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BQ24055 not terminating charge when appropriate

Timothy Starr
Expert 2445 points

Hello-

We are using the BQ24055 our system, and sensing a thermistor in the battery pack: NTC: 10K, Beta: 3435

Test conditions:

  1. Battery is connected to the system which has the TI charger.
  2. Wall Charge is plugged into the system
  3. System is in use – so there is a load current also. The battery charge curent is 600mA and the system load is 200mA.
  4. System works as expected, charger turned on and battery charging starts
  5. System is in use for more than 10 hours with the charger connected.

Observations and Enquiries:

  1. Charger is still on beyond 10 hrs. Is this not contrary to the specified time out?
  2. Measured voltage at TS pin, is about 0.42V, whereas on page 7 of data sheet, it states that charge enable threshold is 175mV maximum. Is this  expected behavior?

to elaborate:

  1. Voltage at TS pin is 0.42V and timer of 10hrs has expired, but the charger is still ON, though as per datasheet in this scenario it should go into Fault condition after 10hrs if charge termination does not happen and the charger has not entered into TTDM.  Why is the charger still ON?
  2. We need to have  clarity on reset of 10 hour safety timer implementation in the charger.
  3. TS pin has two features: when TS pin is pulled low or floated/driven high. A low disables the charger and high puts the charger in TTDM. Thresholds are not clear for high , LOW and Float condition of TS pin.
  4. What is the process for enabling or disabling the charger?

I can send a schematic if that would help, but I would prefer not to post it.

Thanks and Best Regards,

Tim Starr on behalf of VC@AR

  • 0
    TI__Mastermind 20570 points

    The 178 is the HOT temp threshold, the resistance of the thermistor should vary as battery temperature pack temperature changes. Thus the voltage on the TS pin changes as well. Please see the EC table on page 7 for TS thresholds. The TTDM threshold is VTTDM(TS) and the disable threshold is VTS-EN-10K - VTS-DIS_HYS-10K. 

    Regarding on the safety timer, the fast charge timer is fixed at 10 hours and can be increased real time by going into thermal regulation, INDPM or if in USB current limit. The timer clock slows by a factor of 2, resulting in a clock than counts half as fast when in these modes. The operation of these modes can be found in the datasheet feature description section.

  • 0 in reply to Jing Zou
    Expert 2445 points
    Hi Jing-

    If ISET2 is pulled low, does the USB current limit still apply? I don't see how it does, but the data sheet is unclear. Thanks again,

    -Tim
  • 0 in reply to Timothy Starr
    TI__Mastermind 20570 points
    If ISET2 is pulled low, then input current limit is set to whatever current is set on ISET. Thus if ISET2 is pulled low, the IC might not be in USB current limit but in INDPM (IINDPM/VINDPM). You might want to check your adapter power rating.
  • 0 in reply to Jing Zou
    Expert 2445 points
    Hi Jing-

    If the system is connected tot he charger, and is drawing 200mA, how does the battery charger know when the battery is fully charged?
    Related question: what is the criteria for setting CHG/ high impedance?

    Best Regards,

    -Tim
  • 0 in reply to Timothy Starr
    TI__Mastermind 20570 points

    The bq24055 does not have power path function which has separate control for charging and system loading.  Thus, the bq24055 cannot differentiate which current from the OUT pin is to the battery which is to the load. Therefore, if there is a constant load connected at OUT as weill, the charger cannot detect charge done.

  • 0 in reply to Jing Zou
    TI__Guru 71526 points

    Then what has to be added to detect the charge parameters?  I would think with a charger and battery management, most applications will want to know the battery charge status.  Do we have a reference design that would implement this?

    ~Leonard 

  • 0 in reply to LeonardEllis
    TI__Mastermind 20570 points

    This is why we introduced the power path function in bq2407x/bq2403x. Without power path function, the charger itself only monitors the output current at the OUT pin, and cannot differentiate the what is going to the load and what is going to the battery. 

    If the customer would like to have separate control for charging path and system load, they would need to use a charger with power path function.