Re-balance using BMS with low current?

sunrise

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My DIY batteries are out of balance (I have top balanced using the guide in this forum before I put them in use), one cell hit 3.65v (overvoltage cutoff) while other cells are around 3.36v. during charging I did see [BLANCING] on the cell with higher voltage, but I think the bms passive balancing just can't keep up with charging current.

My cells are strapped together with some very strong fiberglass reinforced tape and placed in a very compact enclosure. take them out and rewire to top balance again can be super time consuming.

I am wondering when cells are around 3.2v - 3.3v, if I can hookup a desktop power supply, limit the current to 0.5a and set voltage to 13.8v, and use this to charge the cells, would this give the passive balancing enough time to bring cells balanced?

I also read that I can use a load resister to 'bleed' the high voltage cells, I might try that if the desktop PS thing doesn't work.
 

smoothJoey

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First independently verify the bms sense leads voltage at the jst end of the connector with your dvom.
That is a huge delta.
I would use a capacity tester to bring that one cell down.
Or you could likely just use a resistor.
 

sunrise

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Thanks @smoothJoey, I am pretty sure the balance leads connectors are good. for most of the SOC range, 3.2-3.3v, the cells delta are ~ 0.003v. I had this problem before, and when that happens, 1) the delta are consistently big across different voltage ranges. 2) usually one cell is high and the next one is low.

I had my balance delta set to 0.02v and threshold set to 3.2v(only balance when v > 3.2v)
Should I also lower the balance delta to 0.01v so balancing can kick in earlier?

I am going on a camping trip with the trailer this weekend, after that I will switch off the battery from converter, and use desktop PS to charge it at lower current and see if that would bring cells closer or further apart.

Thanks!
 

smoothJoey

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I had my balance delta set to 0.02v and threshold set to 3.2v(only balance when v > 3.2v)
Should I also lower the balance delta to 0.01v so balancing can kick in earlier?
A balance threshold of 3.2volts is way to low.
You should set to ~3.35-3.375 per cell depending if you float and at what voltage.
Looks like you have middle balanced your cells which is not a thing.
Normally you should only balance during charging.
 

Freep

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I agree with smoothJoey. Manually bleed off current from that one cell. I had the opposite problem because I was in a rush and did something stupid which resulted in one cell being lower than the others. So I took the BMS offline, unplugged the leads from the BMS, cut the power from my battery to the camper and set a desktop charger on just the one cell until it was topped off like the others.
 

sunrise

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A balance threshold of 3.2volts is way to low.
You should set to ~3.35-3.375 per cell depending if you float and at what voltage.
Looks like you have middle balanced your cells which is not a thing.
Normally you should only balance during charging.
I use JBD 120A BMS, which has balance current around 50ma. and 3.2v is there factory default. Note I think balancing only happens during charging, I have not noticed any balancing when batteries sitting idle or discharge.
From my experience watching the BMS app, even I set the voltage threshold to 3.2, balancing won't happen until higher as the delta @3.2v will be very small.
 

sunrise

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First independently verify the bms sense leads voltage at the jst end of the connector with your dvom.
That is a huge delta.
I would use a capacity tester to bring that one cell down.
Or you could likely just use a resistor.
BTW, do you have to disconnect the cell to bleed it off? or a resister can be applied to the pos/neg of the cell while everything (adjacent cells, balancing leads, etc) else are still connected?
 

smoothJoey

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I use JBD 120A BMS, which has balance current around 50ma. and 3.2v is there factory default. Note I think balancing only happens during charging, I have not noticed any balancing when batteries sitting idle or discharge.
From my experience watching the BMS app, even I set the voltage threshold to 3.2, balancing won't happen until higher as the delta @3.2v will be very small.
I have the jbd 8s 100amp bms and it can balance all the time or be limited to just during charging.
I don't care if the default value Its too low.
 

smoothJoey

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BTW, do you have to disconnect the cell to bleed it off? or a resister can be applied to the pos/neg of the cell while everything (adjacent cells, balancing leads, etc) else are still connected?
Nope you can do that in place.
I would enable full time balance and see what it does over a day or 2.
After the pack is balanced change the balance threshold to higher than float and only balance during charge.
Also triggering on 2mv is a pretty small delta.
I have mine to trigger on 15mv.
 

sunrise

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Nope you can do that in place.
I would enable full time balance and see what it does over a day or 2.
After the pack is balanced change the balance threshold to higher than float and only balance during charge.
Also triggering on 2mv is a pretty small delta.
I have mine to trigger on 15mv.
Trigger delta is 20mv, what’s full time balance?
 

smoothJoey

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Trigger delta is 20mv, what’s full time balance?
in the android app
function_setting->balance_enable=true
function_setting->charge_balance=false
parameter_view->balanced_turn-on_voltage=3.35 or higher depending on your float voltage
 

sunrise

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in the android app
function_setting->balance_enable=true
function_setting->charge_balance=false
parameter_view->balanced_turn-on_voltage=3.35 or higher depending on your float voltage
Oh man...
I had no idea this(balance without charging) was possible, This is exactly what I want to achieve with desktop PS.

Thank you so much.

Next week or so when I am not so busy with work, I will start a google spreadsheet on BMS settings, hopefully can help others.
 

sunrise

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@smoothJoey BTW, is there any reason not to have charge balance disabled all the time and set threshold voltage slightly below floating?
seems like this way
1. Passive balancing won't kick in when you are not hooked up with shore power.
2. Once you are plugged in, balancing will eventually bring cells roughly balance, might take a bit time, but I don't actually care as long as it's automatic.
 

JeepDaddy

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using the guide in this forum
Which one?

Use your desktop PS to 3.65v and maximum amps to bring each cell up individually to 3.65 volts and almost no amps. You don't need to break you pack down for this.

Passive balance start voltage should be set to at least 3.5 volts or more. Top balancing needs to be done at the top.

Do you have the capacity (in AH) test results for each cell?
 

smoothJoey

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@smoothJoey BTW, is there any reason not to have charge balance disabled all the time and set threshold voltage slightly below floating?
seems like this way
1. Passive balancing won't kick in when you are not hooked up with shore power.
2. Once you are plugged in, balancing will eventually bring cells roughly balance, might take a bit time, but I don't actually care as long as it's automatic.
A properly balanced battery should only have balance enabled during charging and only above float voltage.
I think I've said this more than once in this thread.
I suggest you read this.
Seems like you are not getting the top part of top balancing.
You didn't answer my question about your charge profile.
 
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Gazoo

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I agree with smoothJoey. Manually bleed off current from that one cell. I had the opposite problem because I was in a rush and did something stupid which resulted in one cell being lower than the others. So I took the BMS offline, unplugged the leads from the BMS, cut the power from my battery to the camper and set a desktop charger on just the one cell until it was topped off like the others.
I have done this as well. There is no need to take the pack apart to either charge or discharge a single cell. And I discovered there is no need to disconnect the BMS. The most important part of this is to make sure there are no loads on the battery when manually balancing individual cells. It might also be a good idea to turn balancing off while doing this. :)
 

Freep

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If the high cell is one of the outside cells you could charge all three at once with the desktop power supply. You just need to set it to10.5 (3.5 * 3) volts and exclude the high outside cell. I'd draw a picture if I was good at that sort of thing.
 

sunrise

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I disabled BMS 'charge balancing' option, per suggested by @smoothJoey above, now the passive balancing is doing its job although slowly, the delta is cut in half already (0.2-0.1v)
I will see how far this would bring cells balanced, if it works reasonably well I'd save the trouble/risk.
 
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