RadioB Tech Xiaoxiang BMS discharge wont turn on cells out of balance

[arcsum68i]

Hey all,

Thanks to many videos and this forum I built a 230ah Eve Cell battery for my RV. I have had it installed for awhile now and have used it on two trips with no issues and mostly love it. Going to be going on another trip soon so I went to charge it up to 100% and all was perfect until it got to 100% and my cells got all out of whack and now the thing wont even let me turn discharge on. I did not top or bottom balance when I built it because I have no way currently of doing it and my cells were perfectly balanced when I first built everything.

What do I need to do to get this working again?

Thanks

 

[HRTKD]

1. You must top balance. This is critical. It sucks to buy a benchtop power supply that you' think you'll use only once. But you need to do it anyhow.
2. I don't see a reason for discharge to be turned off. Do you have the cell low voltage cutoff set too high?
3. The battery is not fully charged. None of the cells even over 3.6 volts. The battery voltage is only at 13.7 volts. Please post the parameters that the BMS is operating under. I don't think you have them set right.
4. While the BMS is not being charged or discharged, pull the balance connector out of the BMS, wait a few seconds and then reinsert it. Maybe a reset will clear up the disabled discharge, but you still have other problems.

 

[arcsum68i]

Noted, just bought one and will have it in a couple of days.

Is this something I am going to need to do regularly?
Should I buy an active cell balancer?

I would like to note, that even at 98% charge everything was balanced with no issues at all. It was not until it got to 100% that things went haywire.

 

[HRTKD]

Noted, just bought one and will have it in a couple of days.

Is this something I am going to need to do regularly?
Should I buy an active cell balancer?

I would like to note, that even at 98% charge everything was balanced with no issues at all. It was not until it got to 100% that things went haywire.

In two years, my cells haven't been taken apart and maintained. I periodically review the balance status and I see almost perfectly balanced cells. If it ain't broke, don't fix it.

If the cells are prepared well and of good quality, an active balancer won't be necessary.

I'm not convinced that your batteries are actually at 100% state of charge.

 

[arcsum68i]

TY, I got the 'Top Balance Tutorial' PDF and will have my benchtop power supply on Wed. I was just re-watching Will's video on active cell balancers not usually being needed if you bought new matched cells from a reputable seller which I did. I think I also recall it being stated that if they were balanced and matched when you got them that you did not need to do anything (but maybe that was just wishful thinking). My cells came from Docanpower and were exactly matched when I got them. I am going to go pull the balance connector like you suggested and will post back to let you know if anything changes.

Thanks again for helping me out, really appreciate it.

 

[HRTKD]

I think I also recall it being stated that if they were balanced and matched when you got them that you did not need to do anything (but maybe that was just wishful thinking).

As I recall, that was an offhand comment. I know there was an offhand comment about not using a BMS that, despite all the times Will recommends using a BMS, people focus on that one comment and take it as gospel.

 

[HRTKD]

My cells came from Docanpower and were exactly matched when I got them.

The only metric most of us can (easily) get is voltage. Yes, the cells all show the same voltage within a few millivolts. But that doesn't tell you the actual state of charge of the cell.

 

[arcsum68i]

Presumably cell 2 is too high compared to the others and maybe that is why discharge is cutoff. I think I recall seeing a 12v light bulb being used to discharge a cell, is that something I should be doing to get them closer to each other?

I did unplug the BMS from the cells and its similar to what I originally posted. I manually measured each cell and come up with the following numbers.
1 - 3.436
2 - 3.496
3 - 3.332
4 - 3.331

Also, here are my settings. Followed a video tutorial for the settings from someone using my exact setup, dont believe I really changed much.

 

[HRTKD]

The cell voltages look better when checked with a multimeter. The difference with cell 2 warrants further investigation. Do you have a bad crimp on the balance lead? Is the balance lead ring terminal not secure/clean?

Of the BMS parameters that I'm familiar with, yours look OK, except for the low temp charge cutoff. -10°C should be 0°C, with the release number set slightly higher than that.

