troubleshooting 6S BMS for electric scooter

I have searched for an inexpensive BMS for a 6S8P battery for my son's electric scooter. I bought a cheap one from ebay but after soldering it together, one parallel string seemed to drop to 3.8V while the others are still at 4.18 or so. I have since charged that string separately to 4.18v and I will be watching it closely. This occurred within a couple weeks of soldering while waiting for a 25.2v power supply to arrive. ( the BMS specs listed a charging voltage of 25.5, not sure if this is the source of my troubles) Once the power supply arrived, I verified the 25.2v before connecting it to the P- and P+ ports on the BMS. None of the parallel stings charged, including the 3.8v string after sitting overnight. The power supply light was green the whole time. I then discharged the pack slightly and reconnected the power supply. This time, the red light illuminated for about 15min, then switched to green. Even when the green light was lit, there was 25.2v on the P- and P+ ports. I'm confused as the BMS was listed as having a balancing function. The power supply is a 2A unit and I realize that it will be a very slow charge but I saw absolutely no increase in voltage on any string after sitting overnight. What gives?

I have ordered another type of cheap ebay BMS to try out separately. In the meantime, I am open for suggestions in a better BMS or approach to troubleshooting.

I don't want to spend a fortune on the BMS as I plan on making a few 6S packs for scooters, ebikes, etc.

Thanks for reading and I look forward to your comments!

What is each cells individual voltages when "charging" ?
If any one of them goes above 4.2 the BMS will disconnect charging directly. Just a guess on whats going on.

daromer said:

What is each cells individual voltages when "charging" ?
If any one of them goes above 4.2 the BMS will disconnect charging directly. Just a guess on whats going on.

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That's sort of what I was thinking initially so I discharged the pack to make sure that all cells were below 4.15. But now that I have the one parallel string charged up to the same voltage as the other cells, I think I will discharge the entire pack to 4v per stage just to see if the BMS will allow charging again.

So after charging the low cells externally, I then discharged the entire battery to 4v per stage, then charged the entire battery through the BMS. Now, the voltages are 4.22 per stage with only one stage reading 4.16.
4.22 seems a bit high doesn't it? I would rather not exceed 4.15 per stage. thoughts?

Not really. A cheap BMS will only start to drain energy from Cells when they hit 4.2V Now with one cell being low the other will be getting above 4.2v That's the problem with BMS's that drain current slowly. sounds like you have a problem with a couple cells. Personally I'd capacity test each cell (not physical but electrically speaking) and see how each cell behaves. If one has higher capacity or one takes longer to fully charge or less time to discharge you have problems.

jdeadman said:

Not really. A cheap BMS will only start to drain energy from Cells when they hit 4.2V Now with one cell being low the other will be getting above 4.2v That's the problem with BMS's that drain current slowly. sounds like you have a problem with a couple cells. Personally I'd capacity test each cell (not physical but electrically speaking) and see how each cell behaves. If one has higher capacity or one takes longer to fully charge or less time to discharge you have problems.

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I have tested each of the 48 cells and they are all close to 2400mah.

Any recommendations on a better BMS for 6S?