bq40z50 External Balancing calculation for Bal Time/mAh

Larry,

I attached a set of equations that are less confusing than the ones in the TRM. We use 1000 ohm series resistors and 150 ohm for the internal FET resistance. Also, there is a firmware bug on the bq40z50 that prevents the bottom cell from balancing in an external balancing configuration, so you should use the bq40z50-R1 which will be released within the next week.

Tom

bq40z cell balancing equations.pdf

Tom,

Does the expression (+)|| mean that I should calculate the total resistance as external resistance in parallel to the 1150 ohm (1K + 150)?

What impact does scaling have on the calculation?

Also, note that slua660a refers to slua420 which has an incorrect FET configuration for the bq40z50.

Thanks,

Larry

Yes, the symbol || means to calculate the parallel resistance. I will have to analyze the scaling question how it is impacted.

Tom,

 

I am upgrading to the bq40z50-R1 and I have my circuit set up like the schematic that you sent with the calculations for external balancing.

In the Technical Reference for R1, it looks like the bit formerly used to distinguish between internal and external balancing is now reserved and set to 1.

 - Is there now no difference between internal and external balancing, except for the external circuitry?

 - It looks like external balancing at rest is no longer excluded – is this correct?

 

I still need some input on the scaling.

 

Thanks,

 

Larry

 

Larry,
When CBM=0, the gauging will balance all cells that require balancing simultaneous. This causes a problem is Cells 1 and 2 need balancing, because the there is only one 100 ohm resistor in the balancing path for Cell 1. This causes the Cell 2 current path to split between the Cell2 balancing loop and the Cell 1 balancing loop and the balancing current will be higher than expected. This results in Cell 1 getting over balanced. We removed the CBM=0 options from the bq40z50-R1 to prevent this from occurring and we are recommending that CBM be set to 1 on all bq30z and bq40z devices. This will force the gauge to use sequential balancing where only one cell is balanced at a time and this will prevent the split current paths. Cell balancing at rest (CBR) is still an option on the -R1 device.

I reviewed the cell balancing / scaling question and it looks like you need to multiply the data flash balancing time parameters by the scale factor to maintain the same level of balancing. It will still balance with the default setting, but it will be more aggressive.
Tom

Hi Tom,

Is it possible to get the recommended part number for the FET used in external cell balancing circuit?

Thanks.

Hi Tom,

Any update for FET?

Thanks.

bq40z50 external cell balancing.pdfHello Hong,

On a 4S LiFePO4 pack we used BSS84LT1G at cell 1 and NTZD3154N at cells 2, 3 and 4.

Larry

I do not have a specific FET recommendation. Any logic level FET should work.

Tom