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Original question:

BQ24773: adapter/Battery absent detection and battery capacity

BQ24773: BATPRES PIN connection

James Zhan
Expert 4365 points
Part Number: BQ24773
Other Parts Discussed in Thread: BQ24780S

Hi team,

According to the datasheet, the PIN 15  of BQ24773 "BATPRES" is input signal PIN, but in the EVM I've found it seems to be output PIN.

If customer want to connect this pin to the MCU to indicate whether if the battery is present, how should they do?

Thanks.

  • 0
    TI__Expert 5330 points

    Hello James,

    The /BATPRES signal on BQ2477x is always an input signal.  The reason that it behaves like an output signal on the EVM is because there is an external comparator (U2) that generates a /BATPRES signal from the battery voltage. 

    If the battery pack your customer is using does not have a signal that can be connected to the MCU for this purpose, then an external comparator such as used on the EVM can be used.  Note that the BQ2477x also has an independent comparator that can be used to detect battery presence as well.  You can refer to the BQ24780s EVM for an example of how the independent comparator can be used instead of an external comparator, saving cost and board space.  (BQ24780s EVM has an external comparator as well, but there are jumper options to select between using the external comparator or independent comparator for /BATPRES)

    Regards,

    Steve

  • 0 in reply to Steve Preissig
    TI__Expert 4365 points

    Hi Steve,

    Thanks for the comments.

    I have more questions as below:

    1. Is PIN 15 BATPRES the only way to detect if battery present in BQ24773? could we just read one of the register and get this information?

    2. Could we left BATPRES PIN open, not connecting to anything? Would this has any risk?

    Please let me know if you have any ideas, thanks! 

  • 0 in reply to James Zhan
    TI__Expert 5330 points

    Hi James,

    The /BATPRES pin is an input pin to the BQ24773.  It is used to inform the BQ24773 that the battery has been removed.  If the pin is driven high externally, this will indicate that the battery has been removed and the BQ24773 to exit LEARN mode and will stop charging.

    The BQ24773 does not have an indicator (either PIN or register) that is an output to indicate when the battery has been removed.  Most of the systems that this device goes into use a multi-cell battery pack, and it is usually the case that a system uC is talking to the battery pack gauge IC over I2C and can sense that the battery has been removed that way.

    If /BATPRES is not used, the pin should be grounded.  Otherwise there is a chance it could float high and indicate to BQ24773 that the battery has been removed, which would force the exit from LEARN mode and charging.

    Regards,

    Steve 

  • 0 in reply to Steve Preissig
    TI__Expert 4365 points

    Hi Steve,

    I still have some follow questions as below:

    1.Could we use 24773 internal comp to compare with Vbat to generate the signal to indicate whether the battery is present or not?

    2.Do you have more information/material about the learn mode? If customer don't use the /BATPRES pin, would it have some risk of using this for charging?

    Thanks.

    James

  • 0 in reply to James Zhan
    TI__Expert 5330 points

    1.Could we use 24773 internal comp to compare with Vbat to generate the signal to indicate whether the battery is present or not?

    This should work.  The BAT+ is high impedance when battery is removed, so it may take some time for the voltage to fall from leakage.  Also, there shouldn't be any leakage that will hold the BAT+ line up, but you would need to double check that.  Generally this should work, but you might have to put a 100K resistor or so on the BAT+ line just to make sure it pulls the line to GND when the battery is removed.

    2.Do you have more information/material about the learn mode? If customer don't use the /BATPRES pin, would it have some risk of using this for charging?

    Learn mode is used in conjunction with a gauge IC to help the gauge learn the battery discharge profile.  If they are using a gauge with their battery pack, the gauge team or battery pack vendor should tell them if they need to use the learn function.  You can read more on learn mode in section 8.3.8 of the datasheet.

    Even if learn mode is not used, a transition of /BATPRES pin from low to high will stop charging and reset the ChargeVoltage() and ChargeCurrent() registers, so it is important to tie it to GND if not used.

    Regards,

    Steve