Understanding a BMS





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  1. #1
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    Default Understanding a BMS

    I'm trying to understand how they work.

    From pictures online the connections to the battery to monitor are all thin little wires. So that tells me they don't charge the batteries individually as I initially thought.
    Its just meant to monitor the voltage.

    So I drew the below on power point.

    Does this mean the BMS will drop the "Fully Charged" mark to 3.15V to prevent the system from trying to bring the weakest cell up to 3.25V ?
    And how does the BMS protect against overload. Seeing the negatives are not connected between the BMS and load (Not shown in the picture) Am I correct in saying there is circuitry in between that will disconnect the battery from the load ?




    Attachment 559667
    Last edited by SBSP; 2020/02/26 at 07:05 PM.

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    Default Re: Understanding a BMS

    A Battery Monitoring System does not charge batteries. It only monitors their condition and state of charge, hence the thin wires. There's a lot of circuitry missing from your drawing and also a lot of unexplainable circuitry. Do you have a proper circuit diagram?
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    Default Re: Understanding a BMS

    Quote Originally Posted by SBSP View Post
    I'm trying to understand how they work.

    From pictures online the connections to the battery to monitor are all thin little wires. So that tells me they don't charge the batteries individually as I initially thought.
    Its just meant to monitor the voltage.

    So I drew the below on power point.

    Does this mean the BMS will drop the "Fully Charged" mark to 3.15V to prevent the system from trying to bring the weakest cell up to 3.25V ?
    And how does the BMS protect against overload. Seeing the negatives are not connected between the BMS and load (Not shown in the picture) Am I correct in saying there is circuitry in between that will disconnect the battery from the load ?




    Attachment 559667
    Why do you have to go and complicate such a simple thing.

    The answer is obvious.
    Cheers

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  6. #4
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    Default Re: Understanding a BMS

    Quote Originally Posted by Olyfboer View Post
    A Battery Monitoring System does not charge batteries. It only monitors their condition and state of charge, hence the thin wires. There's a lot of circuitry missing from your drawing and also a lot of unexplainable circuitry. Do you have a proper circuit diagram?
    Thank you.
    I dont have any drawings, FYI I'm not trying to build a BMS. I just drew how I have it in my head, just to understand how they work on a basic level. Until now I was under the impression a BMS also charge batteries . In a way I still think they do because in my mind a they will manage what ever power source you provide to ensure the batteries are charged and not over charged.

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    Default Re: Understanding a BMS

    Quote Originally Posted by Fluffy View Post
    Why do you have to go and complicate such a simple thing.

    The answer is obvious.
    Because I always overthink stuff Maybe its simple to you but not so simple to me.


    [EDIT]
    I dont need this to turn into a discussion on scientific level. Just wanted confirmation.
    But I think I now understand . They don't charge each battery individually they simply ensure the "weakest" battery don't get stressed by the stronger ones?. Then it just have a few extra stuff to protect the battery from over current ect ect.
    Last edited by SBSP; 2020/02/26 at 09:46 PM.

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    Default Re: Understanding a BMS

    Are there affordable solutions to monitor the state of 8 batteries?

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    Default Re: Understanding a BMS

    Quote Originally Posted by Dirk View Post
    Are there affordable solutions to monitor the state of 8 batteries?
    Your Axpert already does that better than anything else can.

    Don't try and fix what isn't broken.

    Serious.

    BMS monitoring at cell level is only really a Lithium battery thing, except for maybe uber huge commercial systems with batteries the size of Jeeps.
    Cheers

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    Default Re: Understanding a BMS

    Quote Originally Posted by Dirk View Post
    Are there affordable solutions to monitor the state of 8 batteries?
    Yes, a cheap DVM and a willing body and a piece of paper and a pen.

    Record every batteries voltage once a month. Enter the data in a spreadsheet.

    If the data goes pair shaped, call an electrician for help. (Highly highly unlikely)
    Cheers

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    Default Re: Understanding a BMS

    Quote Originally Posted by Fluffy View Post
    Your Axpert already does that better than anything else can.

    Don't try and fix what isn't broken.

    Serious.

    BMS monitoring at cell level is only really a Lithium battery thing, except for maybe uber huge commercial systems with batteries the size of Jeeps.
    So when you have an axpert and you want to make use of LIFEPO4 instead of SLA, then you need them right ?

    Remember someone without the know how like me will go and buy an axpert and a couple of LIFEPO4 batteries from aliexpress expecting them to just work.
    Part of the reason why I'm asking.

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    Default Re: Understanding a BMS

    Die BMS basies doen n paar goed, en ja, daai din draaitjies drae krag rond ook, maar in milliamps.

