BQ25792: 12 cells LIFEPO4 Battery connection on BQ25792(1-4 Cell) IC

Hi Sasidhar,

I have been answering through TI FAE Ray in our internal forum.  Below are the latest responses.

Packs in parallel simply increase the capacity and therefore the required charge current to reach full charge within the safety timer limits.  The 4 series means 4 x 4.2V = 16.8V (actually a little higher) is the charger's max battery charge voltage.  

If the series and parallel LiFePO4 pack voltage is below the 16.8V max charge voltage and the charge current is high enough to reach full charge within the safety timer limits, then, with software changes I mentioned above, this IC can charge the pack.

I recommend setting PROG pin resistor to 10.5kohm which sets fSW=1.5MHz and default VBATREG=12.6V and ICHG=1A.  Then using host software, write to REG0x01-02 to set VBATREG to 12.8V and then REGx03-04 to set ICHRG to the highest charge current possible given the input power as explained below.  The host also either needs to disable the watchdog timer in REG0x10b:20 or periodically write to the WD bit in REG0x10b3.  If not, the registers return to defaults.

You can predict maximum charge current using an efficiency power balance eff=Pout/Pi=(12.6V*(ICHG+ISYS))/(VBUS*IBUS), where ISYS is the system load current, VBUS is the output of power source and IBUS is the power source max output current.  The datasheet has efficiency curves starting on page 20.  The charger limits its input current in order to prevent collapse of the input power source per the IINDPM register.   IINDPM is configured each power up to the lower of the value per on D+/D- communication or ILIM_HIZ pin resistor divider setting if EN_LIM bit is in REG0x14b1 is set.  I recommend disabling EN_ILIM bit or setting ILIM_HIZ resistors for the highest input current limit=3.3A in your power source can provide that much.  If D+/D- are left floating and EN_LIM=0, IINDPM = 3.0A so you will likely want to write REG0x06-07 IINDPM registers to 3.3A.  If the charge current is slightly larger than 18Ahr/12hr-safety timer = 1.5 A then you can leave the safety timer alone but if not, then you will need to increase the safety timer to 24hours in REG0x0Eb2:1.

But, since you disclosed that you have a solar panel, I recommend changing to p2p BQ25798 as it has an embedded MPPT function.  There is also an appnote that you might find helpful for further optimizing solar panel operation at https://www.ti.com/lit/pdf/sluaai1.  When using a power source with high output impedance, like a solar panel, the charger relies on the its VINDPM feedback loop, not IINDPM feedback loop as mentioned above.  The VINDPM loop reduces input current in order to prevent power source voltage collapse below a preset threshold.

Regards,

Jeff

Hi Jeff,

Thanks for the information on IC. Our requirement is 12.8V 18AH.

How to I configure 12 Cells LiFePO4 Battery (Each Cell Voltage = 3.2V Typ) in series and parallel combination? 

BQ25792 IC supports 1-4 cell maximum as per datasheet, Can i use BQ25792 IC for 12 cells?

Hi Jeff,

Thanks for the Info.

I have not connected the battery and disabled the battery charger(EN_CHG set to 0). I have tested the buck boost functionality of the BQ25792 IC.
Whatever input i am giving i am getting the output as input given(If my input is 5V, SYS Pin Output is 5V).

After that i have enabled the BQ25792 Charger (EN_CHG set to 1) and tested again. Whatever input i am giving i am getting the output as input given(If my input is 5V, SYS Pin Output is 5V).

Please suggest the solution for this.

Hi Sasidhar,

The BQ25792 does not have a VBUS to SYS passthrough mode.  Are you testing using the BQ25792EVM from TI?  If so then it is damaged.  If not and is your own board, please provide your schematic and board layout files.

Regards,

Jeff