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BQ24650: BQ24650 charging 4 Li-Ion batteries series connected problem

Michael Remennik
Prodigy 40 points
Part Number: BQ24650

My name is Michael.

I have designed a charger based on BQ24650 that shall charge four Li-Ion batteries connected in series. The chosen constant voltage level is 16.8V

I have encountered the problem that the charger doesn't enter constant voltage mode. When the batteries voltage is close to 16V, the charger start working in "Discontinuous Conduction Mode" - about 1 sec of 

continuous conduction mode with full charging current of 2.6A and State 1 pin in LOW, and then about 2...3 sec of no charge current  and both state pins in HIGH, then the same cycle repeats infinitely.

The same schematic configured for charging two batteries in series works correctly. The type of used batteries is 18650. The DC voltage applied to the charger is 19V. The used coill is 6.8uH, 7mOhm DC resistance,7A saturation current.

If somebody knows the solution, please help me!

  • 0
    TI__Mastermind 24075 points

    Hi Michael,

       When you observe the repeating cycle, can you take a waveform of VCC (measure close to VCC of IC), SRN, STAT1 and PH?

  • 0 in reply to Kedar Manishankar
    Prodigy 40 points

    Hi Kedar,

    In order to check if the problem occurs due to our incorrect design, we performed the test using TI evaluation board BQ24650EVM-639, configured it for 16.8V constant voltage by replacing R10 with 200K resistor.

    The behavior of the charger was the same. You can make the same test with EVB and measure all needed signals.

    Regards,

  • 0 in reply to Michael Remennik
    TI__Mastermind 24075 points

    Hi Michael,

        The evaluation module will show me the correct behavior if I do the test. If there is an issue on your test setup I will need to see the waveforms of the behavior you are describing. 

  • 0 in reply to Kedar Manishankar
    Prodigy 40 points

    Hi Kedar,

    I have taken the scope screenshots.

    They appear in the attached pictures.

    What maybe a reason for this behavior?

    PH Waveform 1

  • 0 in reply to Michael Remennik
    TI__Mastermind 24075 points

    Hi Michael,

    Can you perform the following tests

    • Change power supply input voltage to 24V and retest
      • Check whether you have any CC limit on the actual power supply itself and remove if so
    • Keep initial 19V supply
      • Do you have long wires in between battery and SRN?
      • Monitor VFB with multimeter throughout harge profile, and log DC value at which you observe the STAT flashing
      • If possible, lower your charge current to 1A and re-test

  • 0 in reply to Kedar Manishankar
    Prodigy 40 points

    Hi Kedar,

    Today I decided to replace the batteries block being charged and started monitoring VFB.

    I have to say that with the new batteries block the charger behaved as needed and stopped charging with STAT2 LOW.

    For your information the results of VFB measurement and corresponding charging currents.

    1. Constant current value is 2.5A.

    2. Starting from VFB= 2.09929V the current starts dropping.

    3. At VFB = 2.09962V Ichrg =1.5A.

    4. At  VFB = 2.1006V1 Ichrg = 1A.

    5. at VFB = 2.10160V Ichrg = 0.5A.

    6. At VFB = 2.10196V Ichrg = 0.25.

    7. At VFB = 2.10200V the charging was stopped.

    As is now clear, the problem was in the batteries block.

    I will try another batteries block and repeat the test.

    Than you for your assistance.

    Best regards,

    Michael Remennik

  • 0 in reply to Michael Remennik
    TI__Mastermind 24075 points

    Hi Michael,

       Yes, the above results accurately describes expected behavior of the charger in the CV loop, as output voltage is regulated while charge current tapers down until termination current is hit. Glad the issue was solved with replacement.