.8v as compared to the "series connected" max charge voltage of 3.65. Today I took each cell *individually* just short of 4 volts to 3.95v [I]just for initialization[/I] down to .05C absorb. Done - no drama, but we'll see down the road if this was a bit too high for the preventative-maintenance procedure for GBS as opposed to say Winston. Heh, which one doesn't want to do any more often than necessary.

                    The great news is that sure enough - just doing an individual "top charge" like this (notice I said nothing about balance), resulted in a very tight top-charge terminal voltage spread of no more than 0.025v even when later charging and stopping at my normal lower voltages. RESTED "cell voltage" spreads are even tighter at about 0.019v!

                    I guess I'm lucky - my cells seemed to be matched very well for both capacity, internal resistance, and there isn't any oxidizing gunk under the cell link bars.

                    At the bottom end when discharged down to when the first cell hit 2.8v *under a 0.2C load*, the spread was larger by about 0.130v between high and low terminal voltages. Ie, a low terminal voltage of 2.8v, and a high terminal voltage of 2.93v. Since terminal voltage under charge/discharge isn't *truly* reliable as an soc indicator, they actually *could* be balanced if one takes into account the load current and each cell characteristic, but I won't go there to keep things simple. Maybe something like a Victronic BMV-700 / 702 is in my future. For now, my pack-level powerwerx inline meter and ITS-12 lvd set for 12.8v are doing ok.

                    (EV'ers may want to do their own balancing thing, but if I DID do a bottom balance, I would do so AFTER a top-charge initialization.)

                    Still, I'm manually keeping an eye on things so I don't get complacent. Not for the guy next door in other words.

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