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JBD BMS not balancing?

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Batvette

Solar Wizard
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Feb 25, 2021
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This is a simple 4 cell 280ah 12v bank in a mobile situation. Currently charging solely with a heavy duty power supply via generator about twice a week.
This battery has been in service over a year, cells were top balanced.
The issue Im having is that when charging, cell 1 is much higher voltage than the others, cell 3 is lower. See screenshot 1.
Screenshot 2 shows voltage at rest, perfectly balanced.
2 more images show relevent settings.
Screenshot_20230328_220440_xiaoxiang.jpgScreenshot_20230329_021628_xiaoxiang.jpgScreenshot_20230328_220624_xiaoxiang.jpgScreenshot_20230329_022045_xiaoxiang.jpg
Shouldnt the BMS have these balanced better at this point? Its not practical for me to change positions of the cells, theyre put together with double sided tape. Maybe wasnt the best option lol.
Any help is appreciated.
 
Get a DC power supply capable of delivering 5-10 amps. Manually charge each cell up to 3.65V without disconnecting it from the pack.

Check the "charge balance" option in the manual. I think you may have to enable it.

The BMS may be balancing, but some cells may have a higher self-discharge rate. And if the rate is too high the BMS balancing will not be sufficient. In such case you'll have to maintain the top balance manually. For my camping pack, this is like once each 2nd year.
 
The balance current of the JBD is very low (usually only around 150ma) and it's passive balancing.
With large cells it simply won't keep up.
Maybe you should add an active balancer. These usually have 2 to 5 amps of balance current.
 
'Charge Balance' is not a great name for the function. It means charging current direction must be detected to enable balancing.

On some BMS's this means either with charging (ON) or no charging (OFF). What it should be is either only during charging ('ON') or during charging or discharging ('OFF').

Because balancing also typically should only be done above 3.40v on a cell, this generally means it only happens during charging since to get above 3.40v on a cell you pretty much need to have some charging current to boost cell voltage above that level.

When you have 'charge balancing' turned off, with a BMS that then only allows balancing during discharging, and start of balancing set above 3.4v, this pretty much prevents any balancing from happening.

In this situation, when charge balancing is turned 'OFF', you only get some balancing on a cell that trips cell overvoltage protection, where charging current is stopped and cell voltage is bled from overvoltage value down to 3.4v.

This is why 'charge balancing OFF' should allow balancing during discharging or charging. A BMS should also allow balancing dump even when charging balancing turn 'ON' when charging is stopped due to a cell overvoltage. This allows an overvoltage cell to be quickly discharged to reset the BMS charge disable.

Turn 'charge balancing' ON.
 
Last edited:
RCinFLA said:
'Charge Balance' is not a great name for the function. It means charging current direction must be detected to enable balancing.

On some BMS's this means either with charging (ON) or no charging (OFF). What it should be is either only during charging ('ON') or during charging or discharging ('OFF').

Because balancing also typically should only be done above 3.40v on a cell, this generally means it only happens during charging since to get above 3.40v on a cell you pretty much need to have some charging current to boost cell voltage above that level.

When you have 'charge balancing' turned off, with a BMS that then only allows balancing during discharging, and start of balancing set above 3.4v, this pretty much prevents any balancing from happening.

In this situation, when charge balancing is turned 'OFF', you only get some balancing on a cell that trips cell overvoltage protection, where charging current is stopped and cell voltage is bled from overvoltage value down to 3.4v.

This is why 'charge balancing OFF' should allow balancing during discharging or charging. A BMS should also allow balancing dump even when charging balancing turn 'ON' when charging is stopped due to a cell overvoltage. This allows an overvoltage cell to be quickly discharged to reset the BMS charge disable.

Turn 'charge balancing' ON.
Click to expand...
Ok thanks for that it was off because I saw a thread in here last year advising that I will try it for awhile.
 
RCinFLA said:
'Charge Balance' is not a great name for the function. It means charging current direction must be detected to enable balancing.

On some BMS's this means either with charging (ON) or no charging (OFF). What it should be is either only during charging ('ON') or during charging or discharging ('OFF').

Because balancing also typically should only be done above 3.40v on a cell, this generally means it only happens during charging since to get above 3.40v on a cell you pretty much need to have some charging current to boost cell voltage above that level.

When you have 'charge balancing' turned off, with a BMS that then only allows balancing during discharging, and start of balancing set above 3.4v, this pretty much prevents any balancing from happening.

