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BQ25170J: Termination current error

João Rego
Intellectual 286 points
Part Number: BQ25170J

Tool/software:

Hello,

I am charging a battery using a charger current of 480mA (ISET). Based on the datasheet, the charger termination current should be 10% of this value (48mA) with an error of 10%. However, I am getting an error higher than this 10%. The IC is terminating the charging process around 70mA (this value is constant). Due to this error, the fuel gauge never reaches 100%.

I have another PCB with this charger IC working properly, stopping the charging process at around 45mA.

Is this error normal? What can I do to fix it?

Thank you.

Best regards,

João Rego

  • 0
    TI__Guru* 75876 points

    Hi João 

    The termination current should be 10% of the fast charge current with a 10% accuracy.

    How are you measuring the termination current? is this the Gas Gauge or external meter.  

    Note that current can also be monitored using, ISET pin.

    Is the system connected to battery during charging? 

  • 0 in reply to Bill Johns
    Intellectual 286 points

    Hi Bill,

    So, worst-case scenarios are 43.2mA and 52.8mA (for a charging current of 480mA).

    I am measuring the current through the Fuel Gauge (average value).

    Yes, the system is connected indirectly to the battery (the system is connected to the fuel gauge which is then connected to the battery). The 70mA that I am referring to is the fuel gauge current measurement current plus the system current (ICHG =IFuelGauge + ISYS).

    How can I use the ISET pin to monitor the current? Is its voltage proportional to the charger current?

    Thank you.

    Best regards,

    João Rego

  • 0 in reply to João Rego
    TI__Guru* 75876 points

    Hi João 

    João Rego said:
    How can I use the ISET pin to monitor the current? Is its voltage proportional to the charger current?

    The current out of ISET pin is a 1 to 300 sample of output current.  When in current regulation the voltage at ISET will be 1.00 V and termination will be 100mV.

    The deglitch time is 30mS, current will need to be below 10% continuously to terminate.

  • 0 in reply to Bill Johns
    Intellectual 286 points

    Hi Bill,

    As suggested, I measured the voltage at ISET. Although the voltage was 1.097V during charging, the charging process was terminated at 0.153V. This is far from the expected 0.1097V. 

    What I did next was to change the battery (same reference though). In fact, with this new battery, the charger ended at the expected current value (0.113V at ISET pin). 

    Is the deviation normal from battery to battery? What can I do to fix it?

    Thank you.

    Best regards,

    João Rego

  • 0 in reply to João Rego
    TI__Guru* 75876 points

    Hi João 

    What can you tell me about the battery you are charging?

    Which gauge are you using?

    Resistance between charger and battery has caused problems in the past, see it a lot with current meters (shunt resistance).  Gauge will have a sense resistor.  The battery pack will probably have a protections module with some resistance also.

  • 0 in reply to Bill Johns
    Intellectual 286 points

    Hi Bill,

    I am using the LP-443440-1S-3 battery. Its internal typical resistance is 115mOhm (165mOhm maximum). I have attached the datasheet.

    Regarding the fuel gauge, its internal resistance is 30mOhm.

    BAK_Industries-LP-443440-1S-3-datasheet.pdf

    Thank you.

    Best regards,

    João Rego

  • 0 in reply to João Rego
    TI__Guru* 75876 points

    Thanks for the information.

    I am going to contact you using the PM feature, would like to move the conversation there.