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BQ25505: Solar Charging

Henrik Leegaard
Prodigy 70 points
Part Number: BQ25505
Other Parts Discussed in Thread: BQ24210

Hi

I have a system application where I have implemented a small solar panel to keep the Li-Ion battery charged when the system is in idle.
For this purpose I have used the BQ25505 as charging device.

The open circuit voltage of the solar panel can be up to 5.5V when exposed for bright sunlight.
Due to this I have added a shunt regulator across the solar panel in order not to violated the specifications of BQ25505.

But will the BQ2505 connect a load from VIN_DC to GND in case the battery is fully charged and have hit the VBAT-OV level?
If so, can I removed the shunt regulator circuit (U208, R216 and R217) and save some cost?

Furthermore the MPPT point of the used solar panel is approx. 4.3V.
I have put a resistor divider from VIN_DC to get the VOC_SAMP input pin to approx. 4.3V, but my question is know if the output voltage of the solar panel is below 4.3V (when the panel is exposed to sunlight on a clouded day or exposed to artificial light) will the battery then receive any charge at all?
Or do the output voltage of the Solar panel need to exceed 4.3V before the battery charging is started.

This use case is not clearly explained in the specification for BQ25505.

The Li-Ion battery for the system is connected to VBAT_SEC pin (VSYS domain) and the system loads are connected directly to this power domain.
The battery can also be charged by other means (Inductive charging) as the solar charging is just a mean to prolong service life between main charging's.

Looking forward to hear your comments


Regards

Henrik

  • 0
    TI__Guru**** 165886 points

    Hi Henrik,

    The BQ25505 turns on a FET to GND from VIN_DC if VMPP > VBAT_OV.  The pull down FET has RDSon in the 10 ohm range.  If the solar panel is still high enough impedance, the pull down FET should be able to pull down the input.  

    The MPP resistors, connected at VOC_SAMP, set a fixed percentage of the open circuit voltage as the MPP.  With the resistors above, VMPP = 86%*VOC.  For VOC=5.5V, that is 4,7V, which should trigger the pull down FET at VIN_DC.

    The shunt resistors are quite low.  What is the ISC for the panel?  If it is high (meaning a lower impedance panel), I would not recommend this device without the shunt regulator.  Maybe the BQ24210 would be a better fit.

    Regards,

    Jeff

  • 0 in reply to Jeff F
    Prodigy 70 points

    Hi Jeff

    Thank you for the commmets

    The ISC for the Solar panel in used is approx. 40 - 45mA at bright sunlight.

    If I understand you correct, it woyuld be better to connect VOC_SAMP diectly to VSTOR to get the MPPT point 80% of the VSTOR voltage.
    And keep the Shunt regulator to clamp the voltage when the battery voltage exceed the VBAT_OV level in order not to viaolate the maximum input voltage of BQ25505

    But you are right the BQ24210 might be a better fit.
    If this device was used I would set the max charge current to approx. 50 - 100mA and leave the VDPM pin open for Battery tracking mode.

    Regards

    Henrik

  • 0 in reply to Henrik Leegaard
    TI__Guru**** 165886 points

    Hi Henrik,

    80% is the typical MPP% for solar panels.  If your panel is different (i.e. has a higher MPP%), then you can set it higher.  Your shunt's resistor are quite low which will load the panel. You might only need a zener clamp to protect the charger input for over-voltage.  The charger's internal pull down resistor should protect the battery if its ISC is only 40-45mA.  During the time when the pull down protection is on, the battery will not charge.   

    The BQ24210 is a linear charger so its maximum output current = its input current.  You could only charge up to 40-45mA even in battery tracking mode. 

    Regards,

    Jeff