In this situation, when charge balancing is turned 'OFF', you only get some balancing on a cell that trips cell overvoltage protection, where charging current is stopped and cell voltage is bled from overvoltage value down to 3.4v.

This is why 'charge balancing OFF' should allow balancing during discharging or charging. A BMS should also allow balancing dump even when charging balancing turn 'ON' when charging is stopped due to a cell overvoltage. This allows an overvoltage cell to be quickly discharged to reset the BMS charge disable.

Turn 'charge balancing' ON.
Click to expand...
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.
This is because the internal shunt has some sort of threshold, that below it it can't sense the charge current. From the various JBD models I've used, this is around 400mA.
So, let's say that you charge with 200-300mA, the BMS will not show/sense its charging and the balancing will still occur.

As the balance current is very low in the JBD as I mentioned, you might have to go as low as the balance current (or even less, like 100mA) in order to avoid cell over voltage protection, and still be charging.

I have used this method a few times, but the downside of this is that it takes a very long time to balance, depending on the cell capacity and how much they cells have drifted. But it's still a good method for battery packs that are sealed or if you don't want to open and manually rebalance.
 
meetyg said:
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.
Click to expand...
That is likely because current measurement accuracy and possible DC offset on the shunt voltage amplifier op amp.

There is a similar offset/error margin put into the charge disable function that overrides charge disable when discharging current gets greater than a few amps to prevent overheating of turned off series charge control MOSFET due to its body diode voltage drop. It uses shunt current measurement to check for possible charge current direction to re-disable charging when overridden for high discharge current pass.

Because of this, and slow turn off time on series pass charge control MOSFET's, there may be a short squirt of charging current when discharge current changes to a charge attempt by inverter/charger. If cells are poorly balanced and you repeatedly keep crossing this charge detection threshold current, the multiple short 'squirts' of charging current can cause a cell overvoltage that keeps rising, due to the short charge current squirts, to cell damaging level.

Most BMS's don't put strong gate drivers for the series pass MOSFET's which have a lot of gate capacitance. This slows their turn off / turn-on time.

This video shows the situation.

General BMS charge-discharge Disable function.png
 
Last edited:
Batvette said:
Ok thanks for that it was off because I saw a thread in here last year advising that I will try it for awhile.
Click to expand...
However, you are still going to need to manually intervene to restore balance. The onboard balancer in your BMS will not recover from where you are right now.

Edit.. as previously stated use a bench power supply to raise the voltage of low cells. I also wired up a 5 volt flashlight bulb with a little switch and some wires to alligator clips to be able to burn off power from high cells.
 
As an alternative approach - get a 4S active balancer. But using it once per year will likely not justify the cost.
 
I added an active balancer for this and at first I had it on a switch so I could manually turn it on and off when charging.. then I added a voltage activated relay which turns it off once the pack gets below 3.37v per cell.
 
TomC4306 said:
However, you are still going to need to manually intervene to restore balance. The onboard balancer in your BMS will not recover from where you are right now.

Edit.. as previously stated use a bench power supply to raise the voltage of low cells. I also wired up a 5 volt flashlight bulb with a little switch and some wires to alligator clips to be able to burn off power from high cells.
Click to expand...
I might try the bulb hack you mention. I have a BK power supply 0-18v 5 amp, (though gets very hot if you run at more than 4 amps) can I charge just one cell without disconnecting the bus bars?
 
Batvette said:
I might try the bulb hack you mention. I have a BK power supply 0-18v 5 amp, (though gets very hot if you run at more than 4 amps) can I charge just one cell without disconnecting the bus bars?
Click to expand...
Yes, while watching the relative cell voltages on your BMS app. Be very careful to observe the correct polarity. Red power supply lead to BLACK positive cell terminal.
I must say it out loud before connecting each cell that I want to boost. Red to Black.
 
augiedoggy said:
I added an active balancer for this and at first I had it on a switch so I could manually turn it on and off when charging.. then I added a voltage activated relay which turns it off once the pack gets below 3.37v per cell.
Click to expand...
Mind if I ask how you added the switch to the active balancer, and if you have details on the voltage activated relay as well I'd really appreciate that!
 
Ok revisiting this because I still have an issue, and I think I have a bad cell. They are EVE 280ah I got from Michael Caro about 2 years ago.
Attached are 3 screenshots the first while still charging. I have up to 60a to supply it with but thats the most it will take before cell 4 hits 3.65 and goes into protection. The second image is immediately after charging. The third is a minute later with a small load on the pack.
Should I assume cell 4 is bad?
TIA for your input.



