SLUSDV2A May 2020 – May 2021 BQ25798
The device has a built-in safety timer to prevent an extended charging cycle due to abnormal battery conditions. The user can program the fast charge safety timer through I2C (CHG_TMR bits). When the fast charge safety timer expires, the fault register CHG_TMR_STAT bit is set to 1, and an INT pulse is asserted to the host. The trickle charge timer is fixed 1 hour. The pre-charge safety timer is adjustable 2 hours (POR default) or 0.5 hour. The fast charging timer POR default setting is 12 hours.
The trickle charge, pre-charge and fast charge safety timers can be disabled by setting EN_TRICHG_TMR, EN_PRECHG_TMR or EN_CHG_TMR bit to 0. Each charging safety timer can be enabled anytime regardless of the current charging state. Each timer restarts counting when it is enabled. As soon as each charging stage is initiated, the associated safety timer starts to count, which is illustrated in the battery charging profile chart shown in Section 188.8.131.52.
During input voltage, current or thermal regulation, the safety timer counts at half-clock rate as the actual charge current is likely to be below the register setting. For example, if the charger is in input current regulation (IINDPM_STAT = 1) throughout the whole charging cycle, and the safety timer is set to 5 hours, then the timer will expire in 10 hours. This half-clock rate feature can be disabled by setting TMR2X_EN = 0. If the host disables the half-clock rate while the charger is already running at half-clock rate, the charger keeps running at the half-clock rate and the half-clock rate is not disabled until the charger exit the voltage, current or thermal regulation.
During faults which disable charging or supplement mode, the timer is suspended. Since the timer is not counting in this state, the TMR2X_EN bit has no effect. Once the fault goes away, the safety timer resumes. The pre-charge safety timer and the trickle charge safety timer follow the same rules as the fast charge safety timer in terms of getting suspended, reset and counting at half-rate when TMR2X_EN is set.
The fast charge timer is reset at the following events:
The pre-charge timer is reset at the following events:
The trickle charge timer is reset at the following events: