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BQ25756: Charger misbehaving during disconnecting and connecting back the battery.

sasi vp
Prodigy 30 points
Part Number: BQ25756

Tool/software:

Dear Team,

I have developed a charger using the BQ25756. It works well during the initial operation. The battery voltage is 12.6V and the charging current is 3.5A. However, I am facing an issue where the charger starts oscillating during battery connection and disconnection, and charging stops. I observed that approximately 0.7V develops across SRP and SRN during this condition.

Below are the key component details:

  • Current sense resistor: 5 mΩ

  • RC filter (two stages): 10Ω and 0.1µF

  • Capacitor across SRP and SRN: 470nF

Please advise how to resolve this oscillation issue.

Also, can I replace the 5 mΩ sense resistor with a 10 mΩ one to help stabilize the system?

Best regards,
Sasi VP
sasi@ultralifeindia.com

  • 0
    TI__Expert 5285 points

    Hi,

    How long does the oscillation occur for? When charging stops, does the EN_CHG bit get set to 0? If EN_CHG gets set to 0, and the oscillation stops, can you reset the bit to 1 to enable charging?

    sasi vp said:
    Also, can I replace the 5 mΩ sense resistor with a 10 mΩ one to help stabilize the system?

    I believe so, but you will need to adjust the charge current settings by a factor of 2.

    Best,
    Michael

  • 0 in reply to Michael Bradbourne
    Prodigy 30 points

    Hi,

    I changed the 5MΩ resistor to 10MΩ and also replaced the ICHG resistor, but the oscillation issue still persists when the battery is removed. However, when the battery is reconnected, the oscillation stops.

    I tried sending my schematic diagram via email, but there was a delivery issue.

    Could you please advise on how to resolve this oscillation problem?

    Best regards,
    Sasi V.P.

  • 0 in reply to sasi vp
    TI__Expert 5285 points

    Hi,

    Can you describe what you mean by oscillating behavior? Does the STAT1 and STAT2 pins blink alternately meaning the charger is oscillating between charging and charge done? Or is this a different issue altogether?

    One thing that might help: Does the behavior improve if you set EN_CHG = 0 before removing the battery?

    Best,
    Michael

  • 0 in reply to Michael Bradbourne
    Prodigy 30 points

    Yes, I am observing oscillating behavior where the STAT1 and STAT2 pins blink alternately. As a result, both LEDs appear dimly lit, and a low-frequency sound around 20 ~ 50Hz can be heard.

    I am using the charger in a standalone configuration without I2C control. By default, pin 7 (/CE) is pulled low (connected to GND). To address this, I attempted to pull pin 7 high when the battery is not present by adding two additional contact pins in the battery compartment. This change has led to some improvement.

    However, during the cell balancing process, the BMS MOSFETs disconnect the battery internally. When this happens, I observe the same oscillation issue again.

    Could you please share your email address so that I can send you the schematic diagram for evaluation?

    Regards,

    Sasi VP 

  • 0 in reply to sasi vp
    TI__Expert 5285 points

    Hi,

    I have sent a private message with my email address.

    sasi vp said:
    To address this, I attempted to pull pin 7 high when the battery is not present by adding two additional contact pins in the battery compartment. This change has led to some improvement.

    Can you describe what you mean by some improvement? What changed?

    Can you provide output voltage waveforms of what happens when you disconnect the battery from the system?

    Best,
    Michael