    Die BMS hou die batterye elk dop om te sien wat elke se 0.000v is, so drae een onder n set punt val, dan sal die BMS basies of deur Mosfets of n contactor die load disconnect, en basies sal daai cell basies stadig op bou tot terug op die set punt om die mosfets of contactor oop te maak. In daai tyd sal die bottem-end ballancing ook help om die lae celle op te bou, dis waar die milliamps begin deur stoor deur daai din draaitjies.

    Met my BMS wat ek het op my setup, is hy gestel dat as die celle op n sekere level volts kom, 3.450v dat hy begin deur lek na die selle wat laer is, daai is 200ma. Hy begin dan Cell Difference Balance dan.

    As die unit vol gelaai is op sy AH, dan begin die bms AutoBalance, as hy lae charging amps ontvang, dan doen dit die selfde.

    Die BMS beheer ook die stage of charge, so as die celle begin vol raak of oor n sekere set punt kom, dan sal die bms deur mosfets of n contactor die charge sny tot die levels begin daal het, dis waar die balancers ook in werking kom en dan milliamps af brand van die batterye wat hoog is. Dis wanneer die bms begin warm raak.

    Myne het n klomp leds wat begin skyn wanneer dit besig is met ballancing.

    As jy in die DIY in gaan, dan leer jy rerig wat presies gebeer en wat als doen en hoe dit werk. Ek ken 2 ouens wat diep in die goed is wat ek gereeld hulle brein pick vir inligting en by hulle leer.

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    Default Re: Understanding a BMS

    Ons het eer gister n lekker gesprek gehad oor BMS units. https://powerforum.co.za/topic/5250-...ptional-extra/

    As jy rerig will iets leer, volg Plonkster se post en pick sy brein oor bms en goed. Hy is lopen kennis van die goed.

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    Default Re: Understanding a BMS

    Quote Originally Posted by Gerlach View Post
    Die BMS basies doen n paar goed, en ja, daai din draaitjies drae krag rond ook, maar in milliamps.

    Die BMS hou die batterye elk dop om te sien wat elke se 0.000v is, so drae een onder n set punt val, dan sal die BMS basies of deur Mosfets of n contactor die load disconnect, en basies sal daai cell basies stadig op bou tot terug op die set punt om die mosfets of contactor oop te maak. In daai tyd sal die bottem-end ballancing ook help om die lae celle op te bou, dis waar die milliamps begin deur stoor deur daai din draaitjies.

    Met my BMS wat ek het op my setup, is hy gestel dat as die celle op n sekere level volts kom, 3.450v dat hy begin deur lek na die selle wat laer is, daai is 200ma. Hy begin dan Cell Difference Balance dan.

    As die unit vol gelaai is op sy AH, dan begin die bms AutoBalance, as hy lae charging amps ontvang, dan doen dit die selfde.

    Die BMS beheer ook die stage of charge, so as die celle begin vol raak of oor n sekere set punt kom, dan sal die bms deur mosfets of n contactor die charge sny tot die levels begin daal het, dis waar die balancers ook in werking kom en dan milliamps af brand van die batterye wat hoog is. Dis wanneer die bms begin warm raak.

    Myne het n klomp leds wat begin skyn wanneer dit besig is met ballancing.

    As jy in die DIY in gaan, dan leer jy rerig wat presies gebeer en wat als doen en hoe dit werk. Ek ken 2 ouens wat diep in die goed is wat ek gereeld hulle brein pick vir inligting en by hulle leer.

    Baie dankie, dis presies wat ek wou weet!

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    Default Re: Understanding a BMS

    I have noticed some of our batteries are showing over charged, and others under, the reason for me wanting to monitor it more closely.

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    Default Re: Understanding a BMS

    Quote Originally Posted by SBSP View Post
    So when you have an axpert and you want to make use of LIFEPO4 instead of SLA, then you need them right ?

    Remember someone without the know how like me will go and buy an axpert and a couple of LIFEPO4 batteries from aliexpress expecting them to just work.
    Part of the reason why I'm asking.
    Well in that case, the answer is so complex and complicated that it really cant be answered satisfactorily here.
    Cheers

    ZS5KAD
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    NA TwinTurbo SuperCharger
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    The frogs are starting to notice that the water is getting warm but it is already too hot to do anything about it.....

  18. #15
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    Default Re: Understanding a BMS

    Quote Originally Posted by Dirk View Post
    I have noticed some of our batteries are showing over charged, and others under, the reason for me wanting to monitor it more closely.
    Honestly, call someone who knows.

    Me, in the system you have, I would just let sleeping dogs lye.
    Cheers

    ZS5KAD
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    The frogs are starting to notice that the water is getting warm but it is already too hot to do anything about it.....

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    Default Re: Understanding a BMS

    Quote Originally Posted by Dirk View Post
    I have noticed some of our batteries are showing over charged, and others under, the reason for me wanting to monitor it more closely.

