Problem charging batteries via bms

[TheKing]

I have small electric vehicle which work on 48V 16s 302A CATL battery, with Daly BMS 300A and BLDC 10KWwith Vec 500
Everything was ok, until have trouble with battery. When its start charging with ~SOC 30%, all 16 cells going around 3.350V (+- 0.05). After 30-60min some batteries, 4 pcs going to 4 to 4.2V while charging (screenshot attach). After that BMS turn off charging, suppose some cell is full charged. The result is that I can't fully charge the battery.
Also, I have a problem with the BMS from before, it shows about 75 A consumption or charging, and it has been checked that this is incorrect information (Image attached). The values of the individual batteries are correct, obtained by direct measurement with a multimeter
To summarize:
1. Why can't I charge the batteries through the BMS which is supposed to balance the cells until they are all charged equally?
2. Why does electricity consumption occur when there is no such thing. Can the other information from the App be trusted?
Can anybody help me with my problem?

[Bikemad]

1. Your 302A CATL cells are LiFePO₄, and these cells typically have a maximum charging voltage of 3.65V per cell - Not 4 to 4.2V.

Q: What is the balance function?
Do we need cell balance or not?
A: The working principle and function are as followings:
When your one cell voltage was reached balance voltage(Li-ion 4.18V, LifePo4 3.6V), that cell balance start work, balance resistance starts discharge with 35mA, that cell was in both charging and discharging status, and other cells were not reached balance voltage(Li-ion 4.18V, Life Po43.6V), so they are only in charging status, when the fast cell voltage was reached overcharge voltage (Li-ion 4.25V, Life Po4 3.75V),BMS starts off power protection, all the cells are stop charging, this process will enable your battery charging in balance current, and your battery voltage are in balance status.
However, if your cell voltage difference are in a big range, The equilibrium effect is almost zero,The solution is to replace cells with large voltage gaps.

I suspect that your BMS is either faulty, or it's the wrong type for your battery (it appears to be set for Li-Ion or LiPo instead of LiFePO₄), which is probably why the SOC gauge reads 0% instead of 100% with an average cell voltage of 3.481V:


Are you using the correct 58.4V charger?

Did you fully balance all 16 cells to the exact same resting voltage before assembling the battery pack?

Even with the correct BMS working 100% properly, if your cells were imbalanced by just 1% and left on charge continually to allow the cells to balance, it would take more than 3 days and 13 hours for the BMS to fully balance the cells correctly with just a 35mA drain on the high cells.
As your lowest cell appears to be at ~80% capacity (~240Ah), I suspect you would need to leave it on charge for over 10 weeks for a correct and 100% working BMS to fully balance your pack with a constant current of just 35mA.

I think your BMS needs to be replaced with either the correct type or a non-faulty one.
If you definitely purchased the correct 16S LiFePO4 BMS, then you should contact your BMS supplier and explain the problem to them.
The correct BMS should not allow any cell voltage to exceed 3.75V, as it should disconnect the charging current immediately if any cell reached this voltage.

I strongly recommend that you manually balance the cells by discharging the higher cells down to the resting voltage of the lowest cell.

If you don't have a suitable RC charger for discharging single cells (or multiple cells with a suitable balance lead setup) you can use a 12V 60W headlamp bulb to discharge individual cells at ~2.5A @ 3.4V.
If you're discharging cells using the bulb technique, you must check the resting voltage of the cell after the bulb has been disconnected for a few minutes, as the cell voltage will typically rise when the bulb is initially disconnected.

2. I am not familiar with the operation of the Daly BMS Bluetooth App but I suspect the incorrectly shown charge/discharge current could be due to a faulty BMS unit.

From the screenshot, it would appear that the Charging and Discharging MOSFETs are both turned On, but the Balance appears to be turned Off.
Are you able to manually turn these On and Off via the App?

Alan
 

[TheKing]

Thanks for your reply Sir, it's late now, tomorrow I will read and analyze what you wrote in detail...

my Chinese charger spec in attach 

Regards

[TheKing]

No, I'm not fully balancing cells, I was planning to do with all 16 cells in parallel to stay over 48 hours. I don't have another possibility for now.
I don't know is that the best option ... if you have one, please you're welcome!
I will try parallel 16P first, then with load discharge (Bulb).

