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Battery issue: Dead Cell Group

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    Battery issue: Dead Cell Group

    My friends bike battery was only charging to 53.1v and then shutting off. So he asked me to take a look at it.

    I built this battery probably a year or so ago and it's big. 52v 38ahr. Yes that's right 11 x 14.
    Typically use Sanyo GA 18650 cells rated for 3.45mahr and 10a Max discharge and have never had a battery issue before, Purchase cells from the US so their legit and battery has been working fine for over a year or more. Also use a 30a BMS 14c 52v that are good quality. Been using a K-Weld spot welder and use good pure nickel belt material for my connections and only soldering for the sensing wires and the Battery Neg connect to the BMS and the Pos connections for the XT90 (Motor) and XT60 (charge).

    Problem #1 is it's a 52v battery which should shut-off at 58.8v.

    Checked with a meter at the XT90 (Load plug) and got 48.2v.
    Checked with a meter at the XT60 (Charge Plug) and also got 48.2v.

    Checked voltage between #1 group Neg and #14 group Pos and got 53.1v.

    So I figure BMS tripped so I unplug the sensing wire plug and then replug back in and no change in voltage readings.

    Now I check the XT90 Pos terminal and the #1 group Neg and get 53.1v.
    XT90 Neg terminal and the #14 group Pos and get 48.2v

    Same readings at the XT60 charge plug.

    So now I know there's a break of some sort between the last 2 battery cell groups.

    So I start at the beginning and read from the #1 group Neg to the #1 group Pos and get 4.2v Good.
    #1 group Neg to the #2 group Pos 8.2v Good. (4.0v)
    #1 group Neg to the #3 group Pos 12.3v Good. (4.1v)
    #1 group Neg to the #4 group Pos 16.5v Good. (4.2v)
    #1 group Neg to the #5 group Pos 19.9v OK (3.4v)
    #1 group Neg to the #6 group Pos 24.1v Good. (4.2v)
    #1 group Neg to the #7 group Pos 28.2v Good. (4.1v)
    #1 group Neg to the #8 group Pos 32.3v Good. (4.1v)
    #1 group Neg to the #9 group Pos 36.5v Good. (4.2v)
    #1 group Neg to the #10 group Pos 40.6v Good. (4.1v)
    #1 group Neg to the #11 group Pos 44.8v Good. (4.2v)
    #1 group Neg to the #12 group Pos 49.0v Good. (4.2v)
    #1 group Neg to the #13 group Pos 53.1v Good. (4.1v)
    Here it comes the issue: #1 group Neg to the #14 group Pos 53.1v BAD. ( 0v )

    Every jumper from each group ( 1-2, 2-3 3-4 etc) is fine, no breaks or discoloration, no visibly obvious sign of trouble. All still solidly connected (Spot Welded).

    So now I physically remove the connections between the #13 group and the #14 group. Then I remove the connections between the #14 group itself and test each battery separately and got this.

    #1- .9v
    #2- .9v
    #3- .8v
    #4- 0v
    #5- .7v
    #6- .9v
    #7- .1v
    #8- 0v
    #9- .5v
    #10- .5v
    #11- .1v

    All the sensing wires tested good for continuity so no breaks in connections.
    So basically #14 group not receiving a charge. Could a faulty BMS cause a specific group to 100%+ discharge?
    Individually load tested a couple of the batteries only 3 out of the 11 would hold any type of charge 3.6v or so the rest stayed below 1v.

    What could of caused a total group failure? Gotta be something that in common with that group correct? A cell goes bad and of course it lowers the voltage of the others in the group and the sensing wire doesn't pick up on that and tries to balance the groups?

    Pack is fully charged up ( minus the #14 group ) so Im separately charging (11) new individual batteries to 4.2v before I reconnect them back into the main pack to keep potential roughly the same all around. After I get those all in and everything connected back up (including BMS) I will attempt to top off charge and see if it takes. Odd first time I have seen this and it's killing me.

    Any ideas Im all ears thanks for listening and reading this whole novel.

    Hey guys thanks for all the helpful comments. I got the (11) new cells installed and connected ( spot welded ) back up and plugged in the charger and it went up to 58.2v. I think it will take a few charge/discharge cycles to get the max 58.8v. Everything tested out correctly voltage wise so Im good to go. Still wondering what the cause was.


      Battery left sitting around for a while?


        No, battery was getting weekly usage. My guess is the BMS crapped out and didn't for whatever reason balance charge the last #14 cell group, somehow isolated it let them all extremely discharge down to nothing. Basically making it a 48v battery at that point but the LV shutoff was still for the 52v range. Like I said all connections were rock solid as far as cell to cell was concerned. One for the books.


          Are you still using the same BMS? If so, I guess you'll know in fairly short order if the BMS is bad or not.

          Though I do wonder, as we make bigger battery packs, it seems like more chance for something to go wrong. As you go from 11, 12, 13 to 14, 15+ series groups, each time adds another circuit, voltage sensor and mosfet group to the BMS. Plus as you add more batteries in parallel, more chance to get one random failure and drain the whole parallel group.

          FWIW, I seem to be having a rash of failures, too. Earlier this year, seems the BMS in my X1 flaked out and wasn't charging (or was actively discharging) 3 groups of the 13S battery. Then on a totally different project with a 6s3p pack, I found out yesterday one of the parallel groups was down to 1.8V while the rest were ~4.09. Not yet sure if the BMS flaked out or a bad cell in that group. Then yet another project I had one cell go bad out of a 4P group and zap it down to 0.8V.

          So who knows... more sunspots or cosmic rays all of the sudden?!?


            Be glad you don't have a Tesla with 7000 cells in it. I would imagine it doesn't take many duds there or loose BMS wires to crash the party there either.