JBD BMS not balancing?

[SupraSPL]

When Charge Balance is turned OFF, the BMS will still passive balance the high cells when:
-Cell is above 3.4V
-Difference between cells is more than 0.015V
-Battery is not discharging
-Battery is not charging

Balancing above 3.4V makes sense because above that, the cells start to increase voltage rapidly, which reveals the imbalance and let's the BMS work on it (albeit very slowly). When resting, the cells will fall below 3.4V, which will stop the balancing eventually. If you still need more balancing after this (could take a day) then you can charge back up to 3.5 or 3.6V and let it balance again.

You can do some decent maintenance top balancing this way and you can even tighten them up to .005V instead of .015V in the settings.

Once that is done it probably makes sense change it back to Charge Balance ON, so you get a small amount of balancing on a regular basis during regular use.

 

[RCinFLA]

I agree that the JBD balancing logic is somewhat faulty. But, I have noticed that with charge balance OFF, if you charge the battery with low enough amperage you could still get some balance while charging.

That is because cheap BMS's have problems with DC offset drift on their op amp that amplifies the small DC voltage drop across its current shunt resistor. To avoid the possibility of showing some negative current when there is actually a small amount of positive current from battery they throw a limit value of an amp or two, below which they show zero current. Basically they are masking the operational amp offset drift issue by creating a dead zero current zone for small positive or negative BMS current flow.

This offset is necessary if you turn on charge or discharge disable. When either is disabled the BMS current passes through some MOSFET body diodes, which are the turned off MOSFET's to prevent either charging or discharging. If current gets too high it will over heat the MOSFET due to the body diode voltage drop of about 0.5 to 0.7 vdc. It must also monitor BMS current and override your disabled setting for charging or discharging while the current is high and turn both back to back MOSFET's' 'ON' to prevent MOSFET overheating.

This is also the same function used to shutdown charging for a cell overvoltage or shutdown discharge when a low cell is detected.

When in this 'dead' zone, balancing, no discharge, no charge, functions take their respective default operation best suited for the blind spot.

 

[Cavendish]

Shouldnt the BMS have these balanced better at this point?

Hi there. I hope you've got it solved.
Out of curiosity have you used a multimeter to confirm the cell voltages against the app data?
I am very new to all this. But I'm having the same results (very unbalanced cells shown by the app) however, when checked with a multimeter, all my cells appear to be within 0.002 volts of each other, with the exception of one with at 0.008v difference.

I am running a 4s3p battery bank, which might be the problem as I've just learnt from my supplier that JBD doesn't support parallel battery banks (obviously this doesn't apply to you). But I thought I would write and see if it was of use.

I will start to top balance them all again tomorrow and see if I can make it work.

 

[Batvette]

Hi there. I hope you've got it solved.
Out of curiosity have you used a multimeter to confirm the cell voltages against the app data?
I am very new to all this. But I'm having the same results (very unbalanced cells shown by the app) however, when checked with a multimeter, all my cells appear to be within 0.002 volts of each other, with the exception of one with at 0.008v difference.

I am running a 4s3p battery bank, which might be the problem as I've just learnt from my supplier that JBD doesn't support parallel battery banks (obviously this doesn't apply to you). But I thought I would write and see if it was of use.

I will start to top balance them all again tomorrow and see if I can make it work.

They improved quite a bit with just loosening each stud, moving the flexible buss bar back and forth, then retightening.

Note Im not using bolts in the aluminum posts, hell no. I got some good quality M6 bolts, cut them into threaded rods, chased the threads in the terminals with an end tap, then filled the holes with JB Weld and inserted the rods. So every time you tighten them youre turning an SS locknut on an SS threaded rod.