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BQ24650: Designing a Solar Charger for IoT device

Oliver Schett
Prodigy 50 points
Part Number: BQ24650
Other Parts Discussed in Thread: BQ25570, BQ24210, BQ24250, BQ25505

Hey

So I have read the datasheet of the BQ24650 and I think it is suitable for my school project but I have a few questions. 

So we want to build a device that connects a few sensors that consume about 50mA every 5 minutes, the GSM module we want to use consumes about 1W peak (for a few millisecons) that*s all on 3,3V. The sensors measure every 5 minutes (measuring process takes about 1 minute). Then the device completely goes into sleep mode, consuming only a few µA. We want to use a solar panel with about 0.5-2W ( the voltage of the panel is gonna depend on the charger I want to use) to charge a battery pack. The batterys are 4x 18650 Li-Io in parallel with 3.7V.

So the problem is that I can't really find a charging IC with MPPT and a proper battery protection, that is Standalone (so without I²C interface or something like that). The closest I got is the BQ24650. But can I even drive a System load of the battery pack? Does the charging algorithm get confused when the battery is full but the system is consuming power, considering I connect the system load in parallel to the battery.

And is the BQ24650 even suitable for such an application that's so low powered? At first I thought the BQ24210 is the right choice but since it is linear (so it's to inefficient) and does not really have MPPT it was not suitable for my application. Then I found the BQ25570, but it has no proper charging circuitry for Li-Io batterys and it can just supply 100mA of power which isn't enough to power the GSM module.

I hope someone can help me.

Best regards

  • 0
    TI__Guru**** 166736 points
    Hi Oliver,

    All three of the parts mentioned above perform MPPT in the same manner. They use the charger's VINDPM circuit to regulate the voltage at the charger's input voltage to a fixed voltage, typically 80% of the panel's open circuit voltage (VOC). For the BQ24650, you use a resistor divider from VBUS to the MPPSET pin. For the BQ25570, you can use resistors or simply connect to VSTOR for 80% MPPT. For bq24210, you use only 1 resistor to set VINDPM ( no resistor for battery tracking mode). You can improve the accuracy over panel temp by characterizing the panel and adding a thermistor in series with the resistor as explained here: www.ti.com/.../slua586a.pdf. Therefore, you can use any charger with external VINDPM resistors, like bq24250. The VOC and ISC of panel will determine which charger to use for solar. To charge to 4.2V, you need VOC = 4.2V/0.8V + headroom.

    BTW, you can use multiple bq25505's or bq25570's in parallel if that helps.
  • 0 in reply to Jeff F
    Prodigy 50 points
    OK Thank you,

    So when I'm using the BQ24650 is it ok to connect the system load in parallel to the battery?
  • 0 in reply to Oliver Schett
    TI__Guru**** 166736 points
    Yes. Keep in mind that the charger will not be able to accurately terminate charging because the charger will see any system load current will as charge current.