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BQ24780S: BQ24780s Battery depletion?

Troy.Tsai
Intellectual 365 points
Part Number: BQ24780S
Other Parts Discussed in Thread: BQ24650

Hi,

We're developing battery charger. Our battery voltage is 1.5-2.5V, CC/CV mode charging. We've purchase bq24780sEVM. But our battery voltage always below "battery depletion threshold"(ChargeOption1 Register (0x3BH) bit15:14 )

We need a charger IC which can supply 2.5V(Max) 5-10A in CC/CV mode. Please help me to disable battery depletion threshold or find other charger IC. 

Thanks!

BR,

Troy.T 

  • 0
    TI__Genius 11850 points
    Hi Troy,

    Can you give me some more information about your application? What kind of battery are you charging, and what is your adapter voltage?

    Best regards,
    Angelo
  • 0 in reply to Angelo Zhang87
    Intellectual 365 points
    Yes, Lead–acid battery, Full 2.5V. Adapter can be 12-24V
  • 0 in reply to Troy.Tsai
    TI__Genius 11850 points

    Hi Troy,

    Thanks for that information. The BQ24780S will be fine for your application.

    I just want to clear up one potential misunderstanding. The battery depletion threshold is defined as a set percentage of the voltage regulation limit, as shown below. Therefore, as long as you set your charge voltage register setting to 2.5 V, then you won't have the issue with your battery voltage always being below the battery depletion threshold.

    In the picture above, the "2.486V/cell" approximations are assuming that the charge voltage register is set to 4.2 V/cell, which is standard for Li-ion batteries. These V/cell approximations will be different for you since you're charging a lead acid battery with a charge voltage of only 2.5 V.

    Best regards,

    Angelo

  • 0 in reply to Angelo Zhang87
    Intellectual 365 points

    Thank you for your clarify! Understood!
    I've tried to set charge voltage 2.5V, set current 3A. But when voltage(electronic load, CV mode) under 2.7V(approx), output current only 0.5A.
    I can confirm all current register was correct. When I set charge voltage to 10V /3A, load set 9V(CV mode). Current reach 3A. Everything works fine.

  • 0 in reply to Troy.Tsai
    TI__Genius 11850 points
    Hi Troy,

    In the first case (charge voltage = 2.5 V), what is the voltage on the BAT connector, and did you mean to set the e-load voltage > 2.5 V?

    Best regards,
    Angelo
  • 0 in reply to Angelo Zhang87
    Intellectual 365 points
    Please reference to below condition:

    #1 condition(current doesn't reach expected value)
    Set Charge Voltage register to 2.5V
    Set Charge Current register to 3A

    Set e-load 2V, CV mode.
    1.Start Charging(VBAT=2.5V, correct)
    2.Turn on e-load(voltage drop to 2V, current only 0.5A, current doesn't reach 3A.)
    ----------------------------------------------------------------------------------------------------------------
    #2 condition(Works fine).

    Set Charge Voltage register to 10V
    Set Charge Current register to 2A

    Set e-load to 9V, CV mode
    1.Start Chraging(VBAT=10V, correct)
    2.Turn on e-load(voltage drop to 9V, current reach 2A, correct)
    If I try to set e-load voltage(CV) lower than 2.7V(approx), then current will drop to 0.5A, but still charging.


    Please let me know if any not clear. Thank you!
  • 0 in reply to Troy.Tsai
    TI__Genius 11850 points
    Hi Troy,

    I apologize; I overlooked something in my previous replies.

    BATLOWV_RISING on the BQ24780S is 2.7 V. This means that if VBAT < 2.7 V, the charger will only apply a 0.5 A pre-charge current instead of the full fast charge current. This explains your observations, and I also saw similar results when I tested this on my own EVM. Unfortunately, the BQ24780S will not work for your application since you need 5-10 A of charge current for VBAT < 2.5 V.

    Instead, I would recommend the BQ24650. This device can also charge lead acid batteries, and it has a maximum charge current of 10 A. BATLOWV on the BQ24650 is only 1.55 V, so you'll be able to apply the full fast charge current to the battery when VBAT > 1.55 V. This application note discusses how to use the BQ24650 to charge lead acid batteries: www.ti.com/.../slva437a.pdf

    Best regards,
    Angelo
  • 0 in reply to Angelo Zhang87
    Intellectual 365 points

    Thank Angelo! And thank you for your testing. Now I understand.

    I've checked BQ24650, but in our application , we prefer programmable charger IC. do you have have any recommend programmable IC? Programmable is really useful! It seems most of programmable charger IC BATLOWV only 2.5V(approx).

    BR,

    Troy.T

  • 0 in reply to Troy.Tsai
    TI__Genius 11850 points
    Hi Troy,

    Unfortunately, the BQ24650 is the closest part we have to meeting your requirements. Like you said, the other chargers that meet your requirements (lead acid battery, 5-10 A charge current, and host-controlled) all have BATLOWV around 2.5-3 V, so they won't work for your application.

    Best regards,
    Angelo
  • 0 in reply to Angelo Zhang87
    Intellectual 365 points
    Geti it.
    We'll try to control it manually. Thanks!