After balancing the voltage, I plan to charge the cells with a 12v lead acid charger (that's what I have at the moment), without a BMS, and at the same time I will measure the voltage with a multimeter

The fact that I have to watch the resting voltage after some time of charging/discharging is familiar to me, and I apply it.

Like info, when in some cells the voltage reached 4.1V while charging... since it has been idle for 48 hours it has returned to 3.55V.

I know that a proper BMS should not allow the cell voltage to exceed 3.75V and to me that is the first symptom that something is wrong.
Otherwise, the BMS manufacturer does not have a good service and service network and I expect him to tell me that I need to buy a new BMC.
I have lost trust in the data from the device, so any information about it is reserved.

The charge and discharge MOSFETs are included I think, I'll check when I'm home, and the balance was added to the App a while ago and is not active
Regards,

[Bikemad]

Be very careful when connecting the cells in parallel as the current flowing from the higher voltage cells to the lower voltage cells might be very high when initially connected, so check that the interconnecting wires are not getting too hot due to high current.
Using longer wires between the cells with the higher voltage difference may reduce the current flowing between them.

I would arrange the cells in ascending order based on their resting voltages and then connect them in parallel (one at a time) starting with the lowest voltage cell.
Check the current flow between each cell that is added and those that are already connected in parallel.

I would use solid bus bars between all the negative connections (if you have them) and connect all 16 negative terminals together. Then connect the positive terminals (one by one) using wire until the voltages of the adjacent cells are almost identical.

When all the wires have been swapped for solid bus bars, the pack can then be left to stand for 48 hours.

To charge the fully balanced cells with a 12V lead acid charger I would connect them in a 4S4P configuration as shown below:



The 3 vertical bus bars should be positioned as shown to ensure all cells receive identical charging current/voltage.

Monitor the voltage of each group of 4 paralleled cells during the charge to ensure none of the group's voltages exceeds 3.65V.

Dependant upon the average cell voltage after 48 hours of parallel balancing, your 12V lead acid charger may take a long time to top up the 1,208Ah 4S4P pack to 14.6V (3.65V per cell).

Good luck with your balancing.

Alan
 

[TheKing]

These days it is very cold here, when it warms up I will work and plan to implement every part of your advice.
I'm going to use the solid bus bars between all the negative connections and wire them to the + terminals one cell at a time.
I hope everything will be fine. Very detailed and clearly stated by you. Thanks in advance
Regards,

[TheKing]

Sun 14 h   Mon 19 h

3,52      3,355
3,44      3,355
3,48      3,355
3,37      3,355
3,5      3,356
3,48      3,356
3,37           3,356
3,33           3,356
            
27,49   26,844

Just inform you: In Sunday 14.00h start to balancing cells and after 29h at temp 1 C degree result is in second column. Is it logic ?  The sum of voltage looks with bigger difference than I expect! Cells are connected when the measure is made.
Your opinion ?
Regards,

[Bikemad]

When energy is transferred from one cell to another there will be a certain amount of overall voltage drop due to thermal losses during the transfer process (in the bus bars, connecting wires and the cells themselves).
You also need to consider that some of your cells were previously overcharged, giving an exaggerated pack voltage to start with.

Please note that according to the LiFePO₄ State of Charge curve shown below, a resting voltage of 3.35V per cell indicates that they are almost 100% charged, therefore all of the 8 cells measured still appear to be at a very good state of charge. 



However, I am concerned that you have only shown 8 separate cell voltages when you're supposed to be balancing 16 cells.
You need to have all 16 cells connected in parallel simultaneously for a sufficient length of time to ensure that all 16 cells are perfectly balanced to the same resting voltage.
If you balance two halves of a single pack separately, the two halves could still be at significantly different voltages, and the entire pack would would not be balanced.

If you disconnect the cells and their individual measured voltages still vary considerably, you may have to leave them connected in parallel again for longer to ensure all of the cells are balanced as much as possible.

