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BQ25895: Charge Termination Detection in Aged Batteries

David MS
Prodigy 20 points
Part Number: BQ25895

Hi,

We are considering to use the BQ25895 in a power bank application. Our application needs to know when the battery has been fully charged, which it does by reading bits[4:3] of REG0B to check for charge termination.

Based on our past experience with similar types of ICs, as batteries age, charge termination detection may become increasingly unreliable. The problem might manifest itself as charge termination never being detected even though the battery has been charged for a long time (certainly long enough to be full).

Please let us know if the BQ25895 can reliably detect charge termination in aged batteries and if there are any steps we can take to prevent this problem from happening (either hardware or software wise).

Thank you in advance.

  • 0
    TI__Mastermind 22150 points

    Hi David,

    Charge termination detection is based on the current flowing through the BATFET inside the charger IC and the voltage detected at VBAT. When the VBAT voltage is above the recharge threshold and the IBAT (battery current) value falls below the charge termination threshold, charging will be disabled. When charging terminates, there is usually a small voltage drop seen at VBAT due to the impedance between the VBAT pin and the battery terminal. Ideally this impedance is as low as possible with the battery close to the VBAT pin, but an aging battery may have a higher impedance than its original condition. 

    If the battery impedance has increased, you will see a larger voltage drop on VBAT once charge terminates, since the max charge voltage and termination current will be reached inside the IC when the battery terminal is at a lower voltage than normal. If your battery ages to a point where the charge termination voltage drop causes the VBAT pin voltage to fall below the recharge threshold once the charging cycle finishes, you may encounter a situation where charging cycles start and stop more frequently than desired.

    The recharge threshold can be set to as much as 200mV below the battery charging voltage target to avoid this behavior (software solution). Also, it is recommended to reduce the impedance between the battery and the VBAT pins as much as possible by keeping the battery relatively close to the pins (hardware solution). Both methods in combination help to create a system that is resistant to battery aging effects.

    Beyond this, the host controller can be used to choose different battery charging limits and the charger IC has an integrated safety timer that will disable charging after a set time period if a charging cycle does not terminate as expected.

    Datasheet reference sections:

    8.2.7 Battery Charging Management

    8.2.7.3 Charging Termination

    8.2.7.6 Charging Safety Timer

    Regards,

    James

  • 0 in reply to James Steenbock
    Prodigy 20 points

    Hi James,

    Thank you for the detailed explanation, much appreciated!

    Regards,
    David