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Original question:

BQ24650: bq24650

BQ24650: Precharging issue

Gururaja K
Prodigy 150 points
Part Number: BQ24650

Hi ,

We are using Li-ion Battery pack in solar street light application (operating voltage is 12V to 16.4V). When the battery voltage is below 11V and low sun intensity during early morning time. During this condition charge controller BQ24650 is unable to supply required power to reach 12V within 30 minutes pre-chargin time.  Charge controller will terminate pre-charging. Note: It difficult for the user to reset the device 

VFB pin needs to be configure between 1.54V (Low) to 2.1V (High) and there is constraint that voltage divider values will not match with these voltage  and  it should be within operating voltage i.e 12V to 16.4V. 

Please suggest us solution

Regards

Gururaja K

  • 0
    TI__Mastermind 24075 points

    Hi Gururaja,

        Toggling MPPSET pin low is the best option. It will reset the safety timer, and essentially start a new 30 minute precharge timer which would need to be done until battery voltage can rise above VLOWV = 1.55V (measured on VFB pin). This would require external logic or user control (either a MCU to detect battery voltage and then driving the gate of a FET connected to MPPSET, or a user push button that can drive a FET to pull MPPSET low are some examples). The BQ24650 EVM has a charge disable option whose design can followed.The other option is a POR (power on reset) which would be removing input voltage and re-applying.

  • 0 in reply to Kedar Manishankar
    Prodigy 150 points

    Hi Kedar,

    Thanks for the suggestion. 

    There is one more test scenario, when Battery voltage is less than 11V then battery pack will enter into under voltage cutoff mode. So battery pack voltage will be zero volts. No power to micro controller and rest of the circuit.

    In this condition we need to charge the battery pack bypassing MPPT charger controller circuit till voltage reaches 12.5V then enable MPPT pin to High.

    Regards

    Gururaja K

  • 0 in reply to Gururaja K
    TI__Mastermind 24075 points

    Hi Gururaja,

        Generally it is not best practice to discharge battery so low, but even when pack is off it should still have a conducting path to the battery to charge at low currents through the charge, discharge FETs. When pack is disconnected, BQ24650 employs battery detection as it doesn't detect voltage, and should source 500mA in pulses (see figure 12), which will slowly charge the battery until it reaches a certain voltage at which point the FETs should open, allowing BQ24650 to detect battery voltage and start normal pre-charge. This initial time could be very long depending on how low you discharge your battery. This question also depends more on your battery pack manufacturer specifications than the BQ24650 charge controller itself.

  • 0 in reply to Kedar Manishankar
    Prodigy 150 points

    Hi Kedar,

    In solar street light application, we can't predict sun intensity during rainy season. If street light draws current continuously then battery pack voltage may enter into under voltage cutoff mode. In this condition battery voltage will be zero voltages.

    In this condition we are bypassing BQ24650 charge controller chip (enabling MPPT MOSFET gate) and directly charging battery pack from solar power. Once battery voltage is above 12.5V then battery pack will charge from BQ24650 (disabling MPPT MOSFET gate).

    Above solution is working. Please let me know your feedback for the same

    Regards

    Gururaja K

  • 0 in reply to Gururaja K
    TI__Mastermind 24075 points

    Hi Gururaja,

        Yes disabling MPPSET is a good way of disabling BQ24650 charge and directly charging , but I would check with battery manufacturer to make sure that you are charging at safe currents when at low battery voltages.