• State Resolved
  • Locked Locked
  • Replies 11 replies
  • Answers 3 answers
  • Subscribers 64 subscribers
  • Views 4511 views
  • Users 0 members are here
Support feedback
Options
Options
Related

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

BQ25504: Very low current input from solar cell charge li-ion battery

Le Giang
Prodigy 180 points
Part Number: BQ25504

Hi everyone, 
I'm planning to use bq25504 to charge a 18650 li-ion battery from a solar cell that provided only 2.2mA input.
I'm considering whether it can charge or not because i think the input current from solar cell is too low to charge a battery.
I would like to know with that current the bq25504 can charge or  not?

Thanks and best regard.

  • 0
    TI__Mastermind 21590 points

    Hello,
    What is the capacity of the battery and the open circuit voltage of the solar cell?
    -Raheem

  • 0 in reply to Raheem
    Prodigy 180 points

    Hi Raheem
    The capacity of the battery is 1200mA
    The open-circuit voltage is 4.5V 
    For further concern, as I know the bq25504 only have CV charging and not implement CC charging, so is it OK for charging li-ion?
    If yes is there any problem with that?
    Thank for fast reply

    Regards,
    Le

  • 0 in reply to Le Giang
    TI__Mastermind 21590 points
    Le,
    There are no issues with this. The only thing is that the lower the input current, the longer it takes to charge the battery.
    -Raheem
  • 0 in reply to Raheem
    Prodigy 20 points

    Hello Raheem,

    All the literature on Li-ion batteries say that it has to be charged with a constant current phase, then switch to a top-off phase.

    Would you kindly elaborate on how the boost charger in the BQ25504 works?  Does it have a built-in current limit or does it rely on the

    solar cell being used at the max power point?  It seems the MPPT feature effectively draws whatever current needed to extract max power

    from the solar cell.  Then, is this the same charging current into the Vbat pin?

    I am considering this device for a wireless sensor with a Li battery and solar cell.  I would like to understand if the

    battery is charged properly, so that lifetime and safety are maximized.

    When there is sun light, I would like the BQ25504 to power the sensor from the solar cell and charge the battery.  At night, it would

    let the battery power the sensor.  Is this how the BQ25504 works?  It is not explicitly stated in the datasheet, nor in document SLUU654a.

    Otherwise, are there other harvester devices with a complete Li-ion/Li-Po charger you can point me to?

    Thanks for your help.

    Maurilio

  • 0 in reply to Maurilio Fabiano
    Prodigy 180 points

    That is exacly what i concern about BQ25504 boost charger. 
    Please someone kindly expain how the boost charger work and some VSTOR/VBAT diagram depend on VIN_DC.

  • 0 in reply to Le Giang
    TI__Mastermind 21590 points

    Hello,

    The BQ25504 is a Current Limited Constant Voltage charger. The peak input current is limited to 300mA as shown below. The MPPT draws the maximum current it can from the harvesting device as long as it is below the peak input current. Since the load is connected to the VSTOR pin which is connected to VBAT by a FET, the battery can be charged and also power the load at the same time. When there is little power from the harvesting device, the load is powered by the battery. The FET between VBAT and VSTOR shuts off when VBAT goes below VBAT_UV. The BQ25504 will work for your application. -Raheem

  • 0 in reply to Raheem
    Prodigy 180 points

    Hi Raheem,
    Could you tell me what is the Constant Voltage if I set VBAT_OV=4.2V? Does it 4.2V? If yes, the boost charger off when VSTOR reaches 4.2V then no more constant voltage applies to VBAT pin. So i think it is not contant voltage anymore and the battery couldn't charge to full capacity at 4.2V. 

  • 0 in reply to Le Giang
    TI__Mastermind 21590 points

    If VBAT_OV = 4.2V, then the CV voltage is 4.2V. The boost charger turns off when VSTOR = VBAT_OV and no CV is applied to VBAT because the FET ebwteen VBAT and VSTOR is always on unless VBAT < VBAT_UV. As explained below, charging resumes when VBAT goes below VBAT_OV - VBAT_OV_HYST to keep the battery fully charged as explained below.

    -Raheem

  • 0 in reply to Raheem
    Prodigy 20 points

    Thank you, Raheem.  I figured that much from the datasheet.  I think Figure 12 in the datasheet is also a big clue, but not in a straight forward statement.  The operational scope pictures also take some time to interpret, so thanks again for putting it in words.

  • 0 in reply to Raheem
    Prodigy 180 points
    Thank you Raheem.
    I had used TINA TI to simulate the transient BQ25504 model and I saw VSTOR constant voltage depend on the capacitance of the capacitor in VBAT pin. The lower the capacitance, the lower voltage on VSTOR. The VSTOR increases gradually to VBAT_OV, doesn't it?
  • 0 in reply to Le Giang
    TI__Mastermind 21590 points
    Yes it does.
    -Raheem