When a source is connected to IN or USB, the BQ2416xx runs a Bad Source Detection procedure to determine if the source is strong enough to provide some current to charge the battery. A current sink is turned on (30 mA for USB input, 75 mA for the IN input) for 32 ms. If the source is valid after the 32 ms (V_BADSOURCE < V_SUPPLY < V_OVP), the buck converter starts up and normal operation continues. If the supply voltage falls below V_{BAD_SOURCE} during the detection, the current sink shuts off for two seconds and then retries, a single 128-μs pulse is sent on the STAT and INT outputs and the STATx and FAULT_x bits of the status registers and the battery/supply status registers are updated. The detection circuits retry continuously until either a new source is connected to the other input or a valid source is detected after the detection time. If during normal operation the source falls to V_{BAD_SOURCE}, the BQ2416xx turns off the PWM converter, turns the battery FET on, sends a single 128-μs pulse is sent on the STAT and INT outputs and the STATx and FAULT_x bits of the status registers, and the battery/supply status registers are updated. Once a good source is detected, the STATx and FAULT_x bits are cleared and the device returns to normal operation.

If two supplies are connected, the supply with precedence is checked first. If the supply detection fails once, the device switches to the other supply for two seconds and then retries. This allows the priority supply to settle if the connection was jittery or the supply ramp was too slow to pass detection. If the priority supply fails the detection a second time, it is locked out and lower priority supply is used. Once the bad supply is locked out, it remains locked out until the supply voltage falls below UVLO. This prevents continuously switching between a weak supply and a good supply.