Screenshot_20230612_010636_xiaoxiang.jpgScreenshot_20230612_010657_xiaoxiang.jpgScreenshot_20230612_010931_xiaoxiang.jpg
 
Batvette said:
Should I assume cell 4 is bad?
Click to expand...
Not yet.
This pack is wildly out of balance.
You need to perform a top balance.
Connect all four cells in parallel, attach a power supply with 10 gauge leads, set to 3.5 or 3.6 volts (set this before connecting to the cells and do not touch the setting after connecting to the cells) maximum current.
No alligator clips.
Leave them hooked up this way until amps is less than 0.5.
Disassemble, while reconnecting in series with your BMS, use a cotton swab and isopropyl alcohol to clean every metal surface.
Inspect carefully and ensure there is no debris on any metal surface before finalizing each and every connection.
Buy a torque wrench and actually torque the connections properly.
Cycle for a week and observe behavior. Then you will be able to make a decision well informed about the relative health of your cells to one another. Not before.
 
HRTKD said:
What Tom said! Half the voltage issues seen on the forum are due to poor connections.

What charge profile are you using? How many amps?
Click to expand...
Im charging it at the moment with a 75amp adjustable voltage power supply. Start at around 55 amps taper down to about 30.
When I get around to it it will be hooked up to a redarc 50a dc dc charger with 200watts solar like i had in my last van.
 
TomC4306 said:
Not yet.
This pack is wildly out of balance.
You need to perform a top balance.
Connect all four cells in parallel, attach a power supply with 10 gauge leads, set to 3.5 or 3.6 volts (set this before connecting to the cells and do not touch the setting after connecting to the cells) maximum current.
No alligator clips.
Leave them hooked up this way until amps is less than 0.5.
Disassemble, while reconnecting in series with your BMS, use a cotton swab and isopropyl alcohol to clean every metal surface.
Inspect carefully and ensure there is no debris on any metal surface before finalizing each and every connection.
Buy a torque wrench and actually torque the connections properly.
Cycle for a week and observe behavior. Then you will be able to make a decision well informed about the relative health of your cells to one another. Not before.
Click to expand...
All of the above was done when i assembled it, except for alcohol and a swab.

I prefer spit and my sleeve.

That was about 2 years ago, i will do it again.
 
If you back off the amps (and probably the volts) it will give the BMS more time to balance, assuming that balancing is working. Balancing is turned on, right?
 
HRTKD said:
If you back off the amps (and probably the volts) it will give the BMS more time to balance, assuming that balancing is working. Balancing is turned on, right?
Click to expand...
Yup. Im charging it off a generator in a vehicle so time is a factor.
 
I also have a jbd BMS, and had much better result with charge balance turned off. The reason, as I understand, is as stated that the BMS considers anything below 500mA as not charging.

I have set, in my SCC, boost duration to 120min (starting when boost voltage (3.5v) is reached)

In my setup that 120min is much longer than the time batteries stay within the 3.45-3.5V range (only a couple of minutes).

Not ideal, but works for me.
 
I have another 4 cells and BMS Ive been procrastinating assembling. Im going to take that off the back burner in the next few weeks and do that then I can take this pack offline. And that will take better than a week to top balance those using power from the first pack through an inverter. In the meantime tomorrow Ill open this case up and check for any obvious issues and maybe look at it with my thermal cam while charging at max to see if theres any anomalies.
 
TomC4306 said:
Yes, while watching the relative cell voltages on your BMS app. Be very careful to observe the correct polarity. Red power supply lead to BLACK positive cell terminal.
I must say it out loud before connecting each cell that I want to boost. Red to Black.
Click to expand...
Sorry to jump in. I'm likely going to need to try this sometime. But when you say 'Red to Black' do you mean positive from power supply to positive cell terminal?
To me it didn't read like +/+ , but can't get my head around it the other way.
?
 
Cavendish said:
Sorry to jump in. I'm likely going to need to try this sometime. But when you say 'Red to Black' do you mean positive from power supply to positive cell terminal?
To me it didn't read like +/+ , but can't get my head around it the other way.
?
Click to expand...
Most (if not all) the prismatic cells have black as the positive and most (if not all) power supplies have black as the negative.
Just pay attention and say it out loud right before you connect the power supply cables to the live cell terminals. "Red is positive black is positive, red to Black."
Twice now I've had to replace the output diode on my benchtop power supply.
 
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