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BQ25750: Battery not charging

Predrag Milic
Prodigy 40 points
Part Number: BQ25750

Tool/software:

Hello TI Team,

I have made my own PCB using BQ25750 as part of a Solar Charger system. Still in testing phase, I cant get battery to charge.

Vinput used for testing is 24V. The battery in question is Li-Ion 4S13P, 52Ah, 12.8V.

The schematic is inserted as a pdf in this link: https://chocolate-sibeal-55.tiiny.site

First problem I had was wtih ACUV and ACOV resistors that are in the schematic. With them, the IC wouldn't start (REGN = 0V). When I took them out (ACUV to Vac and ACOV to GND), the REGN voltage jumped to 5V like it is supposed to.

My problem is that all 3 LEDS (Power Good, Stat1 and Stat2) light up. According to the datasheet this is supposed to be some kind of Fault, but reading the Fault Status register (0x24) everything is normal, aka without fault.

When I connect the battery to CON3, it reads 13.08V which is voltage of the battery when it is floating. As I understood from datasheet, using 249K as top Rfb and 29.8k as bottom Rfb, the voltage on battery should be 14.4V, correct?

Second problem I have is that Vout replicates Vac (if I put 24V in, 24 goes out, If I put 48V in, 48V go out), and if I put battery only (without Vac), the voltage is not the same as battery (13.08V), but drops to some level around 8V.

To me it seems like Im missing something to get it to fully work, but I cant figure out what. Any help is appreciated.

Thank you in advance,

Predrag

  • 0
    TI__Mastermind 23470 points

    Hello Predrag,

    Thanks for being patient with this. I'll get back to you later this week if that works for you.

    Best Regards,
    Ethan Galloway

  • 0 in reply to Ethan Galloway
    Prodigy 40 points

    Absolutely. I will make sure to provide any additional information that you require, as soon as possible.

  • +1
    Prodigy 40 points

    EDIT: I managed to get the battery to charge. After reading the registers, I found that the IC was thinking that TS was hot. As it turns out, I misunderstood that if Im not using TS pin, I was supposed to pull it down. After examining the Evaluation module again, I realized my mistake and now i soldered 4k7 resistor to make a resistor divider with the 10k. Now the battery is charging.

    I still have the other problem, where output is the same as input. I need 12V (14.4V ok also) on output from any input that is above 48V, yet now it is alwas the same as input

  • 0 in reply to Predrag Milic
    TI__Mastermind 23470 points

    Hello Predrag,

    Thanks for being patient with this.

    I can't view your schematic by the way. Can you try inserting the schematic as pdf file?

    Predrag Milic said:
    First problem I had was wtih ACUV and ACOV resistors that are in the schematic. With them, the IC wouldn't start (REGN = 0V). When I took them out (ACUV to Vac and ACOV to GND), the REGN voltage jumped to 5V like it is supposed to.

    It sounds like the ACUV and ACOV resistors might be slightly off. You might want to try lowering the ACUV threshold. You can see this FAQ for more information about the ACUV and ACOV pins.

    Predrag Milic said:
    My problem is that all 3 LEDS (Power Good, Stat1 and Stat2) light up. According to the datasheet this is supposed to be some kind of Fault, but reading the Fault Status register (0x24) everything is normal, aka without fault.

    That's interesting. What do the status registers say here?

    Predrag Milic said:
    When I connect the battery to CON3, it reads 13.08V which is voltage of the battery when it is floating. As I understood from datasheet, using 249K as top Rfb and 29.8k as bottom Rfb, the voltage on battery should be 14.4V, correct?

    Yes, 249KΩ and 29.8KΩ should give a 14.4V charging voltage. Also, are you reading 13.08V with a voltmeter of with the charger's ADCs? The ADC's in the charger don't have an accuracy specification. You can see this FAQ for more information.

    Predrag Milic said:
    Second problem I have is that Vout replicates Vac (if I put 24V in, 24 goes out, If I put 48V in, 48V go out), and if I put battery only (without Vac), the voltage is not the same as battery (13.08V), but drops to some level around 8V.

    I don't quite understand what you mean here. Can you try rephrasing this issue?

    Best Regards,
    Ethan Galloway

  • 0 in reply to Ethan Galloway
    Prodigy 40 points

    Hello Ethan,

    I apologize for the delay in answering to you, but I wasnt available during the past 3/4 days. I thank you for your answer and to the TI interest to this problem.

    Now to recap, The status LEDs and the battery charging issue is resolved with the resistor divider at TS pin. Like I said before, it was my misunderstanding of how to handle the TS pin when Im not using it. 

    The only problem I have left is with the output voltage. This could be also one of my misunderstandings of how the IC works, but I need 12V (or 14.4V) on Vout (SYS pin), and right now that voltage is the same as input voltage (VAC pin). If there is 24V on VAC then there is 24V on SYS. If there is 48V on VAC, there is 48V on SYS. This is regardless of weather the IC is charging a battery or not.

    Here is the schematic as a .pdf

     Modul Punjac 1.0.pdf

  • +1 in reply to Predrag Milic
    TI__Mastermind 23470 points

    Hello Predrag,

    No problem with the delay.

    Predrag Milic said:
    The only problem I have left is with the output voltage. This could be also one of my misunderstandings of how the IC works, but I need 12V (or 14.4V) on Vout (SYS pin), and right now that voltage is the same as input voltage (VAC pin). If there is 24V on VAC then there is 24V on SYS. If there is 48V on VAC, there is 48V on SYS. This is regardless of weather the IC is charging a battery or not.

    This is normal operation for the IC. When the the power is good, the IC turns the BATFETs OFF and the ACFETs ON. This connects VAC to VSYS. This FAQ on the power path goes into more detail. Also, here's a link to all the FAQs for reference.

    To get VBAT on the system, you might have to connect your system input directly to the battery. Will that work for your system?

    Also, your schematic looks good. I have a few minor suggestions:

    • A small ceramic capacitor on VBAT and VSYS close to the switching FETs would help filter out high frequency noise
    • I recommend installing a placeholder RC filter from BATDRV to BATSRC in case you need to slow down the BATDRV turn ON.

    Best Regards,
    Ethan Galloway

  • 0 in reply to Ethan Galloway
    Prodigy 40 points

    Hello Ethan,

    Thank you for your reply and your suggestions. I will make sure to follow them through. 

    You have answered all I needed to know. I will make adjustments.

    Thank you once again,

    Best Regards,

    Predrag