You have to compare them at the (roughly) same cell voltages. As @Evwan mentioned, the capacity differences are “magnified” at lower cell voltages since the packs are top-balanced. That 3.6V pack could easily have the same total cell voltage difference as the other one if discharged down to the same voltage.
At 3.6V you can have a very large capacity difference for a very small voltage change since the discharge curve is very flat at around the 50% charge point (near 3.6V).
At under 3.0V you have much larger voltage changes from much smaller changes in capacity. That is, the same amount of self-discharge when under 3.0V (roughly) will result in a much larger voltage change than when at 3.6V.
I agree with @PedroMcJimenez, sounds like something is draining the pack. The odds of all the p-groups self-discharging down at roughly the same rate is very low IMO.
Thanks @Pecos. Very helpful thoughts. I added some supports so that the 2s and 3s sections will have a flat and rigid interface with the main pack.
I’ve been dubious about using hot glue or silicone because the hold is not great in my experience. I’ve started using a high viscosity epoxy to assembly everything, which takes forever but seems to be working well.
In my test the batteries can still be separated but it takes a good amount of force. No damage to the batteries as the epoxy sticks to the 3d print. The glue will be more rigid than hot glue so may be not as good once the pack starts thermal cycling. I’ll post some pics before I start to weld.
I converted a 12s pack to 11s because a group went bad, so I thought I could splice the 11 and 12 balance leads to the jbd smart bms… But instead of seeing identical voltage for 11 and 12 I get this. What am I doing wrong?
Plus you have to change the series count in firmware. Dunno if that can be done in the app, last I tried that didnt work. The uart to usb adapter may be needed
You can do it with certain app versions but not others. The newer auto sensing ones don’t need to be changed, they just do it automagically, but that balance plug looks like an older one, so you’re likely right.
I tried changing it in the app to 11, but I basically get the same issue. I even removed the 12th balance lead from the plugs, but nothing changed except the location of the ghost group.
Voltage is always relative (5V doesn’t exist in a vacuum, it can only be 5V above something else). BMSs have to read the difference between wire A and wire B, and display that.
So the voltage displayed in group 1 is the difference in voltage between the Ground wire and the 1+ wire, group 5 is the difference between wire 4+ and 5+, and group 12 is the difference between 11+ and 12+. So if you short 11+ and 12+, it’ll read as zero. As the others said you’re likely able to program the BMS to account for this and just not show group 12
It might be a bad connection to the balance leads or bad welds? I’m not saying rebuilding the battery is a great idea, but it could be that it’s a balance lead not able to properly read the voltage rather than the cells being cooked. If when you open it up the tabs for those groups kind of fall off or separate from the cells then it’s obviously a weld problem, if the balance lead is heavily kinked or nearly broken or has a weak solder joint then it’s still likely a measurement error, but if you can probe directly to the cell terminals and they’re still reading <2.5V then yeah she’s done
Thanks for your help. I’ll check balance leads first, then p group voltage. I wont be too devastated if it needs to be a 10s battery as it’s only my short board that my son mainly rides now. I’ll have a bit more practice soldering I guess haha.
Makes complete sense now… But I tried altering the cell count to 11 on the app with the 12th +pin disconnected and I wad still getting a similar voltage reading but it moved from the 12th to 11th group.
Yeah I haven’t used that type of BMS at all so I’m not sure how to configure it. I do know some BMSs draw their supply voltage from the top wire so it might get angry if you don’t have 12 plugged in. That’s I think why the pin arrangement matters when setting a BMS up for fewer groups than its max, some of the pins have other functions like supply voltage so you can’t disconnect them at random
