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BQ25798: Charging not working, STAT flashing, fault registers not showing any faults, ADC is unable to measure VAC1, VAC2 and VBAT

David P
Prodigy 20 points
Part Number: BQ25798

Tool/software:

Hello TI,

We are trying to build a charging circuit using the bq25798 IC. It takes power from USB and/or Solar (VAC2). A 2S battery is connected to BAT pins. We would like the bq25798 to run in the default mode (no host) and charge the 2S battery. Schematic is attached below.

We are seeing these peculiar issues:

  • STAT LED blinks at about 1Hz, but fault registers do not indicate any faults (both are 0x00)
  • Charging is not working - neither from the solar panel nor from the USB input (confirmed by querying the registers and measuring the battery voltage over a period of time)
  • We polled the ADC registers and it is showing ~5VDC on VBUS but shows nothing or very low voltage (0.1xx VDC) on VAC1, VAC2, and VBAT. VAC1 should be the same as VBUS. VBAT should be around 7.5 VDC as measured externally by us.
  • When unplugged from USB, the IC is running from batteries, but VBAT_ADC still show 0VDC

Here are some of the register values:

REG00_Minimal_System_Voltage: 7.00 V
REG01_Charge_Voltage_Limit: 8.40 V
REG19_ICO_Current_Limit: 3.250 A
REG03_Charge_Current_Limit: 1.00 A
REG35_VBUS_ADC: 4.864 V
REG37_VAC1_ADC: 0.117 V
REG39_VAC2_ADC: 0.000 V
REG3B_VBAT_ADC: 0.000 V - batteries are present and both BAT and BATP pins are connected
REG20_FAULT_Status_0: 0x00
REG21_FAULT_Status_1: 0x00
REG48_Part_Information: 0x19
REG1B_Charger_Status_0: 0x2b
REG1C_Charger_Status_1: 0x07
REG1D_Charger_Status_2: 0x01
REG1E_Charger_Status_3: 0x00
REG1F_Charger_Status_4: 0x08
REG17_NTC_Control_0: 0x7a
REG18_NTC_Control_1: 0x54
REG2F_ADC_Function_Disable_0: 0x00
REG30_ADC_Function_Disable_1: 0x00
REG0F_Charger_Control_0: 0xa2
REG10_Charger_Control_1: 0x85
REG11_Charger_Control_2: 0x40
REG12_Charger_Control_3: 0x00
REG13_Charger_Control_4: 0x01
REG14_Charger_Control_5: 0x1e

The bq25798 IC is powering nRF9160 and the i2c communication is happening from the nRF IC.

Any help is appreciated. We expected the "default" mode will work out of the box and have been pulling out hair out for quite some time now.

Thank you.

  • 0
    TI__Mastermind 31976 points

    Hello David,

    I haven't seen a setup like this before, I recommend following the schematic checklist:BQ25790_BQ25792_BQ25672_V1p0_Schematic Checklist (14).xlsx

    David P said:
    STAT LED blinks at about 1Hz, but fault registers do not indicate any faults (both are 0x00)

    This fault is causing the device not to charge in default mode, and is likely cause by the low voltage on VAC1 and VAC2.

    Have you measure this value externally?

    Best Regards,

    Christian.

  • 0 in reply to Christian Moyer
    Prodigy 20 points

    Thank you for your comment Christian.

    I think we are getting closer. We poked and prodded the IC yesterday and discovered few things:

    Apparently the ADC (on all channels) is not enabled by default. We have enabled it and it now measures all voltages, albeit not really accurately. 

    We tinkered around with the TS pin and finally disabled the thermal check and now it charges from USB. Hence our resistor combination must have been wrong. I still want to see if it charges from the solar panel. Waiting on sun to poke its head from behind the clouds.

    So far so good.

    One question though - since it seems to have been not charging because of the TS input, how come none of the fault registers indicated thermal problem - too cold or too hot?

    David

  • 0 in reply to David P
    TI__Intellectual 995 points

    Hi David,

    Looking at your schematic, are you using the thermistor of the battery? Please refer to figure 9-12 from the datasheet for reference.

    Best Regards,

    Jibin