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  • Lithium battery BMS Question with Flexmax80


    So I'm reading and reading and I must be missing something. I'm planning to purchase a bunch of Lithium battery cells to wire in series to create a 48 volt battery pack to hook up to a Radian 8048 inverter. (the batteries do NOT have built in BMS)

    I am using a FlexMax80 charge controller and am failing to understand how to get the BMS to shut the controller off when the system is fully charged. Best I can tell, the FM80 does not have any auxiliary control contacts to turn it off remotely.

    I plan on using something like a Rec BMS system (centralized) but noticed that there's no contacts inside the FM80 or way for it to tell the FlexMax80 that a charge is complete or to shut it down for high voltage.

    Best I can figure is to just put a large DC rated power relay in between the charge controller and the battery pack and have the BMS activate that relay as required. But this seems too simple.. so simple it's making me think I have it wrong.

    Is it acceptable to just pull the proverbial plug on a FM80 when its charging a battery? Will disconnecting the battery from the FM80 in such a way damage anything?

    I feel stupid asking such a basic question..



  • #2
    Originally posted by Murby View Post
    So I'm reading and reading and I must be missing something. I'm planning to purchase a bunch of Lithium battery cells to wire in series to create a 48 volt battery pack to hook up to a Radian 8048 inverter. (the batteries do NOT have built in BMS)

    I am using a FlexMax80 charge controller and am failing to understand how to get the BMS to shut the controller off when the system is fully charged. Best I can tell, the FM80 does not have any auxiliary control contacts to turn it off remotely.
    The Flexmax does not need to be "turned off" when charge is complete. It handles all that all by itself.

    The FM80 has an auxiliary output that can be programmed to do a bunch of stuff. Again, though, you don't need to use it to terminate charge.

    You will have to add your own BMS. It will contain a (generally very large) relay to disconnect power if the cells are driven outside their safe operating ranges. This should be a very rare event.

    Comment


    • #3
      Originally posted by jflorey2 View Post

      You will have to add your own BMS. It will contain a (generally very large) relay to disconnect power if the cells are driven outside their safe operating ranges. This should be a very rare event.
      That's what I wanted to know. Are you saying that unplugging the charger from the battery bank while its charging will not harm the charger? I understand the Flexmax80 can charge a lithium battery correctly, but the BMS is there to monitor the individual cells and to make sure the battery bank is not over charged or go below a specific low SOC to prevent damage to the batteries.

      If the battery bank has a cell that starts to get over charged and it can't bleed off the excess, there needs to be a way to disconnect the FM80 or stop it from charging. Since the FM80 has no aux INPUT for this (aux contacts are output only), can I just use a relay on the FM80's output to cut power?

      Comment


      • #4
        The FM80 may survive being disconnected from it's battery and only having the solar PV powering it. But then things get sad at sunset, will it reset itself the next day and turn on ? I don't know - I don't have one. I would not do that disconnecting from either of my 2 charge controllers.

        Batteries.... What ever the balancing amps are, you need a way to throttle the charger back, so the balancers can work without cooking the rest of the cells.
        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


        • #5
          I think I found the answer but still reading.
          Apparently, you can disconnect without damage. That's what this device is for:
          https://www.victronenergy.com/upload...V-100-A-EN.pdf

          Still researching this. Sure wish someone else here with more experience would chime in.

          Comment


          • #6
            Why would you want to disconnect the charger for.? That is silly. If your batteries are charged by noon, you would go on batteries until the next day and all that power that could be utilized from the panels is never used. All that does is wear out your batteries faster. Not a good idea. There is no reason to ever turn off the charger. If done correctly you do not even need a BMS. You just set up the Charge Controller to operate as CC/CV Float charger and set the voltage to 54.4 volts and call it done.
            MSEE, PE

            Comment


            • #7
              Originally posted by Sunking View Post
              Why would you want to disconnect the charger for.? That is silly. If your batteries are charged by noon, you would go on batteries until the next day and all that power that could be utilized from the panels is never used. All that does is wear out your batteries faster. Not a good idea. There is no reason to ever turn off the charger. If done correctly you do not even need a BMS. You just set up the Charge Controller to operate as CC/CV Float charger and set the voltage to 54.4 volts and call it done.
              Well that sounds simple enough.. But what happens when one of the cells charges faster than the others? Why do all these people use BMS's? Even Solar Lithium batteries have built in BMS systems..
              I've seen youtube video's where the guy didn't have BMS and he was doing fine but he seems to be in the minority.

              The battery pack I intend to assemble will consist of almost 50 individual cells configured in 7 parallel-series packs, each pack will be 48 volts nominal and each pack is treated as a "cell" itself. I was planning on a BMS to monitor each of the packs to keep them equalized sort-to-speak.

