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BQ25504: Supercap or LiIon battery?

Fernando Maesso2
Prodigy 90 points
Part Number: BQ25504
Other Parts Discussed in Thread: BQ25505, BQ24210

Hi, 

   

I have four solar cell with 5V - 40mA (at full sun) connected in parallel. Normally only 2 or 3 cells will be sourcing energy due the shadow. My circuitry will be need 5V (I will use TPS61026DRC), with something like 20mA /h and 5 peaks of 20s /h of 120mA. At day the solar cells will be capable to sourcing but at nigth my storage will should be. I thougth to use either 1000mA/h Li-Ion battery or 500F supercap. In other circutry I´m tested a supercap with 500F but it is slow to charge. I saw the BQ25504 can charge supercaps ou li-Ion battery. My questions are: 1 - It will charge the supercap or a battery with a variable input current? 2 - The charging process for Li-Ion battery are similar to other charger? 3 - Are there some special setting to use Li-Ion battery?

Thanks

  • 0
    TI__Mastermind 20570 points

    Hello Fernando,

    The BQ255xx family of devices does not do a typical CC/CV charging profile. It only transfers the input power tot he battery/supercap. As most of the solar source can provide very low power to the battery which is typically much lower than the battery's termination threshold. Thus the typical CC/CV type of provide would be less critical here.

    Please refer tot he datasheet for setting up the charger parameters for Li-ion battery.

  • 0
    TI__Guru**** 165766 points

    Hi Fernando,

    First, the BQ25504 max input voltage is 3.0V so it is not recommended for a 5V panel.  You could use the BQ25505, which can go up to 5V.  Keep in mind both are boost only converters so their output voltage must be set higher than the MPP voltage.  For a 5V panel, the MPP voltage is likely 80%*5V =4V so your final constant voltage (CV) point must be higher than 4V.

    Regarding supercap vs. LiIon charging, most supercaps can be charged at max constant charge current (CC) to CV from complete discharge (0V).  Ideally, for longest life, LiIon batteries are charged at a reduced precharge current until a certain voltage (~2.5-3.0V) then max (fast) CC then CV until the charge current naturally tapers down to a set termination current.  That said, the BQ255xx family only provides CV charging.  The reason CV is okay is because the charger's maximum average input current is limited to about 100mA, which gives a low maximum charge current of VMPP*100mA*efficiency/CVmax, e.g. 4*100mA*.9/4.2V = 85mA. The charge current cannot not be changed and will likely be even lower as the panel gets shade.

    That charge current is a bit low (resulting in long charge time) for either your LiIon battery (1000mAHr/85mA>10Hrs) or supercap (use i=c*dv/dt->85mA*500F*dv/dt).  If you don't need to fully charge the LiIon battery, you might consider the BQ24210 linear charger.  It would be able to charge the battery only up to VMPP=4V but would do so with 4*40mA=160mA.  You could use it for the supercap too but it has lower precharge current at lower voltages so the charge time until the battery voltage gets high enough will be long.  Another option for the supercap only, is to use buck or boost converter (depending on the supercap output voltage) with an opamp to implement MPPT (so the converter doesn't pull too much current and collapse the panel) as explained in the app note below:

       

  • 0 in reply to Jing Zou
    Prodigy 90 points

    Thank you very much!