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LiFeP04 Batteries for Solar & BMS

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  • We can argue these points to death and have. I've moved on.

    With my 40ah 12v 4S bank of GBS, I have also been able to succesfully TOP and BOTTOM balance (not at the same time mind you) a single dinky 20ah gbs cell into that 4S battery. Yep, that single 20ah cell will get hammered, and is of course the limiting factor overall.

    Thing is, nobody would run a battery made like that with a single cell half the capacity of the other three, and just proves a point about balance being a total misnomer based on voltage.

    So real world - aside from doing an initial sanity check upon first charge for a reasonable balance, there is no need to go bananas over minute differences which are going to change anyway as the cells age. That is, if you are buying quality, and not some e-bike online used trash held together with duct tape. THAT mandates a balancer band-aid to protect and disguise the horrid quality.

    In addition, what everyone seems to forget - but Sunking points out too, is that *in our application*, that is a so-called "Sub-C" low-current environment of a solar housebank, or even a small solar project designed correctly, the differences won't be aggravated like they would be in EV use.

    Your gear will start failing long before you reach bottom anyway. And, you may not even have enough solar insolation to trigger the bleeder-boards long enough at the top, which all of us agree are a point-of-failure waiting to happen - again in this sub-c environment of usually no more than about .1 to maybe .2C.

    Unless someone designs their solar system for absolutely no autonomy, and desires to discharge to 80% DOD daily, the bank is too small. Designed properly, a solar housebank or even a smaller project should have a battery that will most likely never see anything higher about 95% SOC, and anything below than about 60% DOD.

    So for the most part, any cyclic solar user will probably always be in a psoc environment, with low currents, in the flat part of the curve, and one may never encounter any slight aging or manufacturing issues that would crop up like it would with EV. Not that I endorse low quality, but I'm just saying that if we don't get things absolutely lab-perfect, we still may never see issues if we've done our autonomy and power budget correctly in the first place.

    I'm not advocating a totally hands-off approach either. Some form of monitoring on a regular basis should take place. If one is buying trash, then you better have insurance. If you buy good quality, and use the cells in our "Sub-C" application, then I'm comfortable with reasonable levels of monitoring.

    Someday we may be able to move beyond the obsession with either top or bottom balancing, and really discover the additional freedoms that come with lifepo4 in a solar cyclic scenario. Unfortunately, we always get sidetracked by non-solar issues.

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    • Thought I should pop into this thread for a look, so impressed that everyone seems to be arguing so nicely, cheers.

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      • Originally posted by Jerry Liebler View Post
        The whole idea of battery management is reducing the risks that operation will be outside the "safe area of operation".
        This topic has been discussed at great length here. You can go back and read the threads. One user advises "take what you want and scrap the rest." That would be my advice. Every user's need and risk tolerance is somewhat unique.

        After lengthy discussion, if I build an off-grid SS, it will have a LFP bank bottom balanced, using quality CC and inverter/charger with programmable thresholds, and individual cell monitoring so I can assess health at a glance. Stay on the flat part of the curve. Maybe additional disconnect if I think it needed, triggered by the cell level monitors. If someone else wants to do different, I say live and let live. Someone comes in here blasting away about top balancing vs bottom balancing at this point, and I remember the words "take what you want and scrap the rest."
        Last edited by Living Large; 08-31-2015, 09:41 PM. Reason: It's fun to be able to edit again

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        • Originally posted by Living Large View Post
          After lengthy discussion, if I build an off-grid SS, it will have a LFP bank bottom balanced, using quality CC and inverter/charger with programmable thresholds, and individual cell monitoring so I can assess health at a glance.
          In the meantime, have some fun and spring for a simple 12v 4S setup consisting of good quality GBS, CALB, or Winston cells. I think you're ready, and the hands-on will provide reassurance that you can handle the larger setups. Try your hand at no-balancers, other than doing an initial sanity-check upon first charge, and stay out of the knees. Put a voltmeter on it once in awhile.

          Mind - BLOWN.

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          • Originally posted by PNjunction View Post
            In the meantime, have some fun and spring for a simple 12v 4S setup consisting of good quality GBS, CALB, or Winston cells.
            Does not have to be that expensive or large, just some 18650 cells does the exact same thing. They don't leave as big as a hole in the ground when they explode
            MSEE, PE

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            • Heh, yeah as most 18650's are NOT lifepo4.

              I'd suggest at least a 40ah battery made up of GBS, Winston, or CALBS.

              On the low end, you could also put together 26550 LIFEPO4 Headways, popular with the DIY wheelchair crowd. Still, I'd just get smaller versions of the typical large-prismatic cells as they are closer in operating characteristics to what you'd use for a real solar bank anyway.

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