Active Cell balancer and BMS equalizer

[Pizzocchero]

Good morning.
I'm not sure if It Is safe to add an
Active Cell balancer in parallel of
a BMS... Did someone test a
System like this?

 

[snoobler]

I plan to, so I hope it works.

 

[tictag]

HOPE is not a strategy!!

 

[snoobler]

I'm pretty confident.

 

[newbostonconst]

I read the other day someone had done it. I agree with it and may add a second Active Cell Balancer to my pack. Report back how it goes.

 

[newbostonconst]

Found someone posting here they had done it.

Overkill Solar BMS Balancing current?

@OverkillSolar Looked at the BMS website. Looked at the manual. Searched the forum, but WAY too many things to sort through. Probably missed it somewhere, but I really did try. Questions: Active or passive balancing? Current? Thanks

diysolarforum.com

 

[Steve_S]

The ability to add a Balancer to a system is not a problem in general.
! Active or Passive Balancing does not fix bad cells !

Passive Balancing is low power stuff and only dissipates excess from a High Cell. (typically < 1.5A)
Active Balancing will transfer High Voltage to Low Voltage cells within a battery pack. (typically 3-6A)
*1 If you have a BMS with Balancing functions internally, an external balancer can interfere with it. (Internal balancing may have to be disabled)
*2 Some BMS' will see an Active Balancer and report it as "charging" which can affect the statistics.
*3 Special Note regarding Cell Capacity. The larger the Cell Capacity, the greater the capability for any Balancer is required. A 50AH cell requires considerably less Amperage Transfer than a 280AH cell. It IS relative.
*4 Balancing will generate heat, any balancing system, including a BMS with it built-in will require free airflow for cooling.

Active Balancing has been shown to help recover some cell capacity on used cells, such as the BYD Packs which have been shown on Youtube by several people. A typical, respectable 8S Active Balancer will run to $200 USD +, there are cheaper ones BUT you get what you pay for. Many variations have been tested and many fail, although it may take time. BE CAUTIOUS ! There are Specific Balancers for Specific Chemistries, as well as some more "general" ones.

WARNING: If an Active Balancer does fail and there is no way to observe such a failure (active indicator), it can result in possible cell / pack / battery damage. This is yet also another piece of equipment in the mix which has the potential to fail, evaluating if you really need & want such a device is specific to your use case.

One of the more popular brands of Well Known Active Balancer is the QNBBM models sold by Deligreen.

[Hot Item] 8s 24V Battery Balancer Prolong Battery Lifespan2-3 Times for 18650 Lto Lithium-Ion and Lead Acid AGM Battery Pack

Metal Coating: Plastic Mode of Production: SMT Layers: Single-Layer Base Material: FR-4 Certification: Ce Customized: Non-Customized

deligreen.en.made-in-china.com

YT Video with QNBBM 8S & Chargery BMS8T (with internal passive balancer OFF) on BYD 24V Pack

 

[newbostonconst]

I don't think the Active Balancers that are rated at say 5 amps are really 5 amps. I think they charge the capacitor at 5 amps and then discharge them at 5 amps but the average rate of power flow is less then 2.5 amps.

I still agree that we need bigger ones then we currently have available for a 280ah battery. Would like to try out a 10 amp one....

 

[Pizzocchero]

Thank you all.
My battery pack (the First i made) Is a Little
Capacity...135Ah (nominal) 12V 4S lifepo4 for
RV appliances. I did connect an overkill 120A.
It seems work fine (protections, monitor app, etc)
The only thing i have to do Is to equalize a voltages mismatch of 100mv appeared on one cell at the top end of charging phase.
I don't know how many time Is necessary,
for BMS, to do that equalization... 4 Day After disconnect charger the difference Is still
around 70mv: 4,461 4,462 4,463-4,396
di i Need an Active balancer?

 

[RCinFLA]

As cells get older they are going to need for balancing current. Resistor dump balancers only provide cell leakage difference correction,.

A typical celll will have 1 to 2 % C per month of leakage current. Worse case spec from manufacturers is 3% C per month. Balancing, at minimum, has to make up the delta in leakage between cells to prevent a growing difference in state of charge. It is not unreasonable to expect a max to min delta in a battery array to have 1% delta in leakage per month between cells. For 280 AH battery that is 2.8 AH difference that has to be made up by balancer.

