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Original question:

Solar panel charging solution for lithium batteries

BQ25505: Help answer questions

rengui sun
Intellectual 680 points
Part Number: BQ25505
Other Parts Discussed in Thread: BQ24210

1. Under the fixed installation of solar panels, does the VOC_CAMP connection method affect the MPPT regulation?

2. How does the bq25505 ensure that the battery is fully charged and does not damage the battery because there is no constant voltage and cross current charging mode of the battery management chip. Can you simply judge whether the battery is fully charged according to the VBAT_OV pin voltage?

3. What is the current that the MOS between VSTOR and VBAT can withstand? The maximum load of VSTRO in the circuit is 350mA. In the dark state, the battery needs to output 350mA current for about 10s.

  • 0
    TI__Guru**** 166626 points

    Hi Rengui,

    Regarding 1, the VOC_SAMP capacitor holds the MPP voltage that is periodically sampled.  The capacitor must be low leakage.  Alternatively, you can apply an external voltage at the MPP value to VOC_SAMP.

    Regarding 2, BQ25505 only performs constant voltage regulation, i.e. it prevents the battery from being "over" charged above the VBAT_OV setpoint.  Because the converter is a peak current discontinuous mode boost converter and because of high impedance sources like solar panels, the converter charges in burst of current not continuous current.

    Regarding 3, PMOS between VSTOR and BAT has RDSon = 0.95 ohm typical and 1.5 ohm maximum.  This practically limits the current to 0.3A-0.5A due to the voltage drop created.

    Regards,

    Jeff

  • 0 in reply to Jeff F
    Intellectual 680 points

    Hi Jeff,

    Thank you for your reply.

    Regarding 3, help recommend the same type of chip with a current limit greater than 0.35A.
    The relevant description of the current limit value 0.3A-0.5A is not found in the manual. How can I calculate it?

    Regards,

    Rengui

  • 0 in reply to rengui sun
    TI__Guru**** 166626 points

    Rengui,

    As I mentioned, the high RDSon causes a voltage drop from BAT_SEC to VSTOR.  Practically, if the battery is at 3.6V, a voltage drop of more than 0.5V=0.5A*1ohm typical would put 3.1V or less at VSTOR.  If your system connected to VSTOR can handle that, then you could still use this IC.

    The next closest IC to your application is BQ24210, which is a buck only charger.  This charger requires a solar panel with MPP > the battery voltage. 

    Regards,
    Jeff