Any advice on attaching the big wires to the BMS board

spacebass
spacebass Registered Users Posts: 87 ✭✭
in Solar Beginners Corner #1
My bms board is rated at 120amps but the battery maximum charge and discharge rates are less than half this each , could I somehow use copper lugs  , mount some M6 posts on the board ? I read they should be welded on , but this process does not seem easy .

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  • Vic
    Vic Solar Expert Posts: 3,208 ✭✭✭✭
    #2
    Have you looked at some of the LiFePO4 battery tear-downs on youtube?

    If not, look at the LithiumSolar teardown of an SOK server battery:
    https://www.youtube.com/watch?v=POf424nczxg

    Around 8 minutes in,  some clear images of the BMS connections, using lugs on fine-strand cable.

    Have never built any battery,   ...   yet.   Good Luck, Vic
    Off Grid - Two systems -- 4 SW+ 5548 Inverters, Surrette 4KS25 1280 AH X2@48V, 11.1 KW STC PV, 4X MidNite Classic 150 w/ WBjrs, Beta KID on S-530s, MX-60s, MN Bkrs/Boxes.  25 KVA Polyphase Kubota diesel,  Honda Eu6500isa,  Eu3000is-es, Eu2000,  Eu1000 gensets.  Thanks Wind-Sun for this great Forum.
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  • spacebass
    spacebass Registered Users Posts: 87 ✭✭
    #3
    Vic said:
    Have you looked at some of the LiFePO4 battery tear-downs on youtube?

    If not, look at the LithiumSolar teardown of an SOK server battery:
    https://www.youtube.com/watch?v=POf424nczxg

    Around 8 minutes in,  some clear images of the BMS connections, using lugs on fine-strand cable.

    Have never built any battery,   ...   yet.   Good Luck, Vic
    Hi Vic thanks. for your answer I have watched a couple of you tubes by that young american guy who is very good but they have not actually addressed the question I have also read that welding cable has thin strands not suitable for lugs , the idea of thin strand cable is a whole new ball park in itself , this small building step seems to have a lot of technicality to it , beyond just the welding skill !!! I have never built a battery before I think I can do it , the You tube videos are very helpful. I am surprised there are not more questions about fixing the cable on the boards.
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  • Vic
    Vic Solar Expert Posts: 3,208 ✭✭✭✭
    #4
    OK,

    Usually, fine strand cable can work fine, with lugs that are intended for use with the exact cable size and stranding.

    Have you tried the DIYsolarpowerforum?:
    BMS (Battery Managment Systems) | DIY Solar Power Forum (diysolarforum.com)

    Good Luck,  Vic
    Off Grid - Two systems -- 4 SW+ 5548 Inverters, Surrette 4KS25 1280 AH X2@48V, 11.1 KW STC PV, 4X MidNite Classic 150 w/ WBjrs, Beta KID on S-530s, MX-60s, MN Bkrs/Boxes.  25 KVA Polyphase Kubota diesel,  Honda Eu6500isa,  Eu3000is-es, Eu2000,  Eu1000 gensets.  Thanks Wind-Sun for this great Forum.
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  • Photowhit
    Photowhit Solar Expert Posts: 6,006 ✭✭✭✭✭
    #5
    We have some people here that are knowledgeable on the care and feeding of lithium battery banks.

    From my limited knowledge, The BMS (Battery Management System) just keeps the cells in close balance with each other, send small amounts of current from those with higher capacity/voltage to those with lower capacity/voltage (I use capacity/voltage because I'm pretty clueless as to how they determine which cells to send current to, unless they are doing it from reading the voltage)

    I've seen pretty big banks built with pretty minimal wiring between the BMS and the cells. Larger wiring would be needed between the actual batteries. I would use a breaker or fuse to protect the wiring between the batteries at some point in the circuit, likely going to the inverter unless multiple strings of batteries are in use.

    I suspect a lithium battery user will be along to give more information.

