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Bike keeps turning off, battery full

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    Bike keeps turning off, battery full

    Hi. I have an ebike which keeps turning off after only a couple of minutes, 48v battery, 250w bafang motor. Battery is probably 6years old. Charger won't put any more charge in, and battery indicator says full on the controller.
    I've pulled the battery apart and tested the following with multimeter.
    Battery measures 52.5V
    Individual readings on the groupings are
    1. 4.02V
    2. 4.03V
    3. 4.06V
    4. 4.02V
    5. 4.03V
    7. 3.8V

    These readings seem ok, maybe one lowish, but is it likely the culprit?

    My controller has a 41V cut out protection.
    I tested the voltage immediately after it stopped, still at 52.X, so that doesn't seem like it (unless the controller gets faulty??)

    Also of interest, when it stops, I can't turn it back on for hours. HoweverClick image for larger version  Name:	20240219_161005.jpg Views:	0 Size:	1.92 MB ID:	166380, if I disconnect and reconnect the battery (bullet connectors give a big spark), it will work however still cuts out soon after.

    What would you suggest from here? Battery obviously could be a goner, but interested if it's recoverable.

    Picture of battery attached if helpful.

    Thanks a lot.


    P.s, not sure why the pic is in the middle of the post??
    Last edited by Mcsydney; 02-19-2024, 02:23 AM.

    Can you rig up some sort of a test load and measure/monitor the groups with a load? Something like an old school high wattage quartz lamp? Or dumb electric cooker? Has to be dumb stuff that is basically just a resistor. Can't have any electronic controls because those won't work on DC and the wrong voltage. I'm thinking you may have one bad group like #6 or #7 that drops fast under load and the BMS then cuts the pack off to protect it. Its not showing that bad after charging or by the time you get to it because its sort of being charged by the other groups.


      FWIW there are 13 groups (vs. 7) in a 48V battery



        Thanks 73Eldo. I wondered about something like that, it really needs real time measurement I guess. I'll have to find something, I get rid of all my old junk these days as I don't have room.
        Out of curiosity, how many bad individual cells would be needed to cut out so quickly? Is it likely to be multiple, 10's, half of the battery?

        Thanks EZguy, noted on the 13, however there doesnt seem to be a way to measure 13, only 7 due the connections (without pulling apart). Am I missing something?


          Is there no access to the back side of the battery? You definitely have 13 series cells. If you just go down the row and measure each top to bottom, that should show the voltage of each parallel group.

          FWIW, I had this same dying/shutdown issue with a battery/controller. I finally pulled the pack apart and found 3 of 13 groups were down around 3.4V while others were up near 4. The BMS failed and wasn't balancing cells correctly. I manually balanced those 3 groups and put in a new BMS. Good as new now.

          Click image for larger version

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            Thanks K442. Yes, can access back..I only measured the voltage at the 7 wires to the BMS.
            Measuring all 13, most are 4-4.04v. One is 3.8v. One is 3.9v.

            Would trying to charge these back to 4v make much difference? It seems so small.

            I'll try put some load on the battery as suggested by 73eldo. I'm a bit unsure how to get 48v on it.
            Is it possible to test the individual rows of 4V with load as I have smaller voltage items I could rig up. I assume the rows would need to be separated to avoid drawing from the other rows?


              Further to this, I've improvised, and managed to rig up a way of testing under load. I think I may have a few dodgy cells, but keen to hear what is acceptable or not. Interestingly, the voltage on the group of cells with the low voltage didn't move much?

              I had the bike freewheeling while balancing on its stand against the bench, and i could throttle on. I did two tests:
              Light Load mean flat out acceleration , but the wheel is just spinning in the air, so not really much resistance except at start up.
              Heavy Load: I jammed some wood for friction in the tyre. I didn't go flat out, because it jammed to hard, but got up to about 10kph to keep the wheel spinning.
              Cell #
              (1 closest to BMS)
              Resting V Light Load V drop Light % change light Heavy Load V drop heavy % change light
              1 3.78 3.76 0.02 0.53% 3.70 0.08 2.12%
              2 4.02 3.93 0.09 2.24% 3.70 0.32 7.96%
              3 3.98 3.93 0.05 1.26% 3.90 0.08 2.01%
              4 3.98 3.84 0.14 3.52% 3.84 0.14 3.52%
              5 4.01 3.81 0.20 4.99% 3.80 0.21 5.24%
              6 3.98 3.85 0.13 3.27% 3.81 0.17 4.27%
              7 3.98 3.85 0.13 3.27% 3.69 0.29 7.29%
              8 4.01 3.78 0.23 5.74% 3.50 0.51 12.72%
              9 4.02 3.63 0.39 9.70% 2.98 1.04 25.87%
              10 3.92 3.87 0.05 1.28% 3.80 0.12 3.06%
              11 4.00 3.86 0.14 3.50% 3.75 0.25 6.25%
              12 4.00 3.93 0.07 1.75% 3.83 0.17 4.25%
              13 4.01 3.94 0.07 1.75% 3.89 0.12 2.99%
              Here's the bike set up.

              Click image for larger version

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              So, from my numbers, the rows in red (2, 7.8.9,11) all had the biggest percentage drops.
              Anyone know what is acceptable?
              If all are suspicious, is it a case of pulling apart and testing cells individually in a tester?

              Also, for my knowledge on the BMS, does the BMS use the worst row as its base line (row11 = 2.98V under load, motor cuts out) = does that mean the BMS thinks the whole battery is 38.74V (13 x 2.98v), which is below the 41V controller cutout?

              Thanks again.
              Attached Files


                A bit hard to say without knowing the exact cells used in the battery. But if you know those, you can look up the spec sheet and see what the nominal numbers are.

                Either way, you have some groups which drop 2-3% under load... seems pretty normal. Then you have some which drop 12 and 25%...that seems a little excessive. If they were low voltage to begin with, then it might be a balance issue. But the 12 and 25% drop cells you show are right at 4 volts open circuit. To me, that points to a bad parallel group, or at least multiple bad cells within that group. Likely the cells have degraded and have high internal resistance.

                On the BMS, it could be monitoring individual groups and shut down if any one drops below a threshold. I think 'dead' is generally considered around 3.3-3.4V and 'minimum safe' is generally around 3.0V, so dipping down into the 2.9 range, or lower, could trigger some sort of shutdown.

                Neat bike, BTW! Spare a few drops of oil on that chain and it will thank you immensely! Hope you can get the battery issues sorted and back on the road.


                  A battery is only as strong as the weakest cell - this battery seems to have more than a few that are a bit shagged

                  Seems like time for a new battery


                    Thansk guys
                    looking at the cost and effort to buy the cells, charger and spot welder, think I might just get a new battery.
                    I was interested as a bit of an education, project. But probably not much point
                    noted on the oil. Its been sitting for a year, unused.


                    • AZguy
                      AZguy commented
                      Editing a comment
                      Building _a_ battery doesn't make sense from an economic standpoint - only good reason to build one is for the hobby aspect...

                      The tools and material for building/repairing aren't inexpensive

                      Repairing one is almost never a good idea... about the only exception would be a nearly new battery that had an early death of one group...