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BQ25798: Configuring my new BQ25798 application

Carl Olen
Expert 1160 points
Part Number: BQ25798
Other Parts Discussed in Thread: BQ34Z100-G1, BQSTUDIO, , BQ25792

Jeff, Wyatt, Shirish,

My system has evolved again.  The batteries are now in a 7S-1P configuration, 8.4V nominal (the extreme voltage range [6.3, 10.5]V).  According to the Panasonic HHR-210AA datasheet, Standard charge is 200mA (16h); Rapid charge is 1.2A (2h).  I want to connect a solar panel as my main energy source, connected to VIN1.  The 28-module solar panel produces a datasheet maximum of at 562.8mA @ 8.92V.  I would also like to be able to use VIN2 as an input for a USB charging energy source so that I can charge in the dark, if needed.  So, the input voltage to the charger will be either 8.92V or 5V depending on whether the USB is detected.  I would like to set the charge current to get as quick a charge as possible in either case.

I will be using the bq34z100-G1 fuel gauge for the battery pack, with no cell balancing for the NiMH batteries.

For the system voltage, I would like 5.4V.  Downstream system loads should be less than 100mA on average.

What do you recommend for setting the jumpers, and if necessary changing some programming resistor values on my EVM and settings for bqStudio?  Please let me know.

Thanks, Carl Olen

  • 0
    TI__Guru**** 165886 points

    Hi Carl,

    The system output can be regulated to a minimum system voltage, SYSMIN, when V(BAT)<SYSMIN but then follows the battery voltage up.  If you need a fixed V(SYS)=5.4V, you will need to add a dc/dc converter following SYS.

    NiMH batteries charged at higher than 1/2 C are supposed to be terminated using dT/dt or dV/dt.  The BQ25792 can provide the constant current (CC) charging but can only terminate with constant voltage (CV).  The BQ25798's ADC does not have an accuracy spec so I recommend using the gauge and host software to implement the dT/dt and/or dV/dt termination.  If using the gauge for termination, the BQ25798's TS pin is not needed so you can either place a resistor divider to set V(TS) to 55% of REGN or you can disable the TS pin in I2C (note that the EVM has 10kohm resistor from TS to ground to simulate a thermistor at 25C).  The EVM PROG pin should be set for 2S.  The SYSMIN register needs to be set to 5.4V as the default for CELLS=2S is higher than that.  Since not using the CV loop for termination, VBATREG should be set higher than the desired final charge voltage, more like protection.  I also suggest either disabling termination or setting ITERM to the lowest setting in I2C.  The default charge current is 1A so that register must change as well.  

    Regards,

    Jeff

  • 0 in reply to Jeff F
    Expert 1160 points

    Jeff, thanks.  I will get my EVM setup going and contact you if I have further questions.

    Carl