SBMS0 Cell Balancing Current

[Paul]

Hello Dacian

1. What is the maximum cell to cell balancing current on the SBMS0 ?

2. Does it use an active or a shunt/passive cell balancing circuit ?

3. Can you send me the circuit diagram for the cell balancing circuit? I am on a 15S 48v LiFePO4 battery bank so I will need to modify it from 8S to 15S . In my country 16S 48vdc systems are common off grid systems. I prefer 15S as its closer to 48v we have no PV VDC restrictions.  

   

Thanks 

Paul

[Jhon]

Hi Paul, I saw your question had not been answered for a bit and I was interested in the answer myself, so I went to google:

From https://github.com/bdlow/sbms0#cell-balancing

"Cell balancing is achieved via a 24 Ω resistor switched across each cell by a FET controlled by the ISL94203; nominal balancing current is therefore 3.6 V / 24 Ω = 150 mA. Cell balancing is only enabled when:

• all cell voltages exceed 3.2V, and
• cell voltage deviation exceeds 10 mV, and
• charging current exceeds 300 mA

These parameters are configurable in the ISL94203; refer the datasheet for details. (TODO: verify if exposed by the SBMS0 config.)"

Hope that helps.

[Paul]

Hello Jhon

Yes I found that and I was going to answer myself on the forum thanks.

1. 150ma starting at 10mV of deviation.  0.075 watts 

2. Uses shunt resistor cell balancing.

3. ?

I don't like this type of cell balancing it is fine for small cylindrical cells but its not effective for 2nd life LiFePO4 cells.

Eg: After a discharge to  3.2 /3.18v 20% soc when recharge again a cell can easily run out by 250mv. 

On a 120ah cell with a cell difference of 250mv this balancing circuit would take 400 hours to correct this cell voltage difference if you were cycling the cells daily. 

The next time the cell difference will be worse. You need about 4 to 5 amp of active balancing for large LiFePO4 cells. 

That's why I no longer use Daly BMS's .

If someone can come up with a circuit to use the single cell balancing triggers from the ISL94203 or BQ76952 or BQ79656 ic's to a common capacitor bank using 12A triacs we can get good cell balancing.  In my country we can get 100ah; 120ah; 280ah 2nd life cells for 650; 775; 2360 ZAR. that's why we are using them. 

[Jhon]

You could potentially add an active balancer, I have never tried that and others might be able to better advise you.  I feel lucky that I was able to import 16 new 272 Ah cells from China (for less than what you pay per cell) and the balancing of the SBM0 seems to be adequate.

[Paul]

Hello Jhon 

Really, I looked into importing them but they were working out more and there was a big gape in the communication. I do import other things from china also I was struggling to find a transporter as they are hazardous freight.
Can you please give me a link to your supplier and I will try again. 

[Jhon]

I used Shenzhen Basen Technology Co on Alibaba.  I had one faulty cell, cells were well packaged, but still got damaged during shipping.  They gave me a discount on the next order, rather then refund or replace the cell and this worked out to be the same as the refund.