What is the function of the charger's /BATPRES pin, and how should this pin be connected?
/BATPRES is an active-low input pin which is used to inform the charger that the battery has been removed. /BATPRES low indicates that the battery is present, while /BATPRES high indicates that the battery is absent. If /BATPRES goes from low to high, then the charger interprets this as a battery removal, stops charging, exits LEARN mode, and exits hybrid power boost mode (on the BQ24780S).
Some battery packs have a BATPRES output pin, which is pulled low by an internal pull-down resistor. This battery pack pin can be connected to the charger’s /BATPRES pin. Therefore, the charger's /BATPRES pin is automatically pulled low when the battery pack is present. In this case, we only need a pull-up resistor on the charger side so that the /BATPRES pin is pulled high when the battery pack is removed.
If you would like to use the /BATPRES functionality, but your battery pack does not have a BATPRES output pin, then you can use the independent comparator to detect whether the battery is present. This can be done by using CMPIN to monitor the battery voltage, and by connecting CMPOUT to /BATPRES. This way, when the battery is removed, then the independent comparator pulls CMPOUT and /BATPRES high, which tells the charger that the battery is absent.
If the /BATPRES functionality is not needed (for example, in non-removable battery applications), then you can simply tie /BATPRES to GND to indicate to the charger that the battery is always present. Do not leave the /BATPRES pin floating. Otherwise, it's possible that the /BATPRES pin may float high, thus stopping charging, exiting LEARN mode, and exiting hybrid power boost mode.
For more information, please refer to the following E2E threads:
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