For a 200 mA resistor dump balancer that is 14 hours per month of active current dump. With resistor dump balancers being active only above 3.4v cell voltage that means 14 hours per month spent above 3.4v. That is a lot of time above 3.4v and can be especially difficult for off-grid PV only charging to accomplish.

As cells get older their leakage increases and diverges in max/min delta. Also cell capacity degrades at slightly different rates. Cell capacity difference is best accomplished with continuously operating active cell balancing.

There should not be any issue with running the active and resistor dump balancers in parallel. Below 3.4v the resistor dump balancer in not in the picture. The only possible interaction I can imagine is around 3.4v where resistor dump first turns on. It might cause a control thrashing between the two balancers but I highly doubt the resistor balancer is a strong enough loading to have any interaction impact. Even if it happens it would be temporary and not result in any damage to either balancer. Compared to normal operational inverter load current jumping around, the resistor dump balancer is very insignificant.

For the continuouly operating active balancer it is this varying inverter current draw that is toughest thing for the active balancer to determine apples to apples cell voltage levels. The cell voltages really needs to sampled at the exact same instant in time so all cells are being measured with same instantaneous inverter load or charge current. A sample and hold circuit can do this and still provide the ability of a single ADC block to round robin the sample and hold output readings. Most microcontrollers with multiple ADC inputs do this.

A by-product of having this sample and hold ability is being able to continuously measure the impedance of individual cells which is a great feature. You need the current to also be captured at same instant in time so it would best if active balancer is within BMS which already has a battery current shunt sensor.

 

[newbostonconst]

LifePo4 shouldn't be charged over 3.65 volts or you can damage them.

 

[Gazoo]

4 Day After disconnect charger the difference Is still
around 70mv: 4,461 4,462 4,463-4,396

LifePo4 shouldn't be charged over 3.65 volts or you can damage them.

 

[Pizzocchero]

Yes... You are right. It is a mistake.
The voltages are 3.xyz

 

[Pizzocchero]

Now the voltages gap between cells Is 65 mv
Is It normal an adjusting Speed so slow?

 

[Steve_S]

Let me simplify something for those getting muddled in the minutia.
These large bulk cells are not like small cells so treat them accordingly.
KISS Applied: Assume a 100AH cell, you can safely assume 100mv floatabilty. For 280AH then say 280mv is within tolerance... 1mv for 1 amp hour is within acceptable ranges.

Active Balancing the larger cells requires more amperage to deal with the capacity. Again a 50AH cell can be managed with a 1.5A balancer but 250AH will need 2.3 or better if you want anything done within a reasonable amount of time... The bigger the battery pack cell the more OOMPH you will need to shift the juice around efficiently.

The standard LFP Prismatics will float up & down when charging/discharging, they also settle once no + energy being applied to them, floating them below determined max voltage allows for resting too (not quite the same though).

 

[Pizzocchero]

Thank you.
Does It mean that my battery pack require a
2A, or more, Active equalizer capability?

 

[Dzl]

Thank you.
Does It mean that my battery pack require a
2A, or more, Active equalizer capability?

I suppose the best way to find out is to just use you pack (assuming you have a BMS) , if you have top balanced and are using a balancing BMS and your BMS cannot keep the cells reasonably balanced, then consider bringing out the big guns (active balancer) to keep the cells balanced.

As far as I know, so long as you use a BMS, no harm will come from your cells being somewhat out of balance beyond diminished capacity and potential nuisance BMS trips. I'm not an expert so if someone disagrees please speak up and correct me. I don't want to give bad info, but basically to me it seems like a non-issue unless it becomes an issue, and then you cross that bridge.

 

[Pizzocchero]

Maybe this difference Is not an issue.
I see the BMS slooooooooly equalize.
The initial 100 mv Is now 64 mv...
But It takes 6 days with battery totally
Disconnected from anything

 

[Dzl]

Maybe this difference Is not an issue.
I see the BMS slooooooooly equalize.
The initial 100 mv Is now 64 mv...
But It takes 6 days with battery totally
Disconnected from anything

Did you do an initial top balance or is your BMS trying to bring your cells in line for the first time?

 

[Pizzocchero]

The second case you wrote