BQ78350-R1: Cell-Balancing

Hi Paul,
Yes, read the documents. You can only configure what is availabe.

Yes, but what I mean is: In the Bq76920 is a register for balancing. This register is for Balancing. If a bit in this register is 1 balancing is active. If a bit in this register is 0 balancing for a particular cell is deactivated.

Q0: The question is, whether I have to set the bits in this register or whether the Bq78350-R1 will do this by itself.




Next thing I want to ask is about the calculation of the components, which are used for external balancing.
Refering to the application note of the bq76920 on page 8, figure 6 you can see a circuit for external balancing with N-channel Mosfets.

The bq76920 will monitor a lithium polymer battery, the maximum cell voltage of this kind of battery will be 4.2 Volt. The maximum input current into the IC is 50 mA.

Example:

4.2 V/50mA= 84 Ohm. Because there a two resistors in series, if a cell is balanced, we divide the 84 Ohms/2= 42 Ohms.


So the Rc, Rcb, Rcc resistors should be greater than 42 Ohms, so ensure that the current will be lower than 50 mA.

This current is used for turning the external FET on. So this current should also be high enough, so It will generate a high voltage at Rcb to turn the external FET on.

Q1: The Z-Diode is to prevent the external mosfet for excessive voltage, right?

Q2: :What is the purpose if the Rga,Rgb resistors?



Q3: Refering to the TIDA-00792 the external balancing is used. I would like to know how big the balancing current in this curcuit it?

Hi Paul,
Q0: Yes, the bq78350-R1 controls the registers in the bq76920. Notice there is nothing else attached to the I2C bus in the diagrams.
Q1: The zener diode prevents excessive gate-source voltage on the FET
Q2: The Rga, Rgb resistor prevent excessive current in the zener diodes
Q3: TIDA-00792 uses external balancing with a 28.7 ohm resistor. The internal path will draw about 4V/2k = 2 mA. The external path will draw approximately 4V/28.7 = 139 mA If you add these you get 141 mA. You can calculate for other voltages.

Hi Paul,
Balancing is done with a small current because it is not usually needed and to avoid heating in the battery pack and the expense and difficulty of dealing with that heat. Additionally balancing is done at low currents during taper when it is easier to distinguish cell voltage from IxR drops in the cells or interconnect. How much balancing current you need is something you need to determine for your system from analysis or test.
If you have a cell which is loaded or self discharges 1 uA more than the others, in 1 day it will be 24 uAh lower than the the others. If it can be recognized as a voltage difference and balanced out, and you cycle the battery every day, and you balance during the last 0.5 hour of charge taper, you will need a balance current of 24 uAh/ 0.5 h x 1/.7 (bq769x0 balance duty cycle) x 1/.5 (alternating between adjacent cells for balance) = 137 uA of balance current.
For this model a few mA of balance current may be sufficient. If any of those system conditions change you may want more balance current.

Hi Paul,
The 0.5 constant is for alternating balancing in 2 sets of cells. Since the bq769x0 supported by the bq78350-R1 is not supposed to have adjacent cells balanced, the bq78350-R1 knows to alternate even and odd cells on the cells to be balanced. If you have a 5 cell pack with a bq76920 and cell 5 is low, the gauge could balance cells 1&3 half of the time and cells 2&4 the other half of the time. If cells 1-4 were all low and cell 5 high, the gauge could balance only cell 5 continuously and the 0.5 factor would not be needed. But with the bq78350-R1 it is the normal factor to use.
If you were doing your own controller with an MCU and chose to balance one cell at a time instead of alternating groups, with 5 cells and one cell low, you would balance each of the other 4 cells one at a time so the factor would be 1/4 or 0.25.
You must determine for your system what is an appropriate balancing current. If your cells are well matched and at the same temperature there may not be much difference in self discharge. If some cells are much hotter than the others due to solar heating or being close to hot circuitry, they may self-discharge faster and you might need more balancing current. Your cell supplier may be able to help with a recommendation or data to assist in your decision.
Again your system usage model will also determine the need for balancing. If it charges daily you may not need much balance current. If the system charges only monthly, you may want a higher balance current.