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Constructing a LiFePO4 BMS (Battery Management System) charger.

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  • Constructing a LiFePO4 BMS (Battery Management System) charger.

    I feel like LiFePO4 has a lot of potential.

    What do I need to learn in order to construct a circuit that can charge a pack of LiFePO4 batteries?

    I understand the basics of physics - but have NO clue how circuits work.

    Is a fundamental circuit board text enough?

    Like -- Fundamentals of Electric Circuits by Charles Alexander

    ... or is this idea too far fetched (like, I need an engineering degree or equivalent)?

    Looking to build one myself...

  • #2
    BMS systems are a very complex system requiring a microprocessor and many A/D converters. It is an engineering nightmare as each cell is monitored for both temperature and voltage, plus the switching mechanisms to charge each dell independently. For now BMS systems are the realm of Electric Vehicles.

    Aside from that Lithium is not really ready for solar as of yet as the price is just to high, getting closer but not yet.

    FWIW this is really the wrong place to get info on BMS. Try the Electric Car Forum. There are hundreds of engineers and scientist on that forum, and well over a 1000 threads on just BMS.
    MSEE, PE

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    • #3
      Originally posted by solarnoobie View Post
      I feel like LiFePO4 has a lot of potential.

      What do I need to learn in order to construct a circuit that can charge a pack of LiFePO4 batteries?

      I understand the basics of physics - but have NO clue how circuits work.

      Is a fundamental circuit board text enough?

      Like -- Fundamentals of Electric Circuits by Charles Alexander

      ... or is this idea too far fetched (like, I need an engineering degree or equivalent)?

      Looking to build one myself...
      You will need a to make a flyback converter for each cell which is isolated from the charge balance controller side - 3-4A per cell, for a 100-300Ah pack. May need a micro on each side of the isolation or possibly an isolated I2C from the battery voltage (say up to 70V) to the control side per cell. The cells all charge in parallel with the charge balance across all cells pushing charge from the total to individual cells that need extra. If any cell is above 3.65 (or something similar) then the charge to all cells is cut (same for discharge)

      An engineering degree will make it easy of if you are savvy enough and and understand the above.

      Go to digikey and the have all the parts - silicon labs for the i2c isolators and wurth for the SEPIC transformer - you'll need schottky diode caps and your choice of a simple micro ... have fun.

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      • #4
        Hi all, instead of making another thread I figured I would bump this one with my question.

        It seems that LiFeP04 seems a better route to take with my project. (environmental concerns)

        My system is just a single 30 watt 12V solar panel. I want to make sure charge control would work properly if I went this route. It doesn't need to be perfect as my application is not that intense nor a long duration.

        Will this battery with it's included PCB with overcharge and over-discharge protection be adequate as a charge controller?

        http://www.batteryspace.com/Customize-LiFePO4-26650-Battery-12.8V-9.9Ah-126.72Wh-7A-rate-with-PCB.aspx

        Or if more is necessary, could this solar charge controller be used in conjunction with this battery/PCB?

        http://www.batteryspace.com/Charge-Controller-120W-12V/24V-Auto-Detection-10A-Rate-for-SLA-or-LFP.aspx

        Thanks for your time. This forum has been a great reference for my research.

        Comment


        • #5
          Originally posted by ashjur View Post
          Will this battery with it's included PCB with overcharge and over-discharge protection be adequate as a charge controller?

          http://www.batteryspace.com/Customize-LiFePO4-26650-Battery-12.8V-9.9Ah-126.72Wh-7A-rate-with-PCB.aspx
          Actually it might just work without an external battery charger. Other than 30 watt panel do you know the rest of the specs? If the Vmp of the panel is between 16 and 18 volts it should be OK. Only thing is you will sacrifice some efficiency, but no big deal in this application.

          What will happen is the battery will pull down the panel voltage on its own to match the battery, and effectively make the panel a current supply which is just fine.

          Now just one caveat, I am assuming the battery built-in regulator will shut off the charge when the battery is fully charged. If you can try to find what the maximum voltage that can be applied to the battery pack. As long as it is equal too or greater than the panel it should work. If needed you could use diodes to lower the voltage which you are going to need at least one to prevent the battery from back feeding and discharging into the solar panel at night or during cloudy times.
          MSEE, PE

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          • #6
            Originally posted by solarnoobie View Post
            ..What do I need to learn in order to construct a circuit that can charge a pack of LiFePO4 batteries?
            The guys at CandlePower forums can help you. The fist thing I'd look at is becoming familiar with safety issues from home-built chargers, or hobby chargers that are not set up properly:

            http://www.candlepowerforums.com/vb/...e-of-batteries

            Then search for 26650 or whatever your project needs with safety as the ultimate goal.

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            • #7
              For small scale research, how about a LifePO4 motorcycle battery with a built-in bms? Something like a battery from Shorai that are supposed to be direct-replacements for motorcycles and their charging systems.

              I'm not sure if these would have any deep-cycle capabilities, but haven't really researched them much.

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              • #8
                Darn, I just bought real real cheap from a company that died, about 200 cell boards and 4 controllers from Elithion, I will need to play with this, since my present BMS is a pseudo BMS as independent boards on each cell that will flow balancing current on each cell that charges over 3.65 (I think).

                http://www.batteryspace.com/Smart-LE...ent-for-L.aspx
                this is my Pseudo BMS!!! (more precisely cell balancer)

                It was real super cheap, like $50 and I tested a few cell boards and they seem good, by watching the LED blink twice.

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                • #9
                  Hobbyking.com has all sorts of hobby grade chargers and balancers that you can build off of. The whole RC community does multi cell charging of Li-ion and LiFEPO4 packs.
                  PowerOne 3.6 x 2, 32 SolarWorld 255W mono

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                  • #10
                    A good LiPo/LiFe battery charger is made by REVOLECTRIX. It comes with a lot of different programs to charge all types of battery sizes while properly balancing the cells for a really reliable charge.

                    I own the Cellpro Multi4 that came with a power supply and 4s adapter. I have a wide range of 3cell 11.4v LiPo batteries from 1.3 to 3.3 mah that I use in my RC planes. Haven't had any issue with it after about 18 months of use.

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