Alan
 

[TheKing]

Hello,
I don't know why, it didn't give me a notification in my e-mail that there is a new post about me on the forum ... so now I accidentally saw it.
To tell you the truth, now I'm sorry why I didn't put all 16 cells together. I thought, of course, I will charge them 4 times with a 12 V charger, and the electric vehicle has 8 cells in the front under the hood and 8 under the back seat.
I think the front cells (not balanced) are lower or almost the same voltage without big difference. And if they are close to 3.350 then I will put all 16 together to balance for 2–3 days, so I can try them without topping up with a 12v charger.
If after 2–3 days of balancing the voltage is 3.30-3.35 then I will not charge them (which would be problematic to get the same voltage) ... but I will immediately try them on the vehicle without BMS.
Thanks again Sir, now the weather is much warmer, I can work outside Best Regards,

[Bikemad]

I don't know why, it didn't give me a notification in my e-mail that there is a new post about me on the forum ... so now I accidentally saw it.

The notification option appears to be turned Off by default, therefore it will need to be manually changed before future notifications will be sent:
Go to the Notifications page for your user profile and make sure the checkbox next to Turn notifications on when you post or reply to a topic is checked.
Then select whether you want the notification to be sent Instantly, Daily or Weekly etc. from the dropdown menu before selecting Only replies or Replies and moderation from the next dropdown menu.

Finally, you may also have to click the "NOTIFY" link in the tab at the top or bottom of this thread to make sure you receive future notifications. If it says "UNNOTIFY" in the tab instead of "NOTIFY", this may have been done automatically already.

Alan
 

[TheKing]

I did it with the notification.
Yesterday I took all the cells out of the vehicle with the intention of balancing all 16 at once. While taking them out, I noticed that there were a few swollen cells, one more and two less (barely noticeable).
This is already another problem.
I plan to test all the cells with a load from a vehicle starter on 12v, 4 cells .... with meters on each cell to show the load voltage.
I have no idea how to load one cell at a time ... Likewise, I don't have such a load, so this is a kind of compromise.
I would like to hear your brief opinion that they are on the right track, or maybe you have better solutions
Best Regards,
Jovan

[Bikemad]

Hi Jovan,

I guess that the swollen cells will be from positions 11~16, as these are the cells that appear to have been overcharged the most:



If you are going to use a car starter motor to load a pack of 4 cells in series, it will need to be turning over an engine to put a high load on the pack.
An unloaded starter motor spinning freely will not draw much current.

Some garages may still have an older style resistor type battery testers that puts a 100A load directly across the battery terminals to check the voltage drop under load. It might be worth asking your local garage to see if they have one, and if they would let you use it.

Alan
 

[TheKing]

Yes, that's right, those 11-16 are swollen. I checked the first 8 cells, they are all balanced and visually ok.
I'll try to find a resistor type tester today, should find something...or some older hard starting vehicle. Ok ... greetings and see you later

[TheKing]

I'm sorry that I'm a little later ... sometimes there are more private obligations, even some travel days through the neighboring countries 

Just to let you know, while I was waiting for the new BMS last days ... I tried to somehow load the cells with a load, I couldn't find any of those old resistors ... so I put a 90A starter for car engine on 12v where all the load cells, group and individually, did not show some significant voltage drop at 90A load. Maybe it's a small load for 300A cells?

Anyway, I assumed that I had to buy at least one cell, and ordered it... In the meantime, with new balance wires (to avoid any bad contact)... I installed the new BMS with active balancing.

To begin with new BMS, let me say that it works... but shuts down when charging because, seems, the swollen cell reaches 3.65v first... Maybe I should raise the upper limit to 3.9 or 4v?

When discharging, it works under a small load of up to 100A ... but when it is higher, sometimes it turns off, sometimes it doesn't!? ... I can't understand the connection why it turns off, all the setting parameters are ok, min/max current, min/max voltage, etc.
Hopefully when I install the new cell, everything will fall into place.
Ok, when there are new events I will let you know ... greetings and have a nice day
Jovan