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BQ24620: Battery charging under load

Alex G
Prodigy 80 points
Part Number: BQ24620
Other Parts Discussed in Thread: TPS62175

In our application, we have a battery that is always powering a few circuits and occasionally gets charged up by an external power source. We would like to use the bq24620 to charge this always-on battery (side question - is there a better part for this application?).

What we are seeing is the bq24620 often indicates "Charge suspend, timer fault, ACOV, battery absent BLINK (0.5 Hz)" according to the STAT pin. Disconnecting and reapplying the external power source (12V) will clear this error and begin charging. The voltage at the VFB pin is around 1.4V-1.5V, which should be well within the limits for "fast charge" rate.

For a bit more context, the "always-on" circuits have a "Run" mode and a "Shutdown" mode. The Run mode consumes approximately 100mA @ 9V, and the Shutdown mode consumes approximately 100uA @ 9V. We are not able to charge in Run mode, only Shutdown mode, and only after disconnecting and re-applying the external 12V after entering Shutdown mode. We would like charging to be active in either Run mode or Shutdown mode, as long as an external 12V is applied.

My question is, could the "always on" nature of the load circuits connected to the battery affect charging? Are we supposed to disconnect the load from the battery while charging? Any other clues to this behavior?

  • 0
    TI__Mastermind 24075 points

    Hi Alex,

       Great question. To clarify, it sounds like you are looking to always power SYSTEM, regardless of if you have adapter (in which case, while adapter is connected the battery should be charged along with supplying SYS) or if you only have battery, in which case it should  directlysupply SYS.

    This describes in a nutshell, the powerpath features of our device. There are a couple of different powerpath implementations so please let me know:

    • Desired input voltage
    • Desired charge regulation voltage and battery chemistry
    • Desired charge, precharge and termination currents
    • Should SYS be regulated, and if so, to what voltage?
    • Is communication interface (I2C/SMBus) required or do you want a standalone charger?

    You will not need to disconnect the load while charging, as the IC will handle these transitions on its own and have clean transition to supplying SYS either from adapter or battery. Let me know the system details above, and I will recommend another part if I don't think BQ24620 is suitable for your application.

    When referring to SYS, this would refer to your "always-on" circuits.

  • 0 in reply to Kedar Manishankar
    Prodigy 80 points

    Hi Kedar, thanks for the reply.

    Yes, your clarification on the powerpath is accurate.

    I am still not sure why we often get an error (blinking on STAT) pin and no charging sometimes. Here is the sequence of events:

    1. Plug in battery - SYS gets powered from BAT (approx 100mA @ 9V)
    2. Enter Shutdown mode - SYS still powered from BAT (approx 100uA @ 9V)
    3. Plug in ADAPTER - charging of the battery commences
    4. Exit Shutdown mode - SYS powered from ADAPTER (approx 80mA @ 12V) - note charging of the battery has stopped and we get blinking on STAT? Why?
    5. Enter Shutdown mode - SYS powered from ADAPTER (approx 80uA @ 12V) - note charging of the battery has still stopped and there is still blinking on STAT? Why?
    6. Unplug and replug in ADAPTER (POR event for the bq24620) - charging of the battery resumes

    Answers to specific questions below:

    • Desired input voltage
      • 6-16V on ADAPTER, 6-12V on BAT
    • Desired charge regulation voltage and battery chemistry
      • 10.8V, 3S LiFePo4
    • Desired charge, precharge and termination currents
      • Charge: 220mA, precharge: 125mA OK, termination: 22mA
    • Should SYS be regulated, and if so, to what voltage?
      • SYS feeds into TPS62175 which regulates to 3.3V (this seems to be an appropriate choice but open to alternative suggestions)
    • Is communication interface (I2C/SMBus) required or do you want a standalone charger?
      • Communication is not required

    Thanks,

    Alex

  • 0 in reply to Alex G
    TI__Mastermind 24075 points

    Hi Alex,

       What is measured VCC voltage and measured SRN voltage, when you have adapter and battery connected to the system?

  • 0 in reply to Kedar Manishankar
    Prodigy 80 points

    Hi Kedar,

    All measurements below are with adapter and battery connected:

    VCC: 11.5V

    SRN: 9.0V

    STAT: blinking

    PG: low

  • 0 in reply to Alex G
    Prodigy 80 points

    Update:

    We found the problem to be with the TS (thermistor input) measurement. We were buffering the thermistor raw input with an op amp and the buffer circuit was affecting the raw value. After fixing the buffering circuit, the bq24620 charging works as expected.

    Thanks Kedar for your replies and confirming for us that charging while the battery is powering a load is OK. We would still appreciate if there is a recommendation for a better charging IC than the bq24620. For now our main issue is solved!

    -Alex

  • 0 in reply to Alex G
    TI__Mastermind 24075 points

    Hi Alex,

       At low charge currents < 500mA a linear charger is recommended over a switching charger, but generally these parts are designed for low cell count (note linear chargers can also do powerpath, which is something to keep in mind for future designs). After seeing that you need 3S LiFePO4 this charger would be good for you.

    Glad you were able to resolve your issue regarding the TS pin. Once we see fault indication the next step is to step through the different fault conditions.