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Thread: Battery isn't charging up like it used to; faulty bms or did my max voltage drop?

              
   
   
  1. #1
    Member 4dollarjoe's Avatar
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    Battery isn't charging up like it used to; faulty bms or did my max voltage drop?

    Hey guys I have an 8 module leaf back with a mini bms. Max voltage is 67.2 v. I had it in a shed over winter. First time charging I got 66.3 v. Now it only charges to 65v.



    I just checked each cell, 4.05v across the board.

    To be clear I have had two instances where the battery has gone under.

    Once was during a cold spell at school. Caused the battery to go under my 48v minimum. I found out once I heard the alarm from the parking lot.

    Second instance was me riding and forgetting about how much the voltage drops once you hit 54v. I got it to charge about 1 minute after the alert.


    There was another instance where the bms has done what I believe was a false lvc event. I accidentally left my audio amp on overnight and it drew phantom power. the bms lights were off and but the pack voltage was reading 52v which confused me.



    I don't think it's an issue with my charger, since the relay led is physically turning off once it's done and I hear the buzzer.




    In addition I have my bms in a case where the radiator would be on most bikes. I am using very long wires to connect things so could impedance possibly be an issue?


    Also do these batteries loose their max voltage with use? I know you can loose some of the capacity, but would something like this drop 2v when I haven't even ridden it 25 times?

    Do you think I could possibly 'dumbcharge' it up to 67.2 v, or is this a bad idea? I also have a power supply I can charge each cell with, but that seems very time consuming and it seems they are already balanced.


    Any suggestions are appreciated.

  2. #2
    Member 4dollarjoe's Avatar
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    Hmm, tested what I believe is the cell which is triggering the hvc event and it only read 4.15v at the exact moment it was done charging. Very weird. Im using 14 gauge wire to connect this and some sections could be 6 feet in length.


    I talked to a guy who works at the battery company next to where I work and he actually found some research notes on these. Even in torture tests the cells could go back up to the 4.2 line. I think somethings funky with the bms

  3. #3
    Empulse R #24 frodus's Avatar
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    Why charge above 4.15VDC? That's much safer than going all the way to 4.2V and is actually recommended.

    It's likely small differences in the tolerance of components on the BMS, and yes, they can change over time. What is the HVC of the MiniBMS?

  4. #4
    Member 4dollarjoe's Avatar
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    For the cell boards I got the leaf version which says hvc is at 4.2 v


    I'm only really concerned because last week I went on a ride starting at 63 or 62v and figured I'd have enough to go to the beach and back. Pack ended up dying 20 miles later; I was riding pretty hard though. Again I did't go in on a full charge. I've had it in the shop I work at so I can install a chain guard and finish the last section of my gas tank. Got it to charge up to 65.7 v yesterday so we will see.

  5. #5
    Empulse R #24 frodus's Avatar
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    You sure it isn't your charger not being able to get up to the correct voltage?

    Check for long charger wires, high resistance connections, etc.

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