bq2415x: What is weak battery detection of a USB input battery charger?

Q: What is weak battery detection of a USB input battery charger?

A:

This is an USB spec. The USB spec states that a battery can not be charged until the portable device enumerates if the battery voltage is above the "weak battery threshold". If the battery is below the weak battery threshold, the portable device is allowed to charge without enumeration for 45 minutes.

The USB spec allows the designer to select the weak battery threshold for their system. The bq2415x default is 3.7V because that is a high enough voltage for most systems to power up and be able to enumerate; however, the user can program a lower value.