    I would check the cabling to the batteries first.

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    Default Re: Understanding a BMS

    Quote Originally Posted by Dirk View Post
    I have noticed some of our batteries are showing over charged, and others under, the reason for me wanting to monitor it more closely.
    The answer to batteries out of balance are going to cost a few rands to solve - ASSUMING THE CABLING IS CORRECT as per Dave:
    1) Check the cells with a hydrometer, and record the readings - then fix that batt manually i.e.charge it separately.
    2) Reshuffle the batts - (if memory serves) the first batt where the Pos cable connects to, move that one space down the line, the last batt where the Neg connected, becomes the 1st batt on the Pos connection.
    3) Get balancers and/or a BMV with Midpoint monitoring ... both together ideal, but costly yet automated.

    Now that I have typed that I can see where Fluffy's one-liners are coming from ... Costs! :-)


    Axpert's SOC measurement is best attempt at calculating the SOC. To make that more accurate will cost a few thousand ... BMV, ICC, RPi, new firmware etc.

    Axpert with lithium's, yeah, the BMS will protect the batteries more BUT (!!!) with all the makes of BMS'es installed on lithium banks, the ideal solution would be to have the BMS'es integrated with the inverters logic, to better control the discharge amps - again, costly solution.

    Or, if your inverter cannot, and you don't want unforeseen cases of the BMS switching off the battery bank at the most inopportune moment, you need to spec the lithium bank to your loads - aka the max current drawn from the bank - or drop the load demands.

    I am looking at a lithium bank where the BMS does not tie in with my inverters logic.
    One "cheaper" make that ties in, Pylons, but too small and I lose out on the systems overall capability - read waste solar power - and getting more Pylons becomes too costly per kWH.
    OR I can go with a BIG bank and not look back, that is cheaper than Pylons per kWH - but then there is the Virus.
    OR I can spend on the makes where the BMS is integrated with my system, but those costs more than a couple of Pylons per kWH.

    So yeah, it gets expensive VERY fast ... question is, is it a Hobby, WANT or NEED?
    Hobby - no price tag.
    WANTS costs more than NEEDS.

    Then again, how much did that battery bank cost you? Is a few thousand rand more worth it to use it more within the manufacturers design specs?
    Are they kept at 25 deg C?

    Decisions decision decision ...
    Last edited by the_terrible_triplett; 2020/02/26 at 11:20 PM.
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    Default Re: Understanding a BMS

    Quote Originally Posted by the_terrible_triplett View Post
    The answer to batteries out of balance are going to cost a few rands to solve - ASSUMING THE CABLING IS CORRECT as per Dave:
    1) Check the cells with a hydrometer, and record the readings - then fix that batt manually i.e.charge it separately.
    2) Reshuffle the batts - (if memory serves) the first batt where the Pos cable connects to, move that one space down the line, the last batt where the Neg connected, becomes the 1st batt on the Pos connection.
    ...
    Here I would agree on point 2 but with a slight difference. Move the batteries with the lowest volts to the positive connection and the highest to the negative. May be it is not worth doing it as Fluffy suggested.

    The problem is perhaps always part of the negative of having 4 batteries in series. They don't charge at the same rate. I would charge the lower voltage batteries one by one with a 12V charger when there is no LS on the time table as TTT suggested in point 1.

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    Default Re: Understanding a BMS

    Quote Originally Posted by ekkekan View Post
    The problem is perhaps always part of the negative of having 4 batteries in series. They don't charge at the same rate.
    Gets WAY worse when people have 8 or more 12v 105ah batteries in series and parallel ... man that is just a disaster.
    Rather get 12v 200ah batteries ... as little as possible batteries connected in parallel.

    6v batteries, 48v system ... I have 8 in series. ;-)
    Parallel is the problem child.
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    Default Re: Understanding a BMS

    Quote Originally Posted by Dirk View Post
    I have noticed some of our batteries are showing over charged, and others under, the reason for me wanting to monitor it more closely.
    How do you know or measure that.

    In a system of batteries in series that are floating (and equalizing) 99.9% of the time, I just cant see there being any discrepancies, unless you have some major problems with faulty batteries. Which I REALLY REALLY doubt.

    When the system is working ie with the grid off. Wait untill it has been supplying the load for about an hour. Now use a meter, it doesn’t even have to be super accurate, and measure the voltage across each battery and write it down.

    They should all be very similar in each series string.
    Last edited by Fluffy; 2020/02/28 at 10:25 AM.
    Cheers

    ZS5KAD
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    NA TwinTurbo SuperCharger
    A V6 and an inline 4

    The frogs are starting to notice that the water is getting warm but it is already too hot to do anything about it.....

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