SLUSDK0A August 2020 – March 2021 BQ25171-Q1
The device has built-in safety timers to prevent an extended charging cycle due to abnormal battery conditions. The precharge timer is fixed at 30 minutes. The user can program or disable the fast charge safety timer through the CHM_TMR pin. When safety timer expires, the charge cycle ends. A toggle on the input supply or CE pin is required to restart a charge cycle after the safety timer has expired.
During thermal regulation, the safety timer counts at half clock rate as the actual charge current is likely to be below the ISET setting. For example, if the charger is in thermal regulation throughout the whole charging cycle, and the safety timer is 10 hours, then the timer will expire in 20 hours.
During faults which disable charging, such as VIN OVP, BAT OVP, TSHUT or TS faults, the timer is suspended. Once the fault goes away, charging and the safety timer resumes. If the charging cycle is stopped and started again, the timer gets reset (toggle CE pin restarts the timer).
The safety timer restarts counting for the following events:
The precharge safety timer (fixed counter that runs when VOUT < VBAT_LOWV), follows the same rules as the fast-charge safety timer in terms of getting suspended, reset, and counting at half-rate.