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BQ24650: BQ24650 with 6V solar panel and 3.7V LIPO battery

Mariem Chaabani
Intellectual 255 points
Part Number: BQ24650

Hello everyone,

First, I hope you are all doing well.

I am currently working on a project that will be using a solar panel interface as well as, for now, a 3.7V lipo battery (at night). So the Solar panel would supply the system and charge te battery in daytime, and the battery will take over at night.

So I thought that the BQ24650 would be adequate for this use case. But I am a little worried as I've seen many posts (without final answers) where people, using a 6V solar panel had trouble charging the battery, and I am using solar panels for the first time.

I need your help in order to answer some questions, and to be sure I am using the right components and values.

1- Does MPPSET voltage must be 1.2V sharp? Or is 1.2V a minimum? Couldn't there be fluctuations in the voltage supplied by the solar panel so that the voltage is not always the same depending on the sunlight intensity? (I think the fluctuation might be in current not in voltage)

2- Here are the characteristics of the Solar panel I am thiking of using:

  • Open Circuit Voltage: 7.7V
  • Peak Voltage: 6.5V
  • Peak Current: 930mA
  • Peak Power: 6.0W
  • Power Tolerance: +/-10%

So for MPPSET,  I should use a divider to get 1.2V from 6.5V right? We calculate using the peak voltage?

3- As the Max charge voltage for 3.7V Lipo batteries is usually 4.2V, then I shoud set VFB to 4.2V (or 4.1V for safety), correct?

4- As I said, I want to use the solar panel at day, and the battery at night (or couldy day) to power the rest of the board, so I should make the same setup as the EVM with D3 between VBAT and VSYS, correct?

                           

That means that VSYS (and I mean the supply going to the board) will vary between 6V (from solar panel) and about 3.1V to 3.7V (with D3 voltage drop, and depending on how much the battry is charged), correct?

4- In this case, as my system uses 3.3V, I was thinking Ishould place a regulator to drop 6V to 3.3V before D3. But in that case, when the battery is charged and the solar panel is still working, wouldn't VBAT take over as 3.7V is greater than 3.3V? I am a little stuck on this issue.

Thank you for your help.

Best regards,

Best regards,

  • +1
    TI__Mastermind 24075 points

    Hi,

    1.   The panel voltage associated with 1.2V on MPPSET pin corresponds to the maximum powerpoint of your solar panel. This is how you calculate the resistor configuration for MPPSET. When solar panel voltage starts to fall, the input voltage regulation loop kicks in to prevent the solar panel voltage drooping further, which allows the solar panel to operate at its maximum powerpoint for as long as possible
    2. Yes, you calculate according to the maximum powerpoint of the solar panel
    3. Set charge regulation voltage to max charge voltage of battery. The charge regulation accuracy is very accurate, and CV charging phase allows for charging real battery to full capacity
    4. Yes, but when battery is connected voltage can range up to max charge regulation voltage. You don't use nominal voltage.
    5.  No, the input to your regulator depends on how diode D3 is biased. If input source is present then you wouldn't have battery as the input source voltage > battery voltage. You need to consider the case when it is night time, and battery voltage < 3.3V. your buck/linear regulator cannot boost the battery voltage to the required 3.3V rail in that case.
  • 0 in reply to Kedar Manishankar
    Intellectual 255 points

    Hello Kedar,

    Thank you for your help.

    Ok, all is clear.

    As an update, while working further on the schematics, I found out that I will be needing 5V power supply for some components (4.5 to 5.5V), so there is no way the 3.7V battery would provide this unless I use a booster, which I won't (we have RF and bossters caused problems before).

    So I think I will be upgrading to a 12V solar panel, and a 7.2V LI-ION battery. This resolves my issue with the regulator as I'll be putting one (or maybe DCDC) for 12V to 5V at VSYS (after D3).

    Thank you again.

    Best regards,