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BQ25756: Solar Application - Part selection query

Satish Deshpande
Prodigy 60 points
Part Number: BQ25756
Other Parts Discussed in Thread: BQ25750, BQ34110

Tool/software:

Hi, We are exploring the suitability of BQ25756 in Solar Tracker system. Below are the specifications:

1. Input power 80W/100W PV panel, VMP=33V & VOC=43V

2. Battery 4AH or 6AH, LiFeO4, 8-cell

3. System load (Battery Charging + DC Motor + MCU with associated sections) ~100W

We would like to understand when system demands full power (100W) and if the PV source is not capable to provide all power, will BQ25756 able to source required extra current from battery? Can you please provide some insights of the operation?

-Satish

  • 0
    TI__Mastermind 31185 points

    Hello Satish,

    Thanks for working on this. I think the BQ25756 should work for this application.

    Satish Deshpande said:
    We would like to understand when system demands full power (100W) and if the PV source is not capable to provide all power, will BQ25756 able to source required extra current from battery?

    Yes, the battery can supplement the system load if the PV source is not capable.

    Satish Deshpande said:
    Can you please provide some insights of the operation?

    The BQ25756 is a non-power-path charger. The system load will need to be connected in parallel with the battery. In this configuration, the battery will begin supplementing the system current when the charging current is not enough.

    Let me know if this makes sense and let me know if you have any other questions.

    Best Regards,
    Ethan Galloway

  • 0 in reply to Ethan Galloway
    Prodigy 60 points

    Hi Ethan,

    Thank you for your response. 

    Since you mentioned about non-power-path charger, I digged further and realised that the system current will be equal to Battery charging current in the case of BQ25756. This I see a drawback in scheme.

    I have gone through BQ25750 datasheet. This has a Powerpath scheme. So do you think this is a better solution?

    In case of BQ25750, will it share the power between PV source and Battery to match the load requirement? Will this be a Auto-Reverse mode? 

    Best Regards

    -Satish

  • 0 in reply to Satish Deshpande
    TI__Mastermind 31185 points

    Hello Satish,

    Your welcome.

    I don't think the BQ25750 will be a better solution. The BQ25750 can't really supplement the PV source or share power between the PV source and the battery. With the BQ25750, the system will always be completely powered by either the PV source or the battery.

    For the the BQ25756, can the customer increase the charge current when the system current increases?

    Best Regards,
    Ethan Galloway

  • 0 in reply to Ethan Galloway
    Prodigy 60 points

    Hi Ethan,

    Thanks for the clarification on BQ25750.

    For the BQ25756 we can increase the charge current looking at the input PV source status. I see another issue, we won't be able to differentiate between actual battery charging current and system (load) current during normal operation since we have only one sense resistor. Any suggestion to handle this scenario? 

    Best Regards

    -Satish

  • 0 in reply to Satish Deshpande
    TI__Mastermind 31185 points

    Hello Satish,

    Satish Deshpande said:
    I see another issue, we won't be able to differentiate between actual battery charging current and system (load) current during normal operation since we have only one sense resistor. Any suggestion to handle this scenario? 

    That's a good question. Does the battery have an ADC to measure the charge current? You could subtract the battery's current ADC from the charge current to get the system current.

    You could also set the battery charge current equal to the system current if the system current is low enough.

    Let me know if either of those suggestions will work.

    Best Regards,
    Ethan Galloway

  • 0 in reply to Ethan Galloway
    Prodigy 60 points

    Hi Ethan,

    Yes, using extra ADC channel will be a good solution. We are planning to use the Battery guage BQ34110 which can provide us the Charge and Discharge current. Let me work on this scheme.

    Second solution won't work as system current is dynamic and hence need to change as needed on the fly. 

    Thank you so much Ethan for your support!

    Best Regards

    -Satish