 

[arcsum68i]

TY, I will correct low temp cuttoff. I have the battery pulled from the trailer and on the bench now waiting for the new toy to arrive.

The crimp and ring terminals are very secure and look like new (they essentially are).

When I do the top balance, will it equalize everything out without me having to do anything else based on my current voltages? IE will I need to individually charge or discharge any of the cells prior to top balancing?

 

[HRTKD]

When I do the top balance, will it equalize everything out without me having to do anything else based on my current voltages? IE will I need to individually charge or discharge any of the cells prior to top balancing?

That's a good question. Since I haven't seen anyone address the question specifically then I'm going to assume that the top balancing implicitly brings the cells into alignment. I would use the step method. Go to 3.55 volts, then 3.60 volts, then 3.65 volts. Checking all cells with a meter each time. The cells should be gently compressed.

 

[arcsum68i]

Figured it out!

In the function setting of the app the 'Switch Function' needs to be turned off. Not sure why or how but I obviously accidently turned that switch on. I was also watching one of Will's videos where he stated if you got new/matched cells the BMS would eventually balance everything out but it would take much longer to complete. I am guessing that was my logic on why I did not initially top balance (in addition to cost of course). I have the bench power supply ordered so I will go ahead and reconfigure my setup and do a top balance because there definitely seems to be some disagreement on the subject.

TY again very much for the help, hopefully this is useful for someone else in the future.

 

[arcsum68i]

So I bench charged everything as per the suggestion. Went camping and even after fully charging I was completely balanced. Figured I was good, but then after being on battery for a short period of time I am back to being unbalanced similar to the original pic I posted. My total voltage diff is now .076, not sure this is even enough to be concerned with but I sure do not like seeing that red on the one high cell.

Any new suggestions?

Should I buy an active balancer to keep things in line?

Is it possible my BMS is just bad?

 

[arcsum68i]

delete

 

[HRTKD]

In your parameters you have "Balancing precision" set to .03. I think mine is set to .015, so half of what yours is using. That means that your BMS is willing to accept twice the out of balance range than mine is. I may be confusing this parameter with "Delta to balance". Look to see if you can find "Delta to balance". If you do find it and it's set to .015 then don't change the "Balancing precision" parameter.

My balance start voltage is 3.4 volts. I don't recall seeing that in your parameter list.

 

[arcsum68i]

I did not see a Delta to Balance setting. Updated my balancing precision to 0.015 and my balance starting voltage is 3.2. Should I change when my balance voltage starts?

 

[HRTKD]

Balance voltage is typically set to 3.4 volts.

 

[mikefitz]

Your charge voltage settings in the chargers may be too high.

You don't want balancing to occur below 3.4 volts.

Are the cells going out of balance when high charge/discharge currents are flowing?

Interconnection issues with cell buss bars and BMS leads can introduce volt drops.

 

[arcsum68i]

Your charge voltage settings in the chargers may be too high.

You don't want balancing to occur below 3.4 volts.

Are the cells going out of balance when high charge/discharge currents are flowing?

Interconnection issues with cell buss bars and BMS leads can introduce volt drops.

Thanks, I have no settings on the charger, it is either on or not but is LIFEPO4 specific.

I have changed my balancing voltage to 3.4.

I am not sure exactly. In this scenario it was perfectly balanced immediately after charging from the last trip. I knew I would be going again 2 weeks later so I left them on the charger (probably should have disconnected the battery and let them discharge a bit). When I checked again they had lost balance.

My buss bars and BMS leads are very solidly connected both to the batteries and to the ring connectors. I checked this last time it was mentioned.

I am currently sitting at a .080 voltage differential, what is the threshold for a proper balance?

 

[arcsum68i]

In case this helps, here are the readings from the app vs actual readings. I will pull current for the next hour and then plug them back in to see if they balance properly with the new settings.

Per the BMS

1. 3.439
2. 3.459
3. 3.379
4. 3.415

Per the volt meter

1. 3.426
2. 3.445
3. 3.366
4. 3.400