              This is a 22 kWh system.


              Comment


              • #8
                Originally posted by Murby View Post
                That's what I wanted to know. Are you saying that unplugging the charger from the battery bank while its charging will not harm the charger?
                I don't know. It generally won't. I know for sure it won't hurt Midnite Solar charge controllers because I've tried it, but that's just one example.

                But it's an odd question, because there's no reason you would ever do that. Charge controllers are designed to remain connected to the battery at all times.

                I understand the Flexmax80 can charge a lithium battery correctly, but the BMS is there to monitor the individual cells and to make sure the battery bank is not over charged or go below a specific low SOC to prevent damage to the batteries.
                Right. CC prevents overcharge. Inverter prevents overdischarge. BMS is a backup to both of them. (Also has accessory functions like battery balance in some cases.)
                If the battery bank has a cell that starts to get over charged and it can't bleed off the excess, there needs to be a way to disconnect the FM80 or stop it from charging.
                When that happens, the BMS disconnects the BATTERY not the charge controller. The switch is in the BATTERY. When that happens, the charge controller will still be connected to the inverter and the rest of the system.

                Comment


                • #9
                  Originally posted by jflorey2 View Post
                  I don't know. It generally won't. I know for sure it won't hurt Midnite Solar charge controllers because I've tried it, but that's just one example.

                  But it's an odd question, because there's no reason you would ever do that. Charge controllers are designed to remain connected to the battery at all times.


                  Right. CC prevents overcharge. Inverter prevents overdischarge. BMS is a backup to both of them. (Also has accessory functions like battery balance in some cases.)

                  When that happens, the BMS disconnects the BATTERY not the charge controller. The switch is in the BATTERY. When that happens, the charge controller will still be connected to the inverter and the rest of the system.
                  If the Flexmax80 is charging the pack and one of the cells in series goes high, the BMS will attempt to compensate by bleeding off some power through a resistor. But what if the BMS can't keep up with the charging rate and it can't communicate with the FM80?

                  This is where I'm confused.

                  Lets say I have 7 lithium batteries in series and number 4 in the middle starts to approach its maximum 4.2 volts while the others are still at 3.9 volts, the BMS will then start to bleed off energy from cell number 4. but what if the BMS can't bleed off the energy fast enough? It needs to be able to tell the charge controller to either slow down or stop for a while right? Except that the FM80 has no way for the BMS to communicate that. I know the FM80 has a set of aux contacts, but they're an output function, not an input.

                  So to solve that, I was thinking about installing a power relay (contactor) to cut power to the battery, (or cut power from the PV array) which would be the same as disconnecting the charge controller all together.

                  Where am I going wrong?

                  Comment


                  • #10
                    Cutting the PV array power is a much safer thing to do, then cutting the battery off of the Charge Controller. The battery powers the CPU in the controller and acts as a voltage regulator to prevent the battery terminal voltage from going too high and destroying the electronics. You would want to leave maybe 1 string of the array connected to allow the BMS to balance the cells.

                    But why not use matched cells and set your controller voltage to a safe voltage?
                    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


                    • #11
                      Originally posted by Mike90250 View Post
                      Cutting the PV array power is a much safer thing to do, then cutting the battery off of the Charge Controller. The battery powers the CPU in the controller and acts as a voltage regulator to prevent the battery terminal voltage from going too high and destroying the electronics. You would want to leave maybe 1 string of the array connected to allow the BMS to balance the cells.
                      Aren't both the BMS and the charge controller powered by the battery? How would they remain turned on at night if not?

                      But why not use matched cells and set your controller voltage to a safe voltage?
                      I'm not sure what you mean by safe voltage. If I have 14 cells in series, and assuming I charge to only 4 volts (for longevity of the batteries), then I have to apply (14x4volts) =56 volts of charge. If one cell loses capacity and starts charging to max voltage faster than the others, wouldn't it go over its 4 volt limit and potentially past 4.2 volts??

                      I have to admit, I've spent a year learning about FLA batteries and I'm constantly finding myself having to stop and rethink for the lithium. Its like muscle memory for my brain and its causing me to get a bit confused sometimes.

                      This is a ~22 kWh battery and even the more expensive REC BMS can only bleed off 0.9 amps per cell so far as I can tell.

                      Comment


                      • #12
                        Li batteries are still in development, and I consider them experimental still. There are some "boxed - all-in-one" packages that are low power, expensive and proprietary.

                        Aren't both the BMS and the charge controller powered by the battery? How would they remain turned on at night if not?
                        That's why I said disconnect the PV, not the battery half.