    Here's someone I have no clue about, hooking up a BMS, using ring connectors rather than solder the wires, state's "...The BMS balance wires are tiny (24 AWG)"

    Using A Battery Management System (BMS) - DIY Lithium Battery — Roam Lab

    Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Magnum MS4024, Prosine 1800(now backup) and Exeltech 1100(former backup...lol), 660 ah 24v Forklift battery(now 10 years old). Off grid for 20 years (if I include 8 months on a bicycle).
    - Assorted other systems, pieces and to many panels in the closet to not do more projects.
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  • Horsefly
    Horsefly Registered Users Posts: 476 ✭✭✭✭
    #6
    spacebass - What brand and model of BMS do you have? Can you post a picture of it, and the wires you are talking about?

    As Photowhit said, the balance leads of a BMS are generally very small. They are only measuring the voltage at each cell and sometimes extracting some balance current to keep cells close.  There are two wires that carry all of the current from the battery. One is generally labeled "B-" and goes to the negative terminal of the assembled battery. The other is either "C-" or "P-" and is the negative lead going out to loads and charging sources. These need to be big enough for the current the battery will be providing. In your case you have a 120A BMS, suggesting these two wires should be sized for 120A.

    Some BMSs have these B- / P- wires already soldered to the BMS main board. Some have threaded pads where you can attach ring terminals. I just don't know what you have.
    Off-grid cabin: 6 x Canadian Solar CSK-280M PV panels, Schneider XW-MPPT60-150 Charge Controller, Schneider CSW4024 Inverter/Charger, Schneider SCP, 8S (25.6V), 230Ah Eve LiFePO4 battery in a custom insulated and heated case.
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  • spacebass
    spacebass Registered Users Posts: 87 ✭✭
    edited August 2022 #7
    Horsefly said:
    spacebass - What brand and model of BMS do you have? Can you post a picture of it, and the wires you are talking about?

    As Photowhit said, the balance leads of a BMS are generally very small. They are only measuring the voltage at each cell and sometimes extracting some balance current to keep cells close.  There are two wires that carry all of the current from the battery. One is generally labeled "B-" and goes to the negative terminal of the assembled battery. The other is either "C-" or "P-" and is the negative lead going out to loads and charging sources. These need to be big enough for the current the battery will be providing. In your case you have a 120A BMS, suggesting these two wires should be sized for 120A.

    Some BMSs have these B- / P- wires already soldered to the BMS main board. Some have threaded pads where you can attach ring terminals. I just don't know what you have.
    Dear Horsefly thank you for your interest , I am baffled why people introduce sense wires into the conversation when I clearly define in the title 'big wires'.
    I uave decided to solder the wires onto the board as the posts I ordered are too big for the holes. I attach a picture as requested and look forward to any comment You may have.I am going to attach 16mm sq cables U.S 5 gauge I think, multistrand but open to suggestions there.I also wonder why there are two solder points on each side seem to be coming from the same track , is that a capacity thing ?
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  • spacebass
    spacebass Registered Users Posts: 87 ✭✭
    #8
    spacebass said:
    Horsefly said:
    spacebass - What brand and model of BMS do you have? Can you post a picture of it, and the wires you are talking about?

    As Photowhit said, the balance leads of a BMS are generally very small. They are only measuring the voltage at each cell and sometimes extracting some balance current to keep cells close.  There are two wires that carry all of the current from the battery. One is generally labeled "B-" and goes to the negative terminal of the assembled battery. The other is either "C-" or "P-" and is the negative lead going out to loads and charging sources. These need to be big enough for the current the battery will be providing. In your case you have a 120A BMS, suggesting these two wires should be sized for 120A.

    Some BMSs have these B- / P- wires already soldered to the BMS main board. Some have threaded pads where you can attach ring terminals. I just don't know what you have.
    Dear Horsefly thank you for your interest , I am baffled why people introduce sense wires into the conversation when I clearly define in the title 'big wires'.
    I uave decided to solder the wires onto the board as the posts I ordered are too big for the holes. I attach a picture as requested and look forward to any comment You may have.I am going to attach 16mm sq cables U.S 5 gauge I think, multistrand but open to suggestions there.I also wonder why there are two solder points on each side seem to be coming from the same track , is that a capacity thing ?

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