Hi all

I am in the process of designning a battery measurement and balancing system I was thinking of using a TI BQ76930, I am following the design guidelines found on pages 4-7 (http://www.ti.com/lit/ug/tiduar8b/tiduar8b.pdf). The main issue I am having with this design is identifiying the following properties I am not entirely sure if these are decided by myself or are figures that need to be calculated.
The following is the batteries I am thinking of using: https://eu.nkon.nl/lg-18650-hg2.html (For this battery pack I will be using 40 batteries, there will be 4 batteries in parallel and then 10 of these parallel batteries in series to provide 36V and 12Ah). The properites I am unsure mainly came up when decideding the current sensing resistor on page 4: SCD and OCD, for these parameters they have a current, time and threshold voltage, I am not sure where I get this information for the batteries I am using. Another property I am unsure of is the balancing current I should be using can anyone direct me as to what current I should typically use to balance?

Any help and guidance would be greatly appreciated.

Kind Regards

Art

 

I would use the Ocd maximum as 20Amps and the Scd as 30Amps, so using their formula that means the I sense resistor is 10X bigger, that makes it 5.0 mOhms( milli ohms).

 

I would use the Ocd maximum as 20Amps and the Scd as 30Amps, so using their formula that means the I sense resistor is 10X bigger, that makes it 5.0 mOhms( milli ohms).

ok great thanks so much may I ask, what values you used for the threshold voltages and times did you use the same times. Also with balancing may I ask what resistor value should I use? Thanks
Art

 

I used threshold voltage of their formula, but using your batteries Maximum short circuit current, this will get you started, you might have to alter the values depending on the cells.

 

I used threshold voltage of their formula, but using your batteries Maximum short circuit current, this will get you started, you might have to alter the values depending on the cells.

Thanks sorry for the balancing current should I be using the max short circuit current, all the values I have seen are in the mA range, sorry I am very new to this so any help would really be appreciated

 

Balancing current is for charging up the cells, usually you charge a cell up at 10% of its rating, so thats 300mA with a 3Ah cell, but the battery management pcb should be doing this for you on page 5?

 

Balancing current is for charging up the cells, usually you charge a cell up at 10% of its rating, so thats 300mA with a 3Ah cell, but the battery management pcb should be doing this for you on page 5?

oh sorry from what I can understand the balancing details are on page 6, where you need to determine the balancing resistance so should I be using a balancing current of 300mA and voltage of 3.6V? Thanks Art

 

I would use the limit design based on your cells, so 300mA is for 10%, if you cant get any datasheet for the cells, i would go with the pcb design in the datasheet, which sets it at 150mA i think? so they're going for 5% charging current.

 

I would use the limit design based on your cells, so 300mA is for 10%, if you cant get any datasheet for the cells, i would go with the pcb design in the datasheet, which sets it at 150mA i think? so they're going for 5% charging current.

oh yeah the following is the datasheet I can't find anything explicitly about percentage charge