                        To prevent runaway single cells, you must use matched batteries. Hopefully, they will age gracefully together. If not, you have a problem. What's needed (and does not exist yet) is a standard for BMS systems to be able to easily control a Charge Controller and throttle it back, and not need to chop off all charging completely.

                        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


                        • #13
                          Originally posted by Murby View Post
                          Aren't both the BMS and the charge controller powered by the battery? How would they remain turned on at night if not?
                          OK you still have a lot to learn and I would discourage you from using lithium as they are for advanced users only. Not to mention FLA will last a lot longer and tough as nails. As to your question you cannot disconnect the batteries from the controller because the controller gets all its power from the batteries. You can damage the controller disconnecting the batteries with the panels connected. You have to disconnect the panels, not batteries.


                          Originally posted by Murby View Post
                          I'm not sure what you mean by safe voltage. If I have 14 cells in series, and assuming I charge to only 4 volts (for longevity of the batteries), then I have to apply (14x4volts) =56 volts of charge. If one cell loses capacity and starts charging to max voltage faster than the others, wouldn't it go over its 4 volt limit and potentially past 4.2 volts??

                          This is a ~22 kWh battery and even the more expensive REC BMS can only bleed off 0.9 amps per cell so far as I can tell.
                          Like I said you have a lot to learn. Unless you have a bad cell, all cells charge equally and discharge equally. If they are Balanced from the start, they stay in Balance. You just said the BMS uses Vampire Boards, aka Bleeders. What you do not understand is how a Vampirte Board works. It only turns on when the first cell reaches 4.2 volts, and when it turns on only bypasses 0.9 amps. Problem is you may still be charging at say 50 amps. So when the first Vampire board turns on and bleeds 1 amp bypass leaves 49 amps flowing through your fully charged battery. Oops! Your BMS just destroyed the cell.

                          Now stop and think. if you Balanced the cells properly initially. and you only charge to 56 volts (4.0 vpc) Your BMS never does anything because no cell ever reaches 4.2 volts now does it? [B]The real question is how do you intend to do the Initial Bulk Balance? [/B]You cannot do that with a BMS as it is not capable of balancing a battery. [B][U]Do you have the equipment and knowledge to do the initial balance? [/U][/B]

                          Think of this no commercial EV manufacture allows a customer to ever fully charge the battery. I build racing golf carts and help a lot of people build DIY EV conversions. Very few would ever consider using a BMS because the BMS is the number 1 cause of failures. But there is a catch. We know what we are doing.

                          So until you figure out how to do the initial Balance and have the equipment to do it, you have no biz working with Lithium Batteries.
                          MSEE, PE

                          Comment


                          • #14
                            Originally posted by Murby View Post
                            If the Flexmax80 is charging the pack and one of the cells in series goes high, the BMS will attempt to compensate by bleeding off some power through a resistor. But what if the BMS can't keep up with the charging rate and it can't communicate with the FM80?
                            Then the BMS will disconnect the battery.
                            Where am I going wrong?
                            You think the BMS is just a balancer. It is not. It is a battery _management_ system that includes circuitry to disconnect the battery to protect against overcharge and overdisharge.

                            At this point do NOT try to integrate a BMS with a raw battery. You'll (at best) quickly destroy the battery. Instead get a battery like the SimpliPhi that has a BMS built in.

                            Comment


                            • #15
                              Originally posted by Mike90250 View Post
                              Li batteries are still in development, and I consider them experimental still. There are some "boxed - all-in-one" packages that are low power, expensive and proprietary.

                              That's why I said disconnect the PV, not the battery half.
                              Looks like disconnecting the PV array is the way to go.. Thanks for that advice. I think a Kilovac relay should do the trick as they are rated for up to 900 VDC at 500 amps and are reasonably priced at around $130 each and have coils that draw only 30ma @48 volts. Not bad at all.
                              Using the "String Sizing Tool" I found through this forum (I think), I'm only going to have a single array anyhow so all I need to do is break one feeder line from the array.

                              To prevent runaway single cells, you must use matched batteries. Hopefully, they will age gracefully together. If not, you have a problem. What's needed (and does not exist yet) is a standard for BMS systems to be able to easily control a Charge Controller and throttle it back, and not need to chop off all charging completely.
                              Well, a lot of controllers do come with the CAN BUS communications, just not my FlexMax80.. so not as flexible as the name implies I guess.

                              The battery pack is a lot more storage than I need so I think I'll be able to set the per cell charge at a very low voltage to extend the life of the battery. Maybe 3.9 or 4 volts or something and then set the cut off a bit higher so its not drained so much.

                              Thank you again for taking the time to help me out.. the information you provided has really been valuable.



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