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Simplified lifepo4 charging and care

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  • #46
    Originally posted by Sunking View Post
    Now for the Dan's and Karrak's out there, you need all the help you can get in the form of automation and need a Nanny Device to control your life. Karrak needs it for his commission, and Dan needs it because no one can think for themselves.
    There might be an imaginary universe made up by Sunking or maybe a parallel universe where NASA uses LFP batteries in its spacecraft, where LFP batteries don't go out of balance, where LFP batteries only last two and a half to five years, where charging an LFP battery to 100% will give you half the lifespan compared to charging it to 90%, where there is a 10% difference between charging and floating an LFP cell at 3.4 V/cell and 3.45 V/cell and where I make a commission out of selling cheap Chinese electronic goods. In reality, none of this is true.

    As for automation, I admit I am lazy. I would rather type in the Internet address of my or my friend's battery monitor on my phone or computer to check up on our batteries rather than grovel around on the floor in the shed with a multimeter or in the case of my friend's system, drive 150km and then have to measure sixteen voltages after having to take the plastic shield off his battery. I also like science, so I am getting logged data that will inform me how well LFP batteries work rather than just relying on hearsay.

    Simon

    Off grid 24V system, 6x190W Solar Panels, 32x90ah Winston LiFeYPO4 batteries installed April 2013
    BMS - Homemade Battery logger https://github.com/simat/BatteryMonitor
    Latronics 4kW Inverter, homemade MPPT controller
    Off-Grid LFP(LiFePO4) system since April 2013

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    • #47
      The thing I get most confused about with LifePO4 is the lack of charge controllers out there. Floating is generally considered bad with LifePO4, but there are very few charge controllers designed to not float a LifePO4 battery. The dinky little Genasun controllers and the Victron 100/30 100/50 series are the only ones I can think of, and I'm really just guessing because they have LifePO4 charging profiles (I have no idea how those profiles work under the hood). If you use the wrong charge controller, you need to wire a relay between the charger and the battery and shut it down when a certain voltage is reached, or you need to monitor it while charging, which is a bad idea IMO because humans make mistakes.

      I'm not even sure how that would work, because charging to 13.8v you'll hit 13.8v, but the resting voltage will drop you back down to 13.6v or lower, so it's like you need a high voltage cutoff combined with a state machine that only turns the system back on once it has dropped below a certain voltage. If you do that, you lose the ability to power things directly from the solar panels too.
      Last edited by createthis; 07-19-2016, 04:12 PM.

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      • #48
        Hmm, would it work to have the float voltage be something really low, like 11.5? Basically, get charged up, then the voltage drops off to basically nothing?

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        • #49
          Originally posted by createthis View Post
          The thing I get most confused about with LifePO4 is the lack of charge controllers out there. Floating is generally considered bad with LifePO4,
          Who told you that silliness?

          A lithium battery charger is a Float Charger silly. Most Charge Controllers that will allow you to set voltages via keyboard down to 00.0x volts will work just fine. The Genasun controller is a float charger you set to 14 to 14.2 volts for LFP.

          The problem is you do not grasp how batteries charge. FLA and LFP use the exact same charge algorithm. Only the voltages change slightly to protect the innocent. A FLOAT CHARGER is a CC/CV charger. They mean the same thing. Every charge controller made is a CC/CV charger. Only thing you have to change is the voltage FLA = 14.4 to 15 volts, LFP 13.6 to 14.2 volts. All you need is a controller you can set the voltage down to 13.6 volts up to 14.2 volts in at least .1 volt increments, .01 even better.

          The only real difference is if you want to go to 100% with Lithium, you will need a way to terminate or turn off the solar controller when current tapers to 3/33 at 3.6 vpc. Genasun just floats below 100% at 14 to 14.2 volts. 14.6 is for 100% saturated to C/33.
          Last edited by Sunking; 07-19-2016, 05:48 PM.
          MSEE, PE

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          • #50
            Originally posted by smily03 View Post
            Hmm, would it work to have the float voltage be something really low, like 11.5? Basically, get charged up, then the voltage drops off to basically nothing?
            Yes, that works if one is using a charge controller designed for lead acid. Two rules: NO temperature-compensation, so disable that. And, IF your controller drops back to 13.6v or less in float, then you won't run into issues.

            I know what Sunking is saying, but I'll still reference the term "absorb", as being 13.8v or higher, and when either a timer or current monitor trips, it falls back to 13.6v float - since we're shoehorning a Pb controller with an LFP battery.

            Ideally, no float at all, BUT if you have a sudden parasitic drain / minor ground short happen, that 13.6v float, might save the day - or at least prevent the bank from hitting the low-voltage disconnect.

            One just chooses which direction they want to take according to their needs.

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            • #51
              Originally posted by createthis View Post
              The thing I get most confused about with LifePO4 is the lack of charge controllers out there. Floating is generally considered bad with LifePO4, but there are very few charge controllers designed to not float a LifePO4 battery.
              If you don't want to float your LFP battery all you have to do is set the float voltage low as smily3 has said.

              I haven't seen any hard evidence that says that floating LFP batteries is bad for them. This information from A123 http://liionbms.com/pdf/a123/charging.pdf seems to indicate that floating LFP batteries is not bad for them. Google "Is floating LFP batteries bad for them?" for more information. I would be interested to see any evidence that backs up the statement that floating LFP batteries is bad.

              Simon
              Off-Grid LFP(LiFePO4) system since April 2013

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              • #52
                Originally posted by karrak View Post

                If you don't want to float your LFP battery all you have to do is set the float voltage low as smily3 has said.

                I haven't seen any hard evidence that says that floating LFP batteries is bad for them. This information from A123 http://liionbms.com/pdf/a123/charging.pdf seems to indicate that floating LFP batteries is not bad for them. Google "Is floating LFP batteries bad for them?" for more information. I would be interested to see any evidence that backs up the statement that floating LFP batteries is bad.

                Simon
                WTF have I been trying to pound into your thick skull for a year?

                There is absolutely no difference between Floating a Lithium at something less than 100%, and two cells in parallel in any EV or laptop.
